Update April 2024
In mid 2019 I converted our off-grid house battery to nickel-iron batteries. 4 1/2 years later, I’ve re-written this review because they didn’t work out. |
| Here are our nickel-iron house batteries, on our verandah |
I have failed to meet our electricity storage needs with our nickel-iron batteries (often abbreviated to “Nife” in reference to the chemical symbols), and I’ve decided to replace them with a different type of battery. I want this review to help others who are considering off-grid PV battery systems, so I will be as clear and extensive as I think is needed.
I decided to try a Nife battery for our house because it appeared they might be the most durable type of battery, with the lowest cost in money and resources per year over their life, along with low toxicity risks. Batteries are usually the weak link and the greatest ongoing expense in an off-grid PV electricity system, and I’ve seen a lot of house batteries die. Claims that Nife batteries can last for decades were attractive to me, and I was willing to manage the technical challenges and the maintenance needs.
However our nickel-iron battery has failed - under my management - to do what’s needed for our PV electricity supply. It has never delivered anything near its rated Amp-hour (Ah) capacity, and its charging voltage has been difficult to fit into our system. Its Ah capacity has now declined to the point where it doesn’t meet our needs.
Below I want to first give a review of what I’ve learned about Nife cells, and what went wrong. Then I will give a longer explanation about batteries for off-grid PV electricity.
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| Here's our shed with our PV array on the roof: about 2.5kW of old panels. |
Our household electricity system
The Nife battery being reviewed here is in our household PV electric system. This has about 2.5kW of old PV panels, charging a 24V battery (currently Nickel-iron, but not for much longer!). PV panels charge through a Plasmatronics PL60 Charge Controller: a switching controller without any MPPT function. Power is distributed at 230V from a Selectronics SE22 inverter (from year 2000) to our house and sheds, and 24V (nominally - ranges from 19 to 32V) DC battery power is distributed in the house for lights, fridge, freezer and small DC loads. We currently get backup power in cloudy weather from a 230V AC generator powering a large battery charger.My experience of Nickel-iron batteries
In summary:- The high charging voltages required by Nife cells have been a challenge for charge controllers, inverters, DC loads and battery chargers.
- Trying to increase charging currents to recommended levels has been difficult. It may be that Nife batteries need a PV array sized to meet the battery’s need for charging current, instead of the household’s need for energy.
- I haven’t found a way to effectively use our solar charge regulator/controller to manage charging, because of the greatly varying current from PV panels and Nife batteries’ voltage characteristics, so I haven’t been able to manage excessive gassing.
- We’ve used a lot of distilled water.
- I’m pretty experienced and capable with off-grid PV systems and their batteries, I have a good grasp of relevant electrical theory, and I’ve tried really hard to make these Nife batteries work for us, but I have been unsuccessful. It’s still my failure, and I’m not saying Nife batteries are no good, but if I can’t make them work, they aren’t for everyone.
Battery Amp-hour (Ah) capacity
The specified capacity of our 24V Nife battery was 200Ah. At its best, it delivered maybe 70 or 80Ah. In the last few weeks (early 2024), it’s been going flat (under 19V, where the inverter shuts down) after discharging 20 or 30Ah from full.I measure the Ah capacity of our battery using our Plasmatronics “PL60 Solar Charge Controller”: I’ve used Plasmatronics regulators for over 20 years, they are very reliable and controllable, and amongst other things they count Ah in and Ah out every day. I set the regulator to change days at 6pm - approximately the end of the solar day, so that next morning I can easily see how many Ah we have discharged overnight. If the battery voltage has dipped below 19V overnight (inverter shut-down voltage), I consider the battery has gone flat. If I know that the previous day was sunny and the battery got full, the Ah discharge overnight is a fair approximation of the total Ah capacity of the battery.
Having our battery go flat frequently requires us to turn off the inverter and the DC fridge and freezer in the evening to reduce discharge overnight. If we don’t turn off loads, the battery can crash to a really low voltage. If the battery goes much under 10V our solar charge regulator shuts down and needs to be “jump started” in the morning for it to allow solar charging to resume.
As far as I am aware, there are 3 things that could possibly cause our battery capacity problems:
Most descriptions of Nife batteries from the literature states that they aren’t harmed by over-discharging or over-charging.
Charge current
It appears that Nife batteries like a high charge current, to “condition the plates”. For example, “Encell” branded batteries are recommended to be given conditioning charges, on installation, at C/2 for 3 hours several times, with discharges between charges. “C/2”, also often written “C2”, means a current equal to the rated Ah capacity divided by 2 - the current that would flatten the battery in 2 hours. In our case, we would need 100A to do this for our 200Ah battery. Encell recommends normal charging to be at C/4 (50A for mine).The old Edison battery brochure (linked at end of post) advises to normally charge at 1.8 to 1.9V per cell, charging at C/5, (40A for mine). If we used 20 cells, this would result in a battery voltage of 36 to 38V, way above the tolerance of most of our loads (inverter, fridges, etc). Edison advises periodic conditioning charges (“boosting”) of 2 to 5 times normal rate (80A to 200A for mine).
Our battery supplier told us, after delivery, that the battery should be normally charged at C/5, meaning 40A for our 200Ah battery. For the first years of the Nife battery here, our solar charge would go over 40A for short periods, but PV supply fluctuates with sunlight and with temperature, and it was normal for charging to be 25A to 40A in the middle of a sunny day, with lower currents early and late in the day. Even at these not-quite-enough currents, we had problems with high voltage causing load shutdowns in sunny times, leading me to remove one cell and operate on 19cells - a common technique. To be clear: the recommended charging current causes battery voltages too high for normal household loads (inverters etc.).
Our supplier told me that our battery wasn’t performing because of the low charge current, so over time, I installed more PV panels. Our charging current has become higher: now peaking at 60A but maintaining 40 - 50A during the middle of a sunny day. This hasn’t improved Ah capacity.
There are some real practical issues with achieving this level of charge current in a household solar electric system: achieving the current, dealing with the consequent voltage, and water loss.
Achieving high currents
40 - 50A is a lot of current for an off-grid PV system like ours. If our battery discharges 60Ah overnight (our average), 40A charge will see the battery full in 1.5 hours (assuming no loads use current and reduce battery charge). For the rest of the day the battery will be full, cycling a little when loads exceed the PV current. A peak charge current of 40A (being conservative) will provide about twice as much energy as we need for our average daily consumption of approx 120Ah (a rule of thumb is total energy = peak x 6h).In other words, while we have had this Nife battery, we have had plenty of PV input to meet our household energy needs, but maybe not enough to meet the battery needs. If we need to increase our current further, we will need to install PV panels that have no purpose other than keeping the battery functioning - we don’t need the extra energy.
If we were to meet the standard proposed in the Encell specs, we would need to periodically do conditioning charges of 100A (C/2). To do this with PV would require more panels than we can fit on our large shed roof, would exceed the capacity of our charge regulator and would test the capacity of our wiring and fuses. To provide 100A with a generator and battery charger would be difficult and very expensive: this would be a very big charger, exceeding the voltage of normal chargers, and costing $1000s.
Charge voltage
Relative to other common batteries, Nife batteries have a high internal resistance, and consequently charge at high voltages and discharge at low voltages. Even with our (perhaps inadequate) charging currents, the charging voltage of our Nife battery has created problems with our household loads, because voltages have been too high for them.Charging 20 cells at 30 - 40A, caused battery voltage to exceed 34V once the battery had charged up a little. At 34V our inverter shuts down (to protect itself) and our DC fridges shut down too. To manage this problem, I removed one cell and ran on 19 cells to lower the voltage, which has worked pretty well. If we were to increase our charge current by installing more PV, this would increase battery voltage, so we would again risk exceeding the voltage limits of our loads.
Water loss
Nife batteries bubble a lot, losing electrical charge energy as hydrogen and oxygen gas.PV charge regulators/controllers use voltage to control charge. In other chemistries, such as lead-acid or lithium, the battery has a low resistance, and gradually rises in voltage as the state of charge increases. Charge current needs to be within a reasonable range for this, as any battery chemistry will increase in voltage with increase in charge current, so a disproportionate charge current will give a high voltage from resistance, instead of state of charge.
With Nife batteries, the high resistance means that charge voltages are more indicative of charge current, than state of charge. For example, I can observe our battery at 18V early in the morning, then leaping to 28V in a few minutes when 2A of charge comes from the PV. Even though it’s at 28V, it’s still flat and can’t support a discharge current. As solar charge increases, the voltage rises, so our nearly-flat battery can be at 32V once it is receiving 40A or so.
This current-driven charge voltage means it’s very difficult to use a charge regulator to limit charging: if you set a voltage in the regulator to throttle charge current (often called maximum boost or bulk voltage), you can find you are cutting down charge when your battery isn’t full yet. Not setting a maximum voltage means unlimited charging which means losing a lot of water: we top up the batteries with 20 to 30 litres of distilled water, every 4-6 weeks. Some Nife battery users would advise this is way too much (I agree), but I have found I can’t find a balance between trying to “fix” our battery with more charge current, and managing water use with voltage-limiting by the regulator.
Electrolyte
Our Nife cells arrived already full of potassium hydroxide electrolyte, so I don’t know what concentration they are. It is conceivable that the electrolyte is not the correct concentration, or that it has been made ineffective by the quality of distilled water I’ve been using, CO2 dissolving in it, or something else. It’s not easy to work this out. I have checked the quality of the distilled water with a test result provided by the seller, but I don’t know how to test the electrolyte in the cells.Other people’s Nife batteries
When I have a problem, I like to ask someone who knows more than me, or who has a different perspective. With Nife batteries, this has been very difficult, because these batteries are so rare (which must mean something…).From what I can find, almost nobody measures the actual Ah capacity of their battery, making it hard to evaluate whether others are having the same problems as me. Furthermore, some people install very large capacity batteries, so that they might find satisfactory energy storage even when the battery isn’t performing to specs.
The only person I know with an off-grid PV electricity system using a Nife battery is my friend Mike Stasse, of Damn the Matrix blog. Mike and I have talked a lot about battery and PV questions.
Mike has a 48V Nife battery, made of 40 cells the same brand and size as mine (specified as 200Ah). His max charging current is around 10A (much lower than mine). Although Mike’s battery has twice the nominal energy storage of mine, and his household energy consumption is significantly lower, his battery still goes flat after a cloudy day. From his description, my estimate is that his battery has a capacity of around 50Ah: similar to mine.
The future of our Nife battery
I can’t solve the problems with this battery while providing electricity to our busy house. One day I might do some experiments to find out if I can get the cells working better, perhaps trying with one cell, but that won’t be easy: changing electrolyte, charging at a suitable current, discharging, measuring Ah of discharge. For now I need to put my Nife battery away somewhere safe, and look after our household with a new battery. I'm not sure if I should store them flooded or dry.
August 2024: I’ve added this update after receiving the very helpful input from other Nife users in the comments below
My understanding of Nickel Iron batteries
Promotion of Nife batteries for off-grid PV household use is generally based on historic accounts of Nife batteries. I don’t doubt that in suitable applications, Nife batteries work and last well.
However, based on my experience and that of other contemporary Nife users, I don’t think that Nife batteries are suited to off-grid PV house use, at least not with current system designs. Here are some key points:
1. The need to deeply cycle Nife batteries.
All the big talk about the durability of Nife batteries describes historic batteries surviving for many decades, but I expect that few or none of these historic batteries were in a cycling pattern anything like that of contemporary off-grid PV domestic power. Reading the Edison user manual (brief, but one of the best sources of information I’ve found) it appears that batteries were discharged in a job (often powering a vehicle, likely at fairly high currents), then taken to the charging station and recharged from a big transformer-rectifier at a specific set current for a specific set time or to a rather high voltage per cell. Discharging would probably be fairly deep, because batteries would be sized to last through a shift (or whatever the job cycle is) with only enough reserve to make it home or finish the job. There was no need to oversize batteries for wet weeks (like we do in off-grid PV), the best battery was just big enough for the job.
2. Separate charging and discharging.
In historic use, charging of Nife batteries was at separate times from discharging. High charging voltages couldn’t harm loads, and charging currents weren’t compromised by clouds, shading or loads like they are in domestic PV systems.
If we imagine a graph of current over time for traditional charging, it would be like a high plateau: straight up to full current from the beginning, flat charging current, then stopping when the battery is full. A graph of charge current over time for our off-grid battery would look like the great dividing range: long flat plains when the battery is full and loads are small, the same when the battery is low and it is dark or cloudy, little rises in morning sun or cloudy light, occasional peaks when the sun comes out with no loads, hilly times when there is sun but loads or clouds are intermittently bringing the currents down. With off-grid PV we can’t charge at a fixed rate, or separate charging and discharging - unless we come up with new tricks.
3. High battery capacity
I think it’s a serious problem to have a high capacity Nife battery. It’s very difficult to charge Nife batteries, from PV, at the required currents. But people on off-grid solar always think about the cloudy weeks (especially here in the mountains), when we want big batteries to keep the lights on and avoid running the generator. So, trying to do the right thing and invest adequately, for example, people install 500Ah at 24V. But to keep Nife batteries healthy, a bigger capacity requires a bigger charge power. To charge 500Ah at C/5 you need 100A at 34V continuous for several hours every day - that’s net charge current. If you want to use power at the same time you need to provide extra current for the loads as well, to maintain 100A of charge into the battery. If there’s no sun and you want to backup charge (without any loads) that’s 4kW of battery charger, requiring a 3 phase 230V/380V ac supply (if you could get/afford a charger that can do Nife voltages). If charging at C/3 is what is needed to recondition the battery, you need 167A at maybe 36V or 38V (6kW output). I can’t see how you could do this with PV or a generator and charger, without phenomenal expense.
4. Solar charge controllers
I don’t think they’re the way to regulate Nife battery charging, because, I think, off-grid household PV isn’t a suitable charging source for Nife batteries. I worked out how to trick my Plasmatronics PL60 into going to 34V, but I never found a way to manage water loss down to a reasonable level. This is because with PV, you’re charging (in between discharging) at a continuously varying rate, so the voltage is swinging up and down, and the charge controller has no way to know when to make the call that the battery is full. You could get a controller to count Amp hours, but I think that wouldn’t work because the Nife cell wants a large, fixed payment every morning, not counting coins all day trying to add up to a full charge.
5. Separating cells to recondition Nife cells that have lost capacity
I have an old Australian made Woods Dialocharge 2460 battery charger (24V, 60A) with an adjusting knob. I used this to charge the Nife battery when it went flat in rainy weather, but the voltage was at the very top of its range, so once the battery voltage started to rise, the current dropped quickly. Certainly no chance of reconditioning 19 (or 20) cells with that.
I have wondered if the Dialocharge could be used on one or a few cells at a time for reconditioning charges, but that would require dismantling our battery (ceasing the supply of power to our house), and taking cells to a mains powered house for days of charging. Then bring the cells home, put the system back together, and see if we can see an improvement. Not easy to do while the house is still full of people and jobs to do and dark nights. If anyone does this, I hope they publish the results.
In summary, the early 20th century industrial Nife batteries that created their reputation weren’t living the off-grid PV household life. They got charged from a big power supply at the right rate, then discharged doing their job, usually in industrial use. It looks to me like they can’t maintain good health in our sort of use, and they just aren’t suitable.
The rest of this blog post
From here on, I offer an explanation of off-grid household batteries, and some of what I have learnt about them. If you are considering installing a household battery of any type, I encourage you to read on.Batteries: a major energy cost
Storing energy in batteries takes a lot of energy. Most of this energy is “embodied” in the batteries in the thousands of processes required to get them working: mining, refining and shaping metals, manufacturing plastics from oil, providing the economic needs of the many different people who design, make, sell, transport, or install them. The best way we have of measuring this energy - and its associated CO2 emissions - is the money we spend on batteries (see my blog post on how money is energy).How much energy - or money - a solar house battery uses is very variable. A small (low cost) battery that lasts a long time will use less money and energy per year than a big battery that dies young.
Battery cost is greatly affected by how you use your solar energy. If you mostly use electricity when the sun is shining and you limit battery use to shallow discharging, you can get by with a small battery that lasts a long time. If you want to do electric cooking at night after a cloudy day, you’ll need a big battery that gets deeply discharged and may not last so long.
To get a feel for the cost of battery storage, consider a traditional lead-acid solar battery just like I’ve had in some of my off-grid solar systems: 12 volts (V), 1000 Amp hours (Ah), costing about $4000 and lasting maybe 10 years. Note that this doesn’t include the cost of transporting or installing them, or building a suitable enclosure, all of which are real energy and money costs.
The daily discharge cycle of lead acid batteries should be only about 10% from full, to get a long life, so each day this battery could comfortably cycle 100Ah at 12V, which is 1.2 kilowatt-hours (kWh). Doing this every day for 10 years gives 3650 cycles, which would total 4380kWh. $4000 divided by 4380kWh gives nearly $1 per kWh of energy stored for a few hours.
Generating this energy from solar PV panels is much cheaper than storing it. If you install 1kW of panels for $2000, that generate maybe 4kWh per day and last for 20 years, their energy costs around 10c/kWh - 1/10th the cost of storing this energy in batteries (I recognise I’m ignoring various real costs such as regulators and inverters - I’m trying to get the principle here).
To summarise: storing energy in batteries costs way more than generating it. If you can reduce your battery size and increase its life - mostly by using less energy and using it when it’s being generated by your PV - you will greatly reduce the energy and money cost of your home energy system.
Living with house batteries
Most of my life I’ve lived here at Mt Glorious, off grid, depending on photovoltaic (PV) electricity and battery storage for our electricity supply. My family and I have learnt to live with the limitations of off-grid PV power, limiting our daily consumption through frugality and efficiency, using power when the sun shines and carefully limiting electricity use when it’s cloudy. For nearly all this time we’ve depended on lead-acid batteries for electricity storage: starting with an old car battery from the kerbside rubbish collection, then old Telecom batteries, and finally a few sets of big, brand new batteries costing 1000$ of dollars. I’ve become comfortable with the mysteries of lead-acid batteries: the need to cycle them from the top (keep them as full as you can), the importance of having enough PV current to give them a good, bubbling equalisation and de-stratification every few weeks, and the lurking dangers of sulphation if they get discharged too low for too long. Lead-acid batteries are like chainsaw chains: they’re never as good as when they’re new, and time gradually fritters them away. If you get 10 years out of them, you should be grateful - 15 years is exceptional, 5 years is not unusual.
As part of our ongoing efforts to create a resilient household, I tried nickel-iron batteries. Now I don't believe they are suitable for off-grid PV household systems, I'm trying a lithium iron phosphate battery now.
Amp-hour capacity
The capacity of a battery in Amp-hours (Ah) is one of its most important characteristics, but Ah means very different things in different types of battery. The ability to tolerate deep discharge without damage has a huge effect on the meaning of a battery’s amp-hour capacity.For clarity: Ah is a measure of battery energy storage, calculated by measuring the current in amps (A) from a battery, for how long in hours (h) it can provide current, until considered to be discharged by going down to a certain voltage. The amount of Amps multiplied by the number of hours gives amp-hours (Ah) - e.g. if you draw 10A for 5 hours, you’ve used 50Ah.
Batteries are sold with an Ah rating: for example our last lead-acid house batteries were 1300Ah cells, meaning you could theoretically discharge them at 13A for 100h and then they’d be flat (but you’d never do this!). An important thing to know is that you get more Ah from a battery if you discharge it slowly (Wikipedia’s entry on Peukert's law describes this relationship and explains how most of the energy isn’t actually lost if the battery is given time to compose itself). Batteries are given their Ah rating based on discharging in a particular number of hours, e.g. my old batteries were rated at 1300Ah if discharged over 100h, but only around half that if you discharged them in 10h. This is shortened to saying the battery has a C100 rating of 1300Ah and a C10 rating of 650Ah.
Lead-acid
The dangers of sulphation!
If you want them to last a long time, lead acid batteries should be kept above 90% full in daily cycling, and never discharged below 70% when you have a long cloudy period (70% full is often described as 70% SOC - state of charge). This is because lead-acid batteries gradually sulphate according to how long and deeply they are discharged - insoluble lead sulphate crystals grow on the lead plates and block the charge-discharge reaction.When our lead-acid house batteries got below 80% SOC due to heavy cloud, we’d start worrying. We’d plan to run a backup generator and charge the batteries up. We weren’t running the backup because the lights were about to go out - our lead-acid batteries were still 70% or 80% SOC with enough storage to run our house for days. We were running the backup because we were worried about shortening our battery life due to sulphation.
So in reality, when you buy 1000Ah of lead-acid batteries, you’re getting maybe max 250Ah of useable storage if you want to get a long life from them, less than 250Ah if you are discharging at high currents. This doesn’t mean that lead-acid batteries are bad - their popularity is because they are relatively cheap, deliver high currents easily, and last a reasonable time if looked after well.
Deep discharging
Most other battery chemistries don’t have the problem of being damaged by deep discharge in the way lead-acid batteries do (though they all have their own problems). For example Lithium batteries can deliver their full Ah capacity repeatedly, though they can be more durable if cycled through a smaller capacity - e.g. between 10% and 90% full. In practical terms, this means that 200Ah of lithium batteries will provide the same daily cycling ability as 1000Ah or more of lead-acid.Overall, this means you can’t usefully compare batteries directly on cost per Ah, because some batteries can be cycled deeply while others can’t, a battery delivers different Ah depending on what current you draw from it and batteries have very different life expectancies which greatly affects cost per year of service.
When the sun goes away
When our mountain goes into the clouds for a few weeks, we tend to stop all the big loads and just run fridge, lights and some electronics. With lithium or Nife batteries we can leave the battery nearly flat, do some backup charging if we are really running out, and wait for the sun to charge the batteries fully again. With lead-acid, we tend to be anxious about deep cycling, and do backup charging with a generator to avoid battery damage.Equalisation, balance charging and regulation
Any normal household solar power system uses some sort of regulator (often called a charge controller) between the solar panels and the battery, to control how the battery is charged. Mostly the regulator protects the battery from over-charging, but some regulators also give some information to the users about the battery state of charge. Depending on the type of battery, this charge regulation is more or less complex, largely regarding the problem of keeping each cell at the same state of charge as its sisters in the battery.For clarification: a “battery” is a group of “cells” joined together, usually in series. However english language now tends to use the word battery to describe a single cell: e.g. a single AAA battery for a torch is really a single cell; a 12V car battery is made of 6 x 2V cells joined together in series.
Each cell in a battery is an individual, each having slight differences. As a battery cycles up and down over time, slight differences between cells can accumulate and become big differences in state of charge. One or two cells in a battery might lag behind the others, gradually becoming discharged while their sisters are full. In a lead-acid battery this could cause a cell or two to gradually sulphate and die young, wrecking the whole battery. To prevent this trouble, from time to time all the cells need to be brought up to being all completely full at the same time. This is called “equalisation”, (often applied to lead-acid) or “balancing” (often applied to lithium), and different battery chemistries require different solutions to achieve this. I’ll run through how different house battery types get equalised.
Equalising flooded lead-acid batteries
Flooded lead-acid batteries (traditional cells with a liquid acid electrolyte that sloshes around in the cell case) are easy to equalise. Every few weeks, a higher than usual voltage is given to the battery, for a few hours, when it’s full. This pushes a higher than usual (for a full battery) current through all the cells in series: in effect the battery is being over-charged. When a flooded lead-acid cell is full - the reaction of lead and sulphuric acid is complete - the electrical energy being pushed through it can no longer be stored as chemical energy. The energy has to go to waste somewhere, and in a flooded cell the energy is consumed in splitting water molecules into hydrogen and oxygen gas, that bubbles up through the electrolyte and out through the cell vent. Gassing like this happens to a lesser extent even when lead-acid cells aren’t full: the fuller they are, the less charging energy is stored and the more energy is wasted as gassing. This gas has a lot of energy in it, which can be released in an explosion if you give it a spark - that’s why you keep sparks and flames away from lead-acid or nickel-iron batteries.While the charge current is flowing equally through all the cells in series, the full cells are losing the energy from the electric charging current as gas, and any cells that aren’t quite full can continue to store chemical energy - charge up - using the same current. Thus all the cells in the battery can gradually equalise - get full - at the same time, even though they may have started at different states of charge.
While this equalisation charge is happening, the flow of gas bubbles rising up between the lead plates in the flooded lead-acid cell has another important purpose: stirring up the electrolyte. The sulphuric acid in these batteries is heavier than the water it is dissolved in, and with time the acid can settle down to the bottom, making the electrolyte more acidic at the bottom of the cell and more watery at the top - this is called “stratification”. This is bad for the cells: the over-strong acid at the bottom can damage the lead; and bad for energy storage: the weak acid at the top doesn’t react as much with the lead so doesn’t store so much energy. Testing the electrolyte in a lead-acid cell with a hydrometer (that measures the density of the acid and thus the SOC) can sometimes show low-density electrolyte (like a discharged battery) when the batteries are fully charged, just because of stratification: the batteries are full but the acid has sunk to the bottom.
To avoid stratification, the solar power system must deliver enough current to adequately stir the electrolyte. In my experience a battery will need PV panels that can give a charge current in Amps, of about 1/20th the Ah rating of the battery. This means a 1000Ah battery needs PV charge of at least 50A, when the midday sun is shining and the regulator is equalising, to keep its electrolyte stirred. This also means that bigger isn’t always better with flooded batteries: if your battery is too big for your PV array, you may not be able to keep the electrolyte stirred, and your battery may die young.
Key points for flooded lead-acid batteries
Here are a few key things for flooded lead-acid cells:
Flooded lead-acid batteries
• Flooded lead-acid batteries are reasonably cheap, easily recycled, low-complexity, and well known.
• Flooded lead-acid batteries are good at losing energy as gas when they are over-charged. Making this gas uses up water so the batteries need topping up. The regulator limits the charge voltage so that batteries don’t need topping up too frequently.
• Because they can lose energy so easily, a flooded lead-acid battery can be equalised by simply pushing extra current through all the cells until any lagging cells catch up with their sisters.
• Lead-acid cells need enough charging current to give them a good bubbling and keep the acid well-mixed. For this, they need enough PV panels charging them.
Sealed lead-acid batteries
Sealed lead-acid (SLA) batteries, often titled valve regulated lead acid (VRLA) batteries, are common in houses. A lot of off-grid solar energy installers recommend sealed lead-acid batteries because sealed batteries don’t need regular topping up with demineralised water, and installers are fed up with customers who don’t maintain their batteries.When SLA cells are working normally, the hydrogen and oxygen gas produced at their plates is re-combined to produce water and heat inside the cell. This system can only handle so much gas, so it’s very important not to over-charge sealed lead-acid batteries and overload the battery with gas. SLA cells also don’t have the problem of stratifying acid because their acid is trapped in gel or a glass mat, so they don’t need to stir their electrolyte with gas bubbles like a flooded cell. It’s very important to carefully regulate the charging of sealed lead-acid batteries, maintaining the right voltages and limiting gas production. Sealed lead-acid cells can’t handle overcharging by losing energy as vented hydrogen and oxygen gas. In a gel cell, excess gassing can cause the gel - containing the acid - to permanently lose contact with areas of the lead plates.
Sealed lead-acid batteries do need equalising by periodically holding them at a higher voltage for longer than their normal charge cycle, but they do this with lower voltages than flooded batteries, carefully controlled by a regulator.
SLA key points
The key things about sealed lead-acid batteries are:• SLA batteries can’t lose energy by venting gas.
• SLA batteries (especially gel batteries) can easily be damaged if they are charged at too high a voltage, so they need strict control of charging voltage by their regulator.
• SLA batteries can lose some energy by internal gassing and recombination. This produces heat, which needs to be limited.
• Because SLA batteries can lose some energy internally, they can be equalised by carefully charging them all until any lagging cells catch up.
• SLA batteries don’t need topping up.
Lithium batteries
I love lithium batteries. They’re great on our electric bikes: they’re compact and light (compared to other battery types), they deliver high currents and some types last a long time. They aren’t damaged by being kept at a low SOC - as long as they’re not too flat. They make e-bikes into the marvellous transport machines they are. |
| Here's a 36V lithium iron phosphate bike battery: 12 x 3.3V cells in series, with the BMS on top, with thin wires going down to keep an eye on each cell. It also needs a special charger that gives the right current and limits to the right voltage |
To keep lithium batteries within their safe bounds, they are managed and balanced by an electronic circuit, often called a battery management system (BMS), that is wired on to the battery. The BMS controls charging and discharging of the battery, shutting the battery’s output down if the voltage gets too low, or shutting down the charge if the voltage gets too high. The BMS also continuously measures the voltage of each cell, so it can shut down the whole battery output if one cell’s voltage gets too low. To equalise - or balance - the cells, the BMS has circuits that slowly discharge the fullest cells during charging, so the less full cells can catch up. This occurs every time the battery is charged to full, so only tiny adjustments are needed.
A lithium battery is very much dependent on its built-in BMS, as well as needing a solar charge regulator like any other battery type. It is a complex electronic device, as well as a chemical energy storage. If there is an electronic failure in the BMS, from component failure, lightning surge, etc., the parts and skills to repair it will be needed for the battery to provide power again.
Lithium key points
The key points for lithium batteries are:• Lithium batteries are easily harmed by being charged at too high a voltage, because they aren’t able to lose any energy as gas, and not much by heat.
• Because of this, lithium batteries can’t be equalised by charging the whole set and waiting for lagging cells to catch up.
• Each cell needs to be individually controlled in voltage, with a BMS that gives individual lagging cells extra charging time and current.
Charging and equalising nickel-iron batteries
Each of the batteries described above - flooded lead-acid, sealed lead-acid and lithium - are more complex in their construction and require more careful and complex regulation than the previous type. Compared to all of them, nickel-iron batteries have the simplest charging needs.Nickel-iron cells are flooded with electrolyte (like flooded lead-acid), so they can easily lose surplus energy as hydrogen and oxygen gas. They are so tolerant of gassing that they hardly need to be regulated. The main purposes of having a regulator on a Nife battery is not to protect the battery, it’s to reduce the amount of topping up with water required, and to avoid system voltages that are too high for the loads (especially the inverter).
This tolerance to over-charging means they can be equalised by simply charging the whole battery and letting any lagging cells catch up.
Nickel-iron cells are not damaged by being left at a low SOC, so a lagging cell won’t be harmed. However cells don’t get much change to lag, because of the over-charging nickel-iron batteries tend to get on any sunny day.
Nickel-iron key points
To summarise:• Nickel-iron batteries easily lose energy as gas
• Equalisation is not usually an issue when Nife batteries are charged generously
• Nickel-iron batteries are tolerant of charging without a regulator (if currents aren’t too high) but a regulator reduces the need to add water and avoids voltages too high for the loads
Battery voltage
Different battery chemistries produce different voltages. Lead acid cells - of all types - produce about 2V. Lithium cells in solar house systems (usually lithium iron phosphate) produce about 3.3V, but the chemistry in lithium 18650 cells (like torches and many bike batteries use) produces about 3.7V. Nickel-iron cells produce about 1.2V each - so you need lots of them.Remember the voltage of a rechargeable cell varies a lot as it does its work each day: it rises when the cells are being charged, rises even more as they get full, and falls when being discharged. Loads need to be able to cope with this varying voltage - off-grid inverters amaze me with their ability to take a varying battery voltage in, and put a steady ac voltage out.
Energy density
Different battery types take more or less volume to hold an amount of energy. Lithium batteries have a very high energy density, being very small for the amount of energy they hold (and very light as well). That's why they're so good in electric bikes. Lead acid take more space per unit energy, and nickel-iron batteries have a low energy density and take a lot of space. Energy density isn't a major issue for a house designed to be off-grid (you just design a space for the batteries), but it can be relevant in a retrofit.Efficiency of off-grid batteries
Nickel-iron batteries gas a lot, even more than flooded lead-acid. This indicates a potentially low efficiency for these batteries - a lot of electric charge goes into them that is lost as gas, and won’t be coming back as electricity. In addition, Nife batteries lose energy through their high internal resistance. When they are charged at 32V but discharged at 25V, there is an energy loss proportionate to the voltage difference.Battery charge efficiency is not usually a significant concern for off-grid household solar power systems. Normal off-grid solar systems are tremendously wasteful of the potential energy output of their PV panels. This is because enough panel power is installed for the house to survive normal times of less sunlight, like cloudy weather. That means that on sunny days, a typical off-grid house has filled its batteries before noon, and the PV panels are mostly switched off (by the regulator) for the rest of the day, except for loads that directly use the power. Batteries that are more efficient, such as sealed lead-acid or lithium, tend to reduce (taper) their charging current earlier in the day, so the energy they don’t waste inside their cells through gassing, is instead wasted by the regulator turning off the power of the PV panels.
For another way of looking at off-grid efficiency: if you add an extra PV panel to help address a power shortage during your few annual cloudy weeks, that panel’s power will be wasted for the sunny rest of the year.
I recognise that battery efficiency can be very useful sometimes: in cloudy weather when the batteries are low and there is limited sunlight for a few days, efficient batteries can help you make the most of what you can harvest.
I argue that we should be cautious about the pursuit of technical efficiency energy- producing and using devices. More efficient panels, batteries or loads need to be looked at in the big picture. The real question for me is what technology and what behaviour results in the lowest total, long-term money and energy cost? This is our best indication of damage to the environment and how much of our life’s effort will need to be spent earning money.
What if the regulator breaks down?
I seem to spend half of my life fixing things that have broken down: I know this is how things are, so I plan for everything to break down and need fixing. I haven’t had a solar regulator break down in this house in decades, but I know this could change today - or more likely in the next thunderstorm. So I think it’s worth thinking ahead about how your solar battery would cope with a failed regulator.The most vulnerable battery to breakdown would be lithium. A lithium battery uses a solar regulator to control the voltage from the PV panels, then has its internal BMS to regulate charge to each cell - there are lots of vulnerable components that could break down. To some extent the PV regulator and the BMS back eachother up: if either fails, the other will protect the lithium cells to a large extent, until the system is repaired. However, failure of either the PV regulator or any part of the BMS would probably put the battery out of action until a fairly major repair is done.
The next most vulnerable battery would be the sealed lead acid battery. Without a fully-working regulator, the charge from a PV array could easily kill an SLA - especially a gel battery - in one sunny day (I haven’t tried this - let me know if you prove me wrong).
A flooded lead-acid battery is reasonably resilient. A modestly-sized PV array could be used to charge a flooded battery without a regulator, if some attention was paid to voltage and the PV switched off at a reasonable time. Over-charging would normally simply result in extra water loss.
Nickel-iron easily wins the regulator resilience race. Unregulated on a sunny day, Nife batteries could create annoying high voltages and bubble a lot, but (unless they have a really oversized PV array cooking them) they won’t come to any harm. I believe some people run them without regulators on purpose.
Durability
Service life makes a huge difference to the energy, materials and money cost per year, or per energy delivered. It also has a big impact on the resilience of your energy system.Electrolyte replacement
Nickel-iron batteries do need major maintenance every 7 to 10 years: electrolyte replacement. I’ve never done this.Their electrolyte is potassium hydroxide, a strong alkali. However the atmosphere we live in tends to make water acidic: carbon dioxide from the air dissolves in water to make carbonic acid. That’s why rainwater is quite acidic and makes stalactites in limestone caves, and is why increasing CO2 in the atmosphere is making seawater more acidic and damaging shelled sea creatures. The battery electrolyte absorbs CO2 from the air and is gradually neutralised - gets less alkaline. When this makes the battery lose its mojo, you need to tip out the electrolyte and put in a fresh batch. This would be a significant job, I imagine taking at least a day or 2. The good thing is that potassium hydroxide is not toxic, and it can safely be diluted onto earth and plants - as long as it is only potassium hydroxide.
I understand that many Nife batteries have Lithium hydroxide added to their potassium hydroxide electrolyte, to reduce gassing and increase efficiency. Lithium hydroxide is toxic, and I wouldn’t be spreading this over my orchard when replacing electrolyte.
More details of my nickel-iron battery experience
That’s enough theory about solar house batteries. Now to my actual experience with nickel-iron, for the benefit of anyone seriously considering giving them a go. Purchase and delivery
I bought my 24V, 200Ah nickel-iron battery from David Bartlett at www.ironcorebatteries.com.au, in August 2019. They cost $5214, delivered to a Brisbane trucking depot, arriving less than a week after I completed payment. The 20 cells were packed in a plywood crate approx 1m x .5m x .5m, weighing about 240kg, which was forklifted onto my friend’s ute.Cells were individually packed in plastic bags, each filled with electrolyte, but we had no spills. Sometimes NiFe cells are delivered dry, and the installer must mix the potassium hydroxide electrolyte and fill the cells - this takes a day or 2. The package also included 20 cell joining straps, made of nickel-plated steel, plus plastic strap covers (to reduce the risk of accidental short-circuits, e.g. by dropping a spanner onto the top of the batteries). Each cell weighed only about 12kg, so they were easy to lift out of the crate and carry into place.
Installation
To fit into my available space, I arranged the 20 x 1.2V cells in 3 rows. I was able to use the same battery box we’ve used for nearly 20 years, this being the 3rd set of batteries to live in it (and before that I had several sets of previous batteries - see why I’m thinking about battery life?).
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| 20 x 200Ah 1.2V nickel-iron cells in 3 rows. This box previously held our lead-acid house batteries: 6 x 1300Ah 2V cells - that took about the same volume. |
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| Here's a copper strap made of flattened 1/2" copper pipe, between 2 cell links, joining 2 rows of cells |
Behaviour of the new NiFe batteries
In practice, nickel-iron batteries are quite different from the lead-acid batteries I’m used to, in two main ways: they have a wide voltage range, and they have limited ability to carry current. I suspect that both of these characteristics are because the nickel-iron chemical reaction is slower than other chemistries.Voltage range
The first obvious difference between Nife and lead-acid batteries is the wide voltage range between full and empty, and between charging and discharging. Battery voltage in our system with 20 cells ranged from 19V to 34V (lead-acid range would be around 24V to 30V), sometimes covering this over short periods - e.g. if I switch on a 700w electric jug on a morning when the battery is deeply discharged, the voltage might drop from 29V to 19V. A normal day might range from about 25V to 33.8V.
Early challenges
It took me a while to learn how to use our new nickel-iron battery, especially given the lack of good information about how to set a regulator for them.For the first few months, I limited the maximum voltage to 31.5V, because this was the highest our DC fridge and freezer could tolerate (with 12-24V Danfoss compressors). While the 2019 drought was on with endless blue skies, this worked alright, but when the 2020 rainy season arrived and we went into the clouds, the battery couldn’t keep things running through the first sunless day - it dropped under 19V under a small load, and got too low to run the DC fridge. I quickly realised I must have been running the battery at too low a voltage, so what I thought was a full battery was actually nearly flat.
One of the clues I had that I’d given the battery too low a charging voltage, was that the cells were losing so little water. I hadn’t topped them up since new, nearly 6 months, because the electrolyte levels were going down so slowly.
Another clue was the battery’s intolerance of high discharge currents. In the morning, before the sun was on the panels, a 700w electric jug (that’s a really low powered jug) would drop the battery under 19V and set off the inverter’s low voltage shut down - with mood-enhancing alarm sound.
Clue 3 was the low charging currents I noticed from the PV supply. The battery could be rather flat, but might only take 10A from panels that could supply 40A (this was because I had set the charge voltage too low on the regulator).
Once I worked out how to set the regulator to suitably high charging voltages (see below), our nickel-iron battery was transformed. For a few years the voltage rarely dipped below 24V at night, usually staying above 25V, even at low state of charge (SOC).
Regulator settings
I use a Plasmatronics PL60 regulator, one that we used for 12 years on our flooded lead-acid batteries. I really like these PL regulators: good quality, made in Australia, programmable, and they count Ah and estimate SOC so you have some idea of how flat your battery is in cloudy weather - many regulators don’t.
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| Here's our Plasmatronics PL60 - after nearly 20 years of working 24/7. On its left are DC and AC circuit breakers. |
When I realised my Nife problems were due to low charging voltages, I found I couldn’t set the PL60 to the high voltages I needed. I called Plasmatronics (people who know their product!) who, although they hadn’t any experience with Nife batteries, came up with a work-around. On their advice, I went into the regulator settings menu and used the temperature compensation setting to add about 2V to all the settings - tricking the regulator into thinking the batteries were very cold and needed higher charging voltages (that’s what lead acid batteries need in cold climates).
If you’re using a PL regulator, this is what I did (while I had 20 cells in series):
SET -> REG -> TCMP -> change setting from 0 to 6. This adds about 2V to charge voltage settings
SET -> REG -> BMAX -> set max boost voltage to 31.8, giving a real life boost voltage of 33.8
SET -> REG -> EMAX -> set equalisation voltage to 31.8 - giving 33.8V
SET -> REG -> ABSV -> set absorb voltage to 31.0 - giving 33.0V.
SET -> REG -> ATIM -> set absorption time to 4 hours.
SET -> REG -> FLTV -> set float voltage to 28.0V - giving 30.0V
Some settings could be reduced if there is too much water loss by gassing - I think this is likely in sunny seasons. Also settings might need to be taken down a notch if the voltage sometimes goes over the inverter’s maximum for a moment - this has happened to me.
The maximum charging voltage is about 33.8V - just under 1.7V per cell. I’ve set this maximum to avoid the inverter having a high DC volts shutdown - maximum DC input to our inverter is 34V, and it shuts down and alarms if it gets to 34V (a Selectronics SE22 with 20 years service here). I think an inverter running on a 24V nickel-iron battery would need a high DC voltage capability of at least 34V.
The 19 cell option
Having read that some people use 19 cells, instead of the standard 20 cells, for a 24V system, I took one cell out of the battery a couple of years after installation. This reduces the overall voltage by about 1.2V at night (when the battery is discharging) and by up to 1.7V in the day when the PV is charging. This strategy solved a lot of the problems I had with the regulator (getting a high enough voltage to charge the battery properly), inverter (going over shut-down voltage) and fridges (having too high a voltage for the compressor). After a while of losing a lot of water, I adjusted the regulator voltage settings down - I don’t have a record of how much.
Backup charging
Occasionally we need to backup charge our batteries, when there is inadequate sunlight to keep minimal loads operating (the fridges being the key load). NiFe batteries make backup charging a challenge, because the high voltages they need to charge at a reasonable rate are not usual for battery chargers.
Normal battery chargers, like our old Woods charger, don't produce a high enough voltage to charge a 20 cell, 24V NiFe battery when driven by a petrol generator. The Woods can however charge half the cells at a time. At 19 cells, the Woods charger works, but at it’s maximum voltage, at reduced current.
At first we used our old Christie charger, a direct charging unit with a small Honda motor and a large car alternator, made to produce DC power straight into a 12V battery. With the nickel-iron battery, we use the Christie charger to charge 7 or 8 NiFe cells in series, then move the clamps along to charge another 7 or 8 cells. This doesn’t add up to 20 cells, so some cells get much more charging than others but they can cope fine with this: they will easily balance out their states of charge in sunny weather; but it's a messy way to do charging.
I've since modified our Christie charger to deliver a higher charging voltage and can now charge 10 cells (half the battery) with full control of charging current (the 12V alternator can't deliver enough voltage for the 24V set). 1/2 hour charging on each half of the battery generally gets us out of trouble on a cloudy day. I planned to use the same modification on a 24V alternator so I could charge the full battery, but never did.
Summary
If you're planning an off-grid power system:
• develop skills: learn to be aware of your solar supply and how full your batteries are
• sometimes it's sunny, sometimes it's not: match your loads to your energy supply
• installing and replacing batteries is a major, ongoing cost
• expect everything to break down some time
• DC lights and fridges can make your system more resilient to breakdowns and cloudy weather
• your expectations are your greatest challenge to living off-grid!
Here are some key points if you’re considering a Nife battery:
• I don’t know anyone with an off-grid Nickel-iron battery that delivers anything like specified capacity, or that is old enough to prove their claimed longevity
• you’ll need to set unusually high regulator settings - check that your regulator can go high enough
• your inverter will need to work at higher than usual voltages
• you’ll likely need to drop down to 19 cells (in a 24V system)
• you’ll need to top up a lot with distilled/demineralised water
• you’ll need to plan on replacing the electrolyte every 7 - 10 years
Extra info and links
Why we should make hay when the sun shines
Using power when it's sunny, and not using it when it's dark or cloudy, is fundamental to using solar energy. This might seem obvious, like making banana bread when you have a ripe bunch. However cheap fossil-fuel-based energy has made us blind to the variability of most natural resources, and perhaps somewhat entitled to having as much energy as we want, whenever we want.
As I described at the beginning of this post, storing solar energy costs about 10 times as much as producing it. You can imagine one kWh of solar electricity, stored in a battery and used at night, is worth about 11 times as much (the cost of generating plus the cost of storing) as a kWh used in the day. Even if some revolutionary battery storage cost 1/10 as much as it currently does (don’t bet on it) - costing the same as PV generation - night power would still have twice the cost of day power.
Of course, off-grid power is much more complex than this. If you’re having a sunny week, each afternoon your regulator might be wasting nearly all your panels’ production because your battery is full and you’ve done all your big electric jobs for the day. That wasted solar energy has zero value (but it did cost money). At the other extreme, if you’re in the middle of a rainy month, your battery is nearly flat, you have a freezer full of beef, your generator has broken down and you insist on vacuuming the carpet, that little bit of energy might cost you a fortune.
These sorts of limits affect energy systems at all sorts of scales. Here’s a fascinating story about what happened to Tasmania in 2016, when they had a hydro power drought and lost their backup power. I note that in the face of this crisis, the Tasmanian government didn’t raise the price of electricity to reduce demand, and have kept the cost of backup diesel generators a secret - perhaps they saw a political imperative not to challenge people’s sense of entitlement to cheap energy.
If our goal is to reduce the cost of our off-grid solar power, or if it’s to reduce our carbon emissions, we should maximise our direct use of PV power while it’s being produced, and minimise our dependence on battery storage. It’s the same: by reducing our long-term cost, we reduce our long-term carbon emissions, and we usually get a more resilient system.
How to manage demand
Successful living with solar power is like growing a vegetable garden. You need to work within a philosophy, build systems, and develop your skills. On a sunny day the gardener might water her lettuces and shade her seedlings, while the solar power user might run the washing machine, pump water and make tea with the electric jug. I think there’s no way around learning to become conscious of the energy supply and matching tasks to it.
As an example, let’s look at how we cook in our house. If there is sun on our PV panels, and our batteries are reasonably full, we cook on a portable benchtop induction cooker, and boil our 700w Birko electric jug. If it’s cloudy and we have a medium amount of cooking to do, we’ll light a charcoal fire. If we want to make a little espresso coffee or re-heat a cup of tea, we can use our LPG gas ring (our 9kg LPG refill is now over 5 years old). For major cooking tasks in the daytime, or dinner every evening, we light our big Rayburn slow combustion cooker - burning wood. We didn’t design our PV system for electric cooking, but we can do most of our daytime cooking using solar electricity that otherwise would be wasted. This electric cooking saves us time and saves charcoal or wood fuel. The LPG is just a very cheap luxury for tiny tasks, and we don’t use it for serious cooking.
When it’s sunny, we pump water, cut firewood with the electric chainsaw, do vacuuming, run the washing machine, use electric saws and planes in the workshop, etc.. Our loads need to stay within the power capacity of the inverter: there is a limit to our maximum power (in watts) as well as our supply of energy (in kilowatt hours). We can use up to 4kWh these days.
In cloudy weather we reduce loads. No electric cooking, essential pumping only, minimal workshop machines. We can stay under 1kWh on these days.
A few times a year we have extended heavy cloud with nearly no solar input for weeks. If the fridge and freezer are doing low voltage shutdowns, we'll turn them off overnight - even a cloudy day usually provides enough solar input to get them to run. We'll leave the inverter turned off most of the day, turning it on for limited periods to charge computers, grind some coffee or briefly run a bench grinder.
DC loads
I like to run our lights, fridge, freezer and a few other small things on DC circuits, direct from the battery. I know the modern way is to run everything at 230V through the inverter, but I think it’s more efficient, safe and resilient to have DC circuits as well.
Having DC circuits is efficient. When we go to bed at night, we turn off our inverter (it's in our bedroom, so that’s easy). This saves about 25Ah of energy overnight (over 0.5kWh), that would otherwise go into running the inverter and powering various little things that are left plugged in, like cordless phones etc.. The fridge and freezer can still run on their DC circuit, and the DC lights can still be turned on. 25Ah may be trivial in sunny weather, but when we’re in the clouds for a month, it makes a huge difference to our energy balance.
Using DC circuits is also more resilient. Like I keep saying: everything breaks down; and this includes inverters. A good inverter costs a few thousand dollar$, most of us don’t keep a spare in the shed, and it could take days or months to get a new one (perhaps more if a virus has shut down China) and get an installer to visit. With DC lights and fridge, a broken-down inverter won’t leave us in the dark with rotting food - although of course other failures could put us in that situation.
DC lights
25 years ago, it was normal for off-grid houses to have DC lights, running straight off the house battery. Now it’s unusual, because PV panels are much cheaper and people are much richer - lights usually run on AC from the inverter. So it’s become harder to find good quality DC lights for a house.
Lately we’ve been buying LED bulbs with a wide voltage range: the seller states 24-36V, but we’ve had no trouble when house battery voltage has dropped to 19V. We’ve been buying from www.12vmonster.com. The bulbs are quite expensive (over AU$20 each), but we’ve had no failures. We mostly use 15w bulbs in our living area.
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| Here's the 15w LED light that lights our dining table - from 12vmonster |
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| These "corn bulbs" from ebay don't last long - sometimes only minutes |
DC fridge and freezer
We use a DC fridge and a DC freezer. These are very efficient. They also allow us to leave our inverter turned off at night, or when we go away from home for a few days, and if we had an inverter breakdown they could keep running.Daylight drive
While I was searching around the web for information about nickel-iron batteries, I found a mob called Living Energy Farms (LEF). LEF take the principle of minimising dependence on batteries to its logical conclusion: many of their home and farm machines are run by a micro-grid that uses DC panel power, direct to loads, without battery storage. They call this “daylight drive” because work can only be done when there is sunlight on the PV panels. They have about 1400w of panels, (maybe 6 x 230w) connected in series, that drive 180V (nominal) DC motors they have installed on a range of machines. Their system is exceptionally resilient and low cost, but would take a fair bit of technical work to set up.Here’s a brief description, and a video.
Living Energy Farm: http://livingenergyfarm.org/
Edison battery
This website: www.nickel-iron-battery.com has some information about nickel-iron batteries, including a historic brochure from the Edison Storage Battery Company, which can be downloaded directly from this link: www.nickel-iron-battery.com/edison_brochure.pdfI have found the old Edison brochure very useful.
Damn the matrix
My friend Mike Stasse was the first (and I think only) person I personally know to get a nickel-iron battery for his house. His blog posts about his experience are worth a look:https://damnthematrix.wordpress.com/2016/05/28/patience-is-a-virtue-they-say/
https://damnthematrix.wordpress.com/tag/nickel-iron/
However be aware that Mike’s battery is made of the same cells as mine, and I think his have similar problems to mine.
The Austrian “Future Workshop”
My friend Peter speaks German, and did a web search for German language info on nickel-iron batteries. He found an interesting resource at: https://zukunftswerkstattverkehr.wordpress.com/2013/03/18/der-nickel-eisen-akkumulator-wunsch-und-wahrheit/
It appears that a thoughtful fellow named Gerald Harbusch has done a lot of study and work on Nife batteries, has come to conclusions in alignment with mine, and has written them in German language. Peter has put the relevant section of Gerald’s website through a translating website and I copy it below as a valuable resource (copy and translation 4 April 2024).
https://zukunftswerkstattverkehr.wordpress.com/2013/03/18/der-nickel-eisen-akkumulator-wunsch-und-wahrheit/
The nickel-iron battery – aim and reality
Published, 18 March 2013
Energy
5 Comments
Tags: battery, accumulator, changhong, iron, kursk, lifetime, lithium, nickel, nickel-iron battery
Verein ZWV [Association ZWV] – ZukunftsWerkstatt Verkehr, Austria
Gerald Harbusch
The world's energy consumption needs to be reduced – a personal blog
Is the nickel-iron battery as good as it is generally described?
Although the nickel-iron battery ekes out a quiet, almost inconspicuous existence, a number of websites have recently been dedicated to this type of battery. For us as ZukunftsWerkstatt Verkehr, this is doubly interesting because we decided, at the end of 2012, to import precisely this battery and enable a small group of people who are interested in nickel-iron batteries to purchase them.
I therefore would like to determine whether the NiFe battery is really as good, environmentally friendly and long-lasting as is generally claimed.
In doing so, I have learnt a lot about this type of battery from the battery manufacturers, Kursk Akkumulatoren from Russia, opinions and data from Changhong Batteries from China and also from various people who have signed up to our list of people interested in buying NiFe batteries on the "solux.pro" website.
To begin with, I would like to briefly list the common pros and cons of nickel-iron batteries.
Advantages of the NiFe battery:
• very robust against deep discharge and overcharging
• very long service life, 20-100 years and more
• large usable operating temperature range
• Insensitive to vibrations
• good efficiency
Disadvantages of the NiFe battery:
• high weight
• very poor availability (only 2 producers worldwide!!)
• low cell voltage (only 1.2V)
• battery can gas during charging (oxyhydrogen explosion), good ventilation necessary
So much for the general perception of NiFe batteries.
Do these points withstand closer scrutiny and examination?
It is a fact that quite a few people are interested in nickel-iron batteries, especially now that the topic of electricity storage from photovoltaic power in batteries and battery banks, stand-alone solutions for houses and backup systems for power outages and night-time operation is becoming increasingly important. Another fact is that there are only 2 manufacturers of nickel-iron batteries worldwide, as far as we could establish.
One is in China, Changhong Batteries, which delivers to the USA and also in minimal quantities to Europe. The other is the company Kursk Akkumulatoren from Russia. They primarily supply the Russian Federation. In this case as well, a few batteries occasionally make their way to Europe.
Although the NiFe batteries from both manufacturers are actually in the same elongated plastic containers, there are apparent differences in capacity, weight and size.
Example 1
Company: Beutilityfree, USA (sells Changhong batteries, China)
Battery: Type 7012
Capacity: 300 Ah with 5-hour discharge
Nominal voltage: 1.2V
Dimensions L, W, H: 138 x 276 x 490 mm
Weight incl. electrolyte: 25 kg
(Price 2010: 302 US dollars per cell, with electrolyte to prepare and to fill yourself)
Example 2
Company: Akkumulatorenfabrik Kursk [Accumulator Factory Kursk]
Battery: Kursk 300M-Y2
Capacity: 300 Ah with 5-hour discharge
Nominal voltage: 1.2V
Dimensions L, W, H: 132 x 169 x 400 mm
Weight incl. electrolyte: 16.0 kg
(Price 2013: 164 Euros per cell, without electrolyte)
Noteworthy: the battery with the same capacity from Changhong is much larger and heavier than the one from Kursk, this while containing only a little more electrolyte (6.1 litres instead of 5.5 litres from Kursk). What does this indicate? The nickel and iron plates in the Chinese NiFe battery consequently have to be larger and heavier. In any case, in my opinion, the Changhong battery has to last longer than the Russian one. As I said, no long-term data is available.
Longevity of NiFe batteries
NiFe batteries are said to be resistant to deep discharge and overcharging. It therefore is alleged to have a battery life of several decades.
Interestingly, the company Changhong, for example, has produced a graph showing the service life of NiFe batteries as a function of the depth of discharge (DoD). But Kursk also writes that their battery lasts no less than 1000 cycles in drive mode (use in mining vehicles = high depth of discharge, DoD >80%). How does this correspond with the [alleged longevity of the] NiFe battery? And what happens when the nickel-iron battery has reached the "end of its life"? Is the battery really done with at this point, or is it just the electrolyte that needs replacing? We have not yet been able to clarify these questions. If it really had become unusable, which I suspect is the case, then the NiFe battery would not last much longer than a good tubular plate lead battery, but at a price that is completely exaggerated in terms of its performance. Not even having taken into account the higher maintenance requirements of the NiFe batteries.
There are a host of various stories about NiFe batteries, which remained buried somewhere for decades (sometimes for 60-70 years!), then were found, refilled with fresh electrolyte and regained their full capacity. These are and were mostly batteries from the time before and during the Second World War when NiFe batteries were used in large numbers for all kinds of primarily military tasks. I am convinced that these stories are true. But, unfortunately, this does not substantiate the claim of a lifespan of up to 100 years that NiFe batteries are said to have. After all, these buried batteries were only in use for a few years and were then "decommissioned". Even the famous electric car from the USA (Electric Baker), which has been running for over 100 years with its original NiFe battery pack, does not prove the longevity of NiFe batteries in any way. Here, too, the car is only used occasionally and, because it is already very old, very carefully (it is driven in slow motion because it can't go any faster!). When not in use, the NiFe battery is resilient indeed and hardly suffers any damage.
If you have a 48V NiFe battery pack in use in your home as a photovoltaic storage bank, which is charged and discharged every day, it seems a legitimate question to ask: how long will this battery bank really last? There is apparently no verifiable long-term experience about the performance of NiFe batteries within the last 30 years, neither about the Russian nor the Chinese NiFe battery. Food for thought!
Another fact is that the NiFe battery is considered susceptible to corrosion. In the past, when the astonishing durability of the NiFe battery was propagated, there was no lithium hydroxide as an anti-gassing additive in the electrolyte. The many statements and discussions about this battery have shown that it is precisely this additive, because of its high corrosiveness, that tends to shorten the lifespan of the NiFe battery now in use. It is possible that the far less corrosive potassium hydroxide solution, the main component of the electrolyte, makes possible the long service life of the NiFe battery. This would also explain the service life diagrams of Changhong and Kursk. Dry NiFe batteries last practically forever. Lithium hydroxide increases the performance of the battery, but also reduces the service life (corrosion!!!). This means that the plates dissolve and the usable surface area becomes smaller and therefore also the capacity.
This has not been proven, but it sounds absolutely logical!
I would like to note at this point that Kursk prescribes a weekly check of the electrolyte level and a quarter yearly check of the condition of the potassium hydroxide solution for its batteries. Together with topping up with distilled water for 40 cells(!!!) and changing the electrolyte every 5-15 years (nobody knows for sure!), the maintenance effort is considerable! These requirements should not be forgotten. They are almost never mentioned when people rave about nickel-iron batteries!
If NiFe batteries are used in Russian mines, for example, as traction batteries for mining haulers, who knows whether they won't simply be replaced after 3-5 years if their performance is depleted due to daily deep discharging and / or the guaranteed number of cycles has been exceeded. Why are there no field reports on this?
The true cost of NiFe batteries
What do nickel-iron batteries really cost? They have a reputation for being more expensive than other comparable batteries, such as lead batteries.
I take a 12V battery bank as a starting point. At ZukunftsWerkstatt Verkehr, a single cell with a nominal voltage of 1.2V and 300Ah (5-hour discharge) costs 164 Euros. This is not a dumping price for NiFe batteries, but it is also not the most expensive battery price for this size.
A 12V bank therefore costs 1640 Euros. But that's not all. In the case of the Kursk company, the battery is supplied dry, without the electrolyte.
According to the manufacturer, the electrolyte consists of 20% caustic potash (potassium hydroxide) and lithium hydroxide (20g per litre). If you ask a chemical wholesaler for these substances, you will be offered various purities, but not exactly what you need. This means that you have to mix the electrolyte in the correct ratio yourself, which is often not so easy for the layman, and for some it is almost impossible. Apart from that, as a private individual, you very often can't buy the desired substances wholesale. But let's get back to the actual cost.
I have made a rough estimate of what the electrolyte would cost. For a 12V bank, the 20% potassium hydroxide solution and distilled water come to around 100 Euros, and the lithium hydroxide (approx. 1kg) comes to around 200 Euros. In addition to the 1640 Euros, the electrolyte costs around 300 Euros. That makes 1940 Euros, i.e. almost 2000 Euros for 12V!
A 48V battery bank (40 cells, a realistic assumption) then costs around 8000 Euros.
Changing the electrolyte would therefore result in downright material costs of approx. 1200 Euros, plus approx. 120 Euros for disposal.
Cycle stability as a basis for comparing different battery types
NiFe batteries can be used for up to 1000 cycles at 100% discharge (at 20 degrees Celsius) (source: Changhong and Kursk!), but over 8000 cycles at only 20% discharge, according to data supplied by Changhong. 30% discharge would therefore be approx. 6000 cycles, which we have used as a reference for comparison. Incidentally, the impetus for this came from a very practically minded NiFe battery enthusiast on our list.
I have therefore adopted this comparative method and evaluated the cost of different batteries. See for yourself!
(Battery banks are all 48V, the basis of comparison is a number of cycles of 6000 or less. All prices are without VAT. DoD is the Depth of Discharge)
1) Nickel iron battery (our purchase)
40 x Kursk 300 Ah cell @ 1.2 V = 14400 Wh
DoD 30% = 4320 Wh (6000 cycles)
Cost = € 6427 (without VAT, incl. electrolyte)
€/Wh = 1.49
2) Lithium iron yttrium phosphate battery
15 x 160 Ah cell @ 3.2 V = 7680 Wh
DoD 60% = 4608 Wh (6000 cycles)
Cost = € 2836 (without VAT)
€/Wh = 0.62
(Note: the LiFeYPo4 battery can be smaller because it allows a greater depth of discharge with the same cycle stability)
3) Lead-acid battery I
4 x TOYO AGM HD 12V 300Ah = 14400 Wh
DoD 30% = 4320 Wh (4000 cycles)
Cost = € 2500 (without VAT)
€/Wh = 0.57
4) Lead-acid battery II
24 x Hoppecke OPzS battery 2V, 300 Ah = 14400 Wh
Lead-acid tubular plate battery, liquid electrolyte, circulation system
DoD 30% = 4320 Wh (4000 cycles)
DoD 80% = 11520 Wh (1500 cycles)
Cost = € 4032 (without VAT)
€/Wh = 0.93
This comparison is herby clearly in favour of the lithium battery. You could certainly buy an even larger LiFeYPo4 battery and would still be very economical, as the number of cycles (= durability) would increase further.
How environmentally friendly is the NiFe battery?
If you look at the components of a nickel-iron battery, you will appreciate that they are much less dangerous and more environmentally friendly than many other batteries. It is therefore tempting to claim that the NiFe battery is one of the most environmentally friendly types of rechargeable battery, especially when its longevity is taken into account. I have already written about the service life, but what about the environmental impact or cost to the environment? The nickel and iron electrodes are in any case neutral ['benign'], or rather, they represent a reusable resource even after the NiFe battery has reached the end of its life. The potassium hydroxide solution (also known as caustic lye) is only slightly corrosive and is not considered a particularly hazardous substance.
The account changes when lithium hydroxide is added to the electrolyte. This material is very corrosive and also toxic. It is a hazardous material. However, manufacturers impose the use of lithium hydroxide because it reduces the gassing (hydrogen is released) that occurs every time the NiFe battery is charged and thus also increases the performance of the cell. At lower temperatures (< -10°), the proportion of lithium hydroxide must even be increased. With our Kursk NiFe batteries (300Ah), the proportion of electrolyte is 20g/1000ml. That is approx. 100g per cell, or approx. 1kg for a 12V bank.
1kg of lithium hydroxide, which needs to be disposed of when the electrolyte is changed. Whether it is ever recycled is questionable.
If you take the idea of lithium batteries a step further (e.g. lithium iron phosphate batteries), you can see that 100% of the lithium in these batteries is recovered because it is very expensive and in high demand. LiFePo4 batteries, which have a cycle stability of 2000 to 7000 cycles depending on the depth of discharge, are therefore becoming increasingly interesting from the point of view of environmental compatibility and also because of the falling prices.
Of course, I have to mention that there used to be no lithium hydroxide in the NiFe batteries and they still worked fine. I personally wouldn't put it in the electrolyte either, like some of our prospective customers. You can imagine the effects. The problem is not the performance of the battery, it's its warranty!
Manufacturer's warranty
Both Kursk and Changhong naturally give a manufacturer's guarantee on their NiFe batteries. A certain amount of time for the unused battery and then a certain amount of time for the used battery. As already mentioned, the second part of the guarantee is cancelled if lithium hydroxide is not added to the electrolyte as instructed. As sellers of Kursk NiFe batteries, we sell the batteries dry, so we have no influence on the electrolyte that you fill into your batteries. For this reason, warranty claims are unlikely to materialise in practice. One solution would be for us to fill the batteries, but we currently lack the facilities to do this and we only sell the NiFe batteries as empty batteries that are not yet ready for use.
The manufacturer Kursk generally produces its batteries for specific applications. In the case of NiFe batteries, these are starter batteries for locomotives, power supply for railways and as traction batteries for heavy underground hauling machinery.
The use as energy storage for solar power is not intended here and will also result in a breach of warranty in this case. This means that anyone who puts NiFe batteries in their cellar to store photovoltaic power will not be entitled to a warranty claim in the event of battery cell failure.
This is also an experience that casts a shadow over the highly praised nickel-iron battery.
Charging technology for the NiFe battery
Enticing as they may be, how do I charge a NiFe battery?
The manufacturer Kursk specifies the constant voltage method for charging its NiFe batteries (see ZWV website – NiFe battery). However, there must also be chargers with proper charging characteristics for NiFe batteries. A NiFe battery charger is not so easy to find. But after a long search, I came across a solar charger that is also suitable for NiFe batteries. The TOYO-MPPT-12-24V-40A has a battery type selector switch and the last setting (9) clearly mentions a nickel-iron alkaline battery, which is what NiFe batteries are. The preset voltages also fit exactly. The TOYO-MPPT-12-24V-40A is supposed to be identical to the VICTRON BlueSolar-MPPT-12-24V-40A. An enquiry with Victron has revealed that most of their chargers and chargers / inverters are freely programmable and are therefore suitable for NiFe batteries. As there are certainly similar devices from other manufacturers, the problem of charging NiFe batteries can be considered as being solved.
A few more lines on the charging properties of NiFe batteries:
The charging voltage for NiFe batteries is specified as approximately 1.45 - 1.60V. The higher the charging voltage, the higher the gassing. More hydrogen is released. If lithium is not added, this value is even higher. More distilled water must therefore be topped up more often.
A charger with MPPT technology (Maximum Power Point Tracking) is highly recommended for charging NiFe batteries, which increases efficiency by a good 20-30%.
Depending on the use / wear and tear and the condition of the electrolyte, we recommend changing it every 5-10 years.
I recently heard from a NiFe battery user that the NiFe batteries produce quite a lot of noise during operation (a large 48V bank). The hissing and bubbling is said to be so loud that you need to wear ear muffs when in the same room as the batteries. These characteristics of NiFe batteries are also new to me, but are a sign that there is almost no practical experience. And this problem has not even been addressed yet.
Conclusion
My very personal opinion when it comes to looking for an energy storage system for a domestic photovoltaic system is, if you take on board what has been stated above, that the nickel-iron battery has lost much of its fascination. My tendency is clearly away from NiFe batteries. A small calculation example at the end:
Assume that a battery bank is charged and discharged daily, that's 365 cycles a year. So, if a lithium battery lasts 6000 cycles at 60% discharge, that's already 18 years of service life. Even more at 50%! And you have to bear in mind that there is no additional effort or cost involved. However, it does need a good battery management system, which also has its price. I don't want to withhold this fact.
A good lead-acid battery that lasts approx. 4000 cycles at 30% discharge is also worth considering. It would have about 12 years of life expectancy. The charging technology is sophisticated and integrated into the electronics of the system, so you don't have to worry about any additional expenses.
Gerald Harbusch, 2013
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https://zukunftswerkstatt-verkehr.at/archiv01/akku-nickel-eisen.html
Verein ZWV [Association ZWV] – ZukunftsWerkstatt Verkehr, Austria
Report: Gerald Harbusch (2013-2017)
Last revised: October 2018 [?]
Nickel-iron battery
The nickel-iron battery has been around for well over 100 years. Can it be the energy storage solution of the future for stationary systems?
You could answer this with 'yes', if you don't wish to use a completely maintenance-free system. A certain amount of maintenance and monitoring is necessary when utilising nickel-iron batteries!
The latest NiFe batteries, from 2016 onwards, have become more modern and have a significantly improved performance compared to the nickel-iron batteries that existed in 2013, when the association ZukunftsWerkstatt Verkehr started to take an interest in nickel-iron batteries from Russian production (battery factory KURSK [Akkumulatorenfabrik Kursk]).A new nickel-iron battery with 48V
The advantage of the NiFe battery is that it is very robust in operation (deep discharging or overcharging does not damage it) and yet it is made of non-polluting, basic materials. Its long service life (if handled correctly) also makes it an interesting and, in the long term, favourable energy storage device that will not cause any problems once it is disposed of. For this reason, many people are willing to buy the nickel-iron battery to store their electricity generated by the means of photovoltaic.
October 2018:
PV system with lithium titanate instead of NiFe battery
The ZWV association has put a PV system into operation and as a 48V PV storage system we have NOT opted for the NiFe battery, but for a lithium titanate battery.
More about the photovoltaic LTO system on a separate page --> PV system ZWV with LTO storage
[ Link -> https://zukunftswerkstatt-verkehr.at/archiv01/pv-anlage-zwv.html ]
We are absolutely delighted with this, as it performs its service completely maintenance-free and inconspicuously in the background and was also considerably cheaper than a NiFe battery. The expected extremely long service life, the compatibility with standard programmable inverters/chargers, which are also suitable for lead-acid batteries, and the extreme operational reliability made the decision in favour of the LTO an easy one!
--> Lithium Titanate Battery
[ Link -> https://zukunftswerkstatt-verkehr.at/archiv01/akku-lithium-titanat.html ]
The advantages and disadvantages of the nickel-iron battery
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https://zukunftswerkstatt-verkehr.at/
Verein ZWV – ZukunftsWerkstatt Verkehr
Ecological, sustainable and affordable mobility
The association 'ZukunftsWerkstatt Verkehr' is a research and development institution in the field of alternative mobility and the provision of the energy required for this. The main aim of the development work is to reduce energy consumption. The association works closely with the company 'Zukunftswerkstatt Energie e.U.'.
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Verein Zukunftswerkstatt Verkehr Austria
A 7564 Dobersdorf, Feldweg 174
Chairperson: Gerald Harbusch
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