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Thursday, 20 October 2016

Makita 7.2V cordless drill repair: lithium batteries

More than 20 years ago I bought a Makita cordless drill. It did a lot of work, but the batteries eventually died. The non-genuine batteries I bought on ebay didn’t last well, so the drill languished for years. The drill was good quality (for its time), with a keyless chuck, so I couldn’t bring myself to throw it out.
I recently repaired it, using a pair of Headway Lithium Iron Phosphate cells from a broken bike battery. It now works like new, but with much more energy storage.
I made a battery pack from two 10Ah 38120 cells, with soldered connections (all my newer Headway packs have bolted connections, which I really like). The 2 cells are connected in series to give about 6.6V, and have small Anderson plugs on a lead to connect to the drill. The drill has wires soldered onto the battery connection spring tabs (the drill needs to be unbolted and split to access these), with Anderson plugs on the outer end.
2 single-cell chargers attached via the Anderson plug and the cell series connection
Recharging is done with 2 single cell 2A chargers which came from Ping, a company which sells bike batteries. We were sent the chargers with some spare cells when we had to repair some Ping packs (fixing Ping batteries is no fun! That’s part of why I like the bolt-together Headway cells so much). Each charger connects to a single cell with alligator clips and has an LED which changes from red to green when the cell is fully charged at about 3.65V. I cut a small window in the battery pack wrapping at the cell series connection to enable each cell to be charged independently.

The little opening in the paper mache at this end is where the single cell charger clips are attached
There is no BMS to protect the cells, but the very high capacity of the cells (5 times the capacity of the original battery) means that the cells should be safe if I charge reasonably often. The Anderson plugs enable the battery to be disconnected to avoid accidental discharge.

The battery pack has a paper mache wrapping. After the connections were soldered, the cells were wrapped in thin polythene to keep them dry (an old plastic bag), then wrapped in several layers of used office paper and PVA glue (slightly diluted). This paper mache forms very nicely into shape, and dries to a tough material.
The battery pack is attached to the drill with rubber bands cut from a bicycle tube. These bind the battery onto a piece of plywood with is attached to the foot of the drill handle with string and PVA glue - a strong, cheap and versatile joining method. 
4.5mm plywood attached with string and PVA glue
bands cut from a bike tube hold the battery very firmly
Now I have a low-spec cordless drill which should last for a few more decades, with occasional battery changes.


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