Adapting A Makita Universal Charger For Other Nimh Batteries

We standardised on 18V Makita NiMH cordless drills at the farm. NiMH is older tech and heavier than LiIon, but the price/performance seemed to be about right there. We’re not hauling these up ladders or carrying them for miles. The trouble with cordless drills, well the NiMH ones, is that you have to stop using the drill as the battery runs down – when you hear the change in note as the power fades, switch the battery. If it’s you who will be paying for the replacement battery you will do that, but if it’s something used by many people that doesn’t happen, because it’s hassle to go get another battery.

Under the harsh load of a drill, that means as the voltage of a cell falls to 0, soon current gets forced backwards through the weakest cell and it dies. So you get a pack with a few duff cells in them, and have to junk the whole thing. The moral of the story is change the battery as soon as the sound changes and the power drops…

battery pack

battery pack

An 18V pack has 15 cells in it, because 15 × 1.2 = 18V. It uses the sub-C shape – same diameter of a regular C cell but shorter. This is a very common form in power tools, presumably the form factor lets manufacturers create the bizarre array of non-compatible battery sizes from a regular standard part. We have a cordless strimmer than uses 12V packs of sub-C cells, which needs 10 cells. So there’s an opportunity here to downcycle the batteries, well, as long as fewer than five of the old cells are duff. The way to look for that is to measure the voltage across the cells of a recently charged pack after it’s been used for a few minutes and the sound of the drill has changed a bit – the dogs will read a lot lower than 1.2V.

Makita power pack connections

Makita power pack connections

The connections to the charger are shown – if using the old battery case as an adapter you can leave the thermal fuse and the thermistor in there. The question is whether this is a delta-V charger looking for the rise in voltage to terminate the charge or if it is looking for the rise in temperature. If the latter I’m going to be out of luck unless I adapt the batteries. Obviously if you’re going to take a chance like this then there’s the risk of burning your house down etc. It so happens it’s a delta-V charger, but I still only run it for an hour max. It works fine for the 12V pack – just patch out the + and – to the original Makita battery terminals after taking all the cells out, leave the other Makita parts where they are.

Makita charger spec showing the voltage range covered

Makita charger spec showing the voltage range covered

The charger is smart enough to deal with the different battery voltage, as long as it’s between 7.2 and 18V

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