Right, got 2 dead batteries, lots of space inside the old pack, what to do next?
Easy! Lithium Ion! I have a stack of used 18650 Lithium Ion cells which I had managed to bring back from the brink. I’m not sure how long they will last but that the heck!
NiCad and NiMH cells are easy, 1.2V each so you need 20 to make 24V. LiIon cells have a fully charged voltage of 4.2V per cell but most of the time people refer to their nominal voltage which is around 3.7V.
5 cells in series (as used in my Milwaukee drill) gives a nominal voltage of 18.5V and a max of 21. Not enough! 7 in series is too much with a max of 29.4V! I’d fear for the drill’s control electronics. So 6 is the ideal number. A nominal of 22.2V (near enough 24V) and a max of 25.2V (again near enough 24V).
The cells I have are only rated at 5A max discharge. I do not know the drill’s actual rating but it is safe to say it is more than 120W. I’d guess it is well over 500W! Given that the average time spent drilling a hole is about 30 seconds I can get away with going a lot higher on the discharge. But I still need to parallel up the cells to get something sensible.
So can I fit a multiple of 6 cells in the old pack? YES! 18 to be precise. Which gives me a sensible current of 30A or just the far side of 700W. Score!
Some hot glue and soldering later…
Now the downside to lithium Ion batteries. If you over charge them or over discharge them they can get very unhappy. At the extreme they can burst into flames, and this is never a good thing inside a house.
Ebay comes to the rescue again with a protection board designed for up to 7 cells in series and a current limit of 20A continuous, 40A pulse. Under £7 but coming from China so it takes the slow boat.
This board will cut off charging when the cells reach 4.2V and will stop discharge if any cell hits 2.4V. I’d prefer it to be closer to 3V at the lower limit as this is the more common lower limit stated by cell manufacturers.
Hey, it fits! Anyone would think I had designed this!
Wiring up the voltage sense wires for each set of cells.
Now we have our first problem. The tags that connect to the drill were mounted to a plastic plate that was part of the cells rather than the outer casing.
The centre two tags go to a 10k thermistor used by the charger. The drill doesn’t care how hot the pack gets!
A bit of cutting later:
The slots I’ve cut are to help the glue grip, which is the next step.
A good clean, some 2 part epoxy and we are good to go!
Oh so close! Will it close? Will it work?
Yup! Hurrah! I even got + and – the right way around.
Oh, result! 300g less than the old pack!