Sunday, July 19, 2015

Cordless drill baterry lesson

I've got an interesting challenge. Black and Decker cordless drill CD12C without charger. Ok machine but obviously useless without charger. After some investigation on Internet I found suitable charge for about 60 EUR. However, believe or not I found also complete new Makita 6271DWPE with two!!! batteries and charger for 85 EUR on web store. This makes absolutely no sense! Some people call this a "business". So, why should I buy the charger for 60 EUR if whole new machine costs just 25 EUR more.

At the same time I found one Makita charger DC1414T. The question popped up: how to charge Black and Decker battery by Makita charger?

Here is the drill itself:

Here is the Makita charger, with spec. from 9,6 V up to 14,4 NiMH / NiCD batteries:

Note 5 terminals on the charger.

Here are some pictures of the Black and Decker battery. While googling it seems like that this is compatible with DeWalt batteries as well. There are only 3 terminals on this battery. After partial disassembling I found out interesting thing. The third terminal is ground (-) of 1/2 of the battery pack and NOT any thermistor as it was mentioned on some forums.

I've tried first to connect battery and charger simple way : positive to positive and negative to negative. Just to see if Makita charger would deal with it. However, the charger detected battery "temperature problem" and it wasn't charging. So, I had to cheat the charger to start charging. Thermistor is a safety part - it will stop charging if the temperature is too high. BUT, can not be that big issue if Black and Decker doesn't have such a feature at all.

I got original Makita battery pack and measured the resistance between thermistor and negative terminal at room temperature and right after charging (warm battery).

Here is a picture of battery schematic including measured values of thermistor.

So, final solution was to fake the thermistor by a fixed resistance in order to cheat the charger. I have used constant resistor of 16 kohm. Here is the final solution and yes! it's charging! (red light blinking)

Conclusion: The best is to have any charger for 9,6 - 18 V for NiMH/NiCd batteries. It's possible to charge ANY type of NiMH/NiCd battery pack using some cables and crocodile clamps. Of course, I'll modify the charger later on by soldering cables inside and make some more "comfortable" connections + clear marking of the terminals.
PS: Be extremely careful to not cross-connect some terminals, you can damage charger or battery or both.