Rebuilding Makita NiMH Battery with Cells of Bigger Capacity. Will it Charge Properly on the Old Charger?

Question

This is for a Makita 6932FD impact driver. It runs off a 14.4V NiMH battery of 2.6Ah. I ordered sub-C cells (with solder tabs, don't worry :P) that are 4.2Ah.

I was reading about DIY NiMH chargers just out of curiosity. The author used a PIC to monitor charge termination based on 3 conditions:

1. dV/dt
2. Temperature
3. Time

So that got me thinking, I believe the highest capacity of OEM battery was ~3Ah. So will the charger be able to fully charge it? Or will it take too long, and get cut off? In which case, I'd have to

• Use it as is
• Unplug then plug in the charger again to "reset" the count
• Get a different charger, probably one for RC batteries as this is the target audience of the cells I bought

Answer 1

NiMH chargers terminate the charge based on dV/dt flattenings, with temperature as a backup/safety measure. So your old charger should charge the new cells just fine, only a bit longer.

However, professional power tools use battery cells that don't have an impressive capacity not because they want it cheap and low-grade, but because they want to have high discharge rate and drive the tool's motor hard. Unfortunately, high-discharge cells do have a somewhat smaller overall capacity, this is a design trade-off. So you need to worry a bit and better check if your new high-capacity cells have good discharge rate and can drive your tool to full design capacity/torque.