Ni-mh batteries: a mystery


I was a little too rough with my battery and under-drained them. Normally when I do this, I drain the voltage so low that the chemical reaction inside the battery becomes out of whack. The result, the battery is ruined. The statistics:
Battery is ni-mh with 8.4 volts with 7 cells at 650 mAH (not sure if batteries are parallel or series).
I drained it to 4 volts.
Charged it (although I thought no point really). Charged in 5 minutes, and the battery got very hot.
Waited one day after charging. I measured the voltage and found 10 volts.

How is this possible?


Hello Emily,

Discharging your batteries to 4 volts is not good for them, but they are probably still okay to use. What are you using to charge your batteries? Charging them in 5 minutes is too fast. A good battery charger will have a charge capacity selector, battery type selector (NiCd or NiMh) and trickle charge. Fully charged NiMh cells have a voltage of 1.3 - 1.4 V, so 10 V for 7 NiMh cells is not unreasonable.

By the way, did you buy these batteries from Pololu? I noticed you said they were 650 mAh, and our batteries are 700 mAh.

- Ryan

I do have batteries from pololu, however, you are right. The batteries in which I am referring to are different. I am using a Ni-Mh battery charger but I have never gotten desirable results with it (the battery charger is not from pololu). As for my battery, I think it is out of commission. I hooked it up to my project and it didn’t have enough current.

Hi Emily,

Sorry to hear that your battery is not working. I don’t really know what happens once a NiMh gets discharged so much that it breaks.

Does your battery charger have a charge capacity selector? If it doesn’t, it is possible that it is meant for charging batteries with a higher capacity. That would explain why they get really hot, but at this point I’m thinking it is more likely due to them breaking.

- Ryan