I have some Ni-Mh rechargeable batteries (size AA/Mignon 1.2V, 2100 mAh) and just started to charge them using an automatic charger with 450 mA output.
Now one of those batteries surprisingly started to make silent fizzy sounds, like if you're frying something in a pan. It started within seconds after putting it in the charger and keeps going for minutes after removing it from the charger again. There is nothing visible from the outside though, I don't see any bubbles or leaking fluids.
- What exactly is the chemical process here that causes this sound (probably something is producing gas) and why does it happen in this one battery and not in the others I have of the same type?
- Is that battery still usable (safely?) or do I need to replace it?
Update: As of now, the battery in question is replaced and returned to a recycling station. I'm still interested in the chemical process behind this noise though.