Ni-Mh battery makes fizzy sounds while and after charging - why and do I need t replace it? 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.
It's not possible to give a proper answer to (a) without examining the battery but it is likely due to a compromised casing.  The noise is likely hydrogen gas leaking out as it's produced. Nickel metal hydride (NiMH) batteries use a metal hydride (MH) for energy storage.  When this Hydride is exposed to moisture in air it reacts to form hydrogen gas: $$\ce{MH_x + xH2O \rightarrow M(OH)_x + x H2 }$$

For (b) I would say that your battery is probably not usable—safely or otherwise—anymore and may present a fire hazard. You should replace the battery as rechargeables don't cost near as much as the device they can leak inside of and destroy or the cost of fire damage to a house.