Vented Rayovac Ni-MH AA incident


Newly Enlightened
Sep 9, 2020
New Mexico, USA
So this was about a month and a half or two months (not exactly sure) ago and thank heavens didn't involve a flashlight, but I will mention it here because it could very well happen in a flashlight if you go too crazy and use garbage quality batteries. Needless to say I was doing some ridiculous experiments that involved pulling 20+ amps (quite near short circuit current) from low-end Rayovac 1350mAh AA rechargeable Ni-MH batteries. All was going well and nothing seemed to have any issues except for the batteries needing to cool down after short periods of usage. The something odd started happening. I noticed on my smart charger (Opus BT-C2400) that one cell is taking in far more juice than the others. Leading up to the incident, it was taking in several hundred mAh more than the other cells but I was too cheap to replace them so yeah... Then one night I was messing around as usual and decided to let the batteries run a tad longer before letting them cool off. Big mistake. One cell out of 4 in series, not sure if it was the one misbehaving on the charger or not, decided to "poof" quietly and started letting out a small amount of smoke. I rushed it to a sink filled with water cause the smell was a bit annoying and I didn't wanna set of the hypersensitive smoke detectors we have here in college (this would cause the fire brigade w/ fire truck to come and I wouldn't wanna have to explain myself and my crazy quarantine electrical misadventures to campus authorities). I leave it for a minute in the water and remove it, and it literally resumed smoking. Put it back and bubbles started emerging from the water. I removed it 10 minutes later and everything was over... Or so I thought. Even when the smell mostly weakened in the student apartment I was in at the time, there was an aftersmell and aftertaste left in my mouth that made me throw up when I tried to sleep an hour after the incident. I assumed the gas is non-toxic since KOH (potassium hydroxide) which is used as an electrolyte in Ni-MH, isn't toxic and is just a lung irritant. So yeah, lesson learned not to use crappy dirt cheap low capacity cells for extreme high drain shenanigans (I switched to Japanese made 2300mAh Energizers and no venting issues have occurred since even when I pushed the cells super hard , and they heat to lower temperatures under the same level of use). I also unintentionally debunked the claims that Mi-MH cells will rupture when they fail, unless of course you have some garbage ventless ones. This is exactly why I do not use li-ion for what I was playing with (don't even ask lol).

I wonder if anyone has an idea as to why one cell had that excessive charging issue that worsened especially quickly in the few charge cycles preceding the incident. I am thinking it was probably slowly failing internally due to being pushed too hard and got high internal resistance, and then finally shorted and caused the venting incident.