I have a home-grown safety incident to report. Just as I was getting used to handling lithium polymer batteries, I became complacent and one bit back.

The battery in question was a 2 cell (7.4v) 4000mah LiPo. One of a pair that I use in my Twinstar.

I charged these both up on the 21st June so they would be ready to use on the following weekend. I charged them at a fairly low current so they took ages, I was in no rush.

When the charger beeped to let me know that the second one was charged, I thought I’d finish watching the telly before unplugging it. Consequently I forgot and left the thing plugged into the charger for the next 24 hours.

When I noticed this the following day, I thought “damn! I bet that has discharged the pack by some percentage”. Without checking the pack, I set the charge program off again to top the pack up.

The only mistake I didn’t make was leaving the charge process unattended. After about 30 minutes of charging, I heard a small crack followed by a fizz sound. Guessing that this could be the battery (I also thought it might be something I had cooking), I had a close look and could see that the pack had swelled up.

I immediately unplugged it and took it outside (carrying it be the lead). I then prepared a bowl of salt water to provide an environment to safely discharge the pack. I dropped the pack in and went back into the house.

About a minute later there was a very bright flash and a moderately loud bang. I ran outside to find that the battery had clearly exploded and jumped out of the bowl of water.

Some kind of black deposit mixed with water was sprayed everywhere. The battery continued to burn for about 5-10 minutes.

My guess now is that the battery must have completely discharged when left plugged in for 24 hours. This would have left the pack in a state where it couldn’t be recovered. I guess that the combination of the battery’s condition and whatever the charger tried to do to inject a charge, resulted in this catastrophic failure. I will be a lot more cautious the next time I suspect that a pack might be discharged by mistake.

The picture above was taken less than a minute after the explosion and about two minutes after it was inside my house. You’ll notice the black charred deposit on the stones either side of the pack. This was where the pack flared intensely for a few seconds after it landed on the stones. The explosion was when the pack was inside the bowl. Imagine that flaring on the carpet.

I count myself fortunate that it didn’t explode in my house or while I was carrying it outside. I hope that you can also learn from my experience and exercise appropriate caution with these packs.