The batteries in question were brand-new Battlepacks made from 20, 3300 mAh NiMH cells. I had two packs of 20 cells each, each with their own 40 amp fuse, wired in parallel. I put them on my trustworthy peak detecting charger, to charge at 3 amps. This is the 2-hour rate, a good safe rate to charge NiMH cells when you're not in a big hurry. Then I went to bed.

When I got up in the morning, both packs had burned. One pack (top in photo) was completely destroyed and was reduced to a pile of cells lying on the ground. The other (bottom in photo) was partly burned. The gaffer's tape holding them in place was burned through. Fortunately, they were charging out in the garage, on a concrete floor.

Some cells in the worst-affected pack are completely burst open, while most have at least bulged enough to split their wrappers open. You can see in the top middle of the picture the end-cap of a completely destroyed cell. There is lots of blackened stuff. In the bottom of the picture you can see the red foam insulation melted on to some cells. The pack was completely wrapped in the red foam, but there are only bits of it left. Wrapping battery packs in foam might not be a good idea, since foam is a thermal insulator. These battery packs did not have the optional heat-saver rings which allow airflow through the pack; I've ordered more packs that do.

The charger, which I've used regularly for 5 years and never had trouble with, doesn't seem to have overcharged them. The display shows that it delivered 3874 mAh of total charge. Since the two packs in parallel have 6600 mAh total capacity and they were mostly run down, it can't have overcharged them.

Charging NiMH cells in parallel is usually OK as long as the packs are matched, but it can in theory cause thermal runaway. Overcharging NiMH packs heats them up (because the energy has nowhere else to go but heat), and as the cells get hot, their voltage goes down and they will accept more charge at a given voltage, leading to thermal runaway. However, it shouldn't be possible for a cool pack to discharge into the parallel hot pack, because the voltages don't change that much.

My guess is that one cell short-circuited making one pack effectively 19 cells, and then all the energy from the 20-cell pack was dumped into the 19-cell pack. Rechargable battery cells can and do short out, so it's always dangerous to have two packs in parallel even when not charging. The right solution is a diode bridge to isolate them, as I explained in my Scooter page. But, I failed to take my own advice on this project.

Be careful charging batteries. If a cell shorts, or your charger malfunctions, or any of a number of things go wrong, they can get hot enough to start a fire. It's a good idea to charge them on a non-flammable surface, like concrete or tile, and without anything flammable over them. Lithium cells are even more dangerous, as they contain lithium which burns violently in air (think magnesium but worse) when hot enough. And you can't put it out with normal fire extinguishers. See the Lithium Battery Alert from the Academy of Model Aeronautics.

The idea of a microprocessor-based peak-detecting charger is scary. What happens when the code crashes, or the microprocessor fails in any one of a million ways? Presumably, most chargers will just keep charging until your batteries explode in flames.