So, my Zivan NG1 flaked out somehow and didn't stop charging at the end of its cycle. It just kept dumping power into my pack all night long.

My flooded Nicads experienced full thermal runaway at the plates, which melted them into slag. Eventually, some of the nylon cell casings began to burn.

I drive my car to work, so when I walked up to it Monday morning and could still hear the charger running, I knew something was wrong. The windows were all fogged over from the inside, and I could smell burning.

I immediately pulled the plug and noticed the electrolyte in the front pack was boiling out the tops of the cells. I opened the car up to look at the back pack. The car was filled with caustic Potassium Hydroxide smoke, so I put on some gloves and a mask and opened all the windows, doors, and sunroof. Once the smoke cleared out a little, I could see a small fire smoldering on the tops of the rear pack. I threw a towel over it to smother it, but I had to get to work so after making sure there was no fire left and the charger was disconnected and out of the car I hopped in my ICE car and went to work.

Upon my return I started cleaning out the electrolyte residue. The middle battery in the rear pack had exploded, spraying electrolyte all over the roof and seats of the car. After I got it cleaned out well enough to get in there, I saw that all three of the NiCads had melted down and were ruined. Originally, I had hopes that the front pack would be salvageable, but even though they looked OK, they appear to be ruined as well. After I take apart both packs this weekend, I might find enough cells to assemble a battery or two, but that's about it.

I had a thermal cutoff sensor, but it appears I placed it in the wrong place (back corner of the front pack) which must not have gotten hot enough to trip it. Or maybe the charger just failed with the charging circuit closed and didn't check it anymore. I should have had a timer on the outlet I use for charging as a safety measure in case of charger failure, but I hadn't gotten around to it yet.

It's lucky I drove the car a bunch Sunday night so that the pack was reasonably depleted and also that I was planning to go in early Monday morning, or I might have had a big fire on my hands and even more damage.

Fortunately, the electrolyte is basic and doesn't damage the metal and plastic like acid would have. The car looks like it will clean up just fine.

But those batteries were hard to obtain, reasonably expensive, and required a lot of work to get serviceable. I'm off the road for the foreseeable future until I obtain replacements, if I can even find them.

So, the moral of the story is this: If you must charge your batteries when you are not directly observing them, have a timer on your outlet to protect against charger failure and extreme overcharging.

Here's the car before the incident: