Here's my new battery charging bunker. It's a concrete block, with flats on top and bottom. I chiseled out a slot for the wiring. I'm going to try the "sand in bag" technique for the top. The idea is that you put the sand in a plastic bag, and a fire will melt the plastic, smothering the battery. I might keep the concrete flat on top of the bag, so it will totally seal. There's a smoke detector on top.
Here's the reason for the new battery bunker. It was a 3S 500, relatively small. I'm pretty paranoid about watching over charging, so I was there when one of the cells popped. I took it off the charger and examined it; within a couple of minutes smoke started coming out, so I tossed it out the door and it burst into flames a few seconds later. I wasn't concerned with the leaves, because it was a rainy night and everything was soaked. I've also got a fire extinguisher next to my bench; you should too!
