glad you are OK, scary stuff.
was this ego battery being charged using a 510 USB charger, or a 510-t USB charger?
and, have you used this USB charger before with an ego battery?
you probably know this already! but dormouse pointed out that 510 USB chargers are OK for ego batteries, but 510-t USB chargers are not OK with ego batteries or normal 510 batteries.
i fried an ego batt using a 510-t charger, it was scary but not like your experience. my battery just got very hot and the power switch melted. it smelt horrible.
i think you'll be much happier when you find out what caused it, it put me off using these things until i found that out. so i hope you get a good answer to what caused it soon.
edit - i hope this never happens with passthrough batteries, i don't even want to think about that.