So I found an old Streamlight SL-20x (5 or 6 years old, never used) in my uncle's basment yesterday. Of course it didn't turn on, so when I got home I plugged the charger in. Instructions said to charge for 10-14 hours, so after about 10 hours of charging I went to go check on it.
When I approached it I heard a quick hissing noise and and very loud pop. It scared the hell out of me lol. I quickly unplugged it and found out that the first battery's top blew off the rest of the pack into different peices and completely exposed the first battery, you could see right into the battery.
I realized it was a Ni-Cd pack and that it is extremely toxic, I immediately threw it away and washed my hands.
I learned to just throw away really old rechargeables and buy new ones.