(Sorry for the cross post, but it seems more appropriate here)
There can be problems with rechargeable lithium-ion batteries in general:
Lithium ion batteries - Provider Wiki
I'm no rocket surgeon, but I found this interesting:
"The reason lithium-ion batteries do catch fire involves tiny lithium particles that form fibers known as dendrites. Over several charge/discharge cycles, these dendrites can accumulate on the battery’s carbon anodes. Once that happens, short circuits can occur, resulting in rapid overheating and combustion. Scientists have previously been able to study the principles of dendrite formation using theoretical models and microscopes, but had not been able to quantify the amount of dendrites formed... "