Thinkpad just ... turned off?

[Miller88]

I was in a class the other day taking notes (as usual). The battery was at 50% charging.

Suddenly it just turns off.

I pressed the power button to get it to turn back on and the lights on the front flash, it crackles and does nothing.

I unplug and remove battery.

I put battery back in and plug the power cord back into the back. When I turn it on I noticed it wasn't charging but running off of battery AND plugged in.

After class I turned it off and it works now ... what is going on with this thing? Could I need yet another AC adapter?