I happened today: while charging a 2800 mAh, 3cell lipo, after some hours (the charger a cheappy one, rated to 1.2A max so it takes its time to charge such a pack), I connect it to a 15V source (2A rated), the charger has 3 leds, A, B and C:

A: on/off indicator
B: charging
C: when the pack is charged, it turns on, B turns off

but now the B led is blinking periodically, once per second or so, and the C led doesn't turn on. The pack has reached more then 12V, and I tested it with two motors and it seems to be fully charged. It never happened.

Any idea?