Try connecting the battery to the charging station and then press and hold the red button. If you hold it down for max. 10 seconds, it should start charging again. If this is the case, allow the battery to charge fully.
no. technically the battery fits, but the copter will not start.
vice versa. p2 batteries cannot be flown in the p3. (after a small modification of the p3, p2 batteries fit).