It is theoretically possible to overcharge the battery, but the ECU will not let that happen. If you look around you'll see lots of posts from people talking about their ICE suddenly spinning up to high RPMs and scaring the bejesus out of them while they were coasting down a hill. That was the ICE reving to a state where it could utilize some of the battery charge and drain it to prevent overcharging.

If you're near full charge, at a stop, the ICE will cycle on and off several times in fairly quick succession to do the same thing--just without the high RPM (spinning up the ICE by the motor causes a pretty good HV batt drain).

Evan E. Fusco, MD
'04 Prius/BC moded to the T
'06 Highlander Hybrid Limited AWD w/NAV