Rechargable Batteries?

how does it know when to stop charging when its fully recharged?

The charger gets feedback from the battery. Inother words the battery no longer has a draw.

This however, is not the case with my cell phone charger. If I put the battery on the charger the little light will be red for 2 hours. Then it turns green to let me know its done. If I put it right back on, it is red for another 2 hours. This charger obviously works on a timer.

The short answer is, they don’t.
The long answer is: If you try to charge a battery with a constant (high) charge and overcharge it, it will overheat and possibly rupture. The chargers usually use a timer (cheap ones) or a resistance detector to tell when the battery is at full amp/hours (As the battery has more energy in it, resistance in hte circuit increases). Some (usually more expensive, such as cell phone or Li-Ion batteries) also have a temperature sensor in them that cuts power to the battery if a temperature threshold is reached.