# Rechargable Batteries? **URL:** \ **Category:** Factual Questions\ **Created:** [May 11, 2000, 2:57am UTC](https://boards.straightdope.com/t/rechargable-batteries/19196 "2000-05-11T02:57:54Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![chief](https://avatars.discourse-cdn.com/v4/letter/c/df788c/32.png) [@chief](https://boards.straightdope.com/u/chief)\ **Post date:** [May 11, 2000, 2:57am UTC](https://boards.straightdope.com/t/rechargable-batteries/19196/1 "2000-05-11T02:57:54Z")
how does it know when to stop charging when its fully recharged? ---
**Author:** ![DAYIUZ](https://avatars.discourse-cdn.com/v4/letter/d/7ab992/32.png) [@DAYIUZ](https://boards.straightdope.com/u/DAYIUZ)\ **Post date:** [May 11, 2000, 3:32am UTC](https://boards.straightdope.com/t/rechargable-batteries/19196/2 "2000-05-11T03:32:35Z")
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. ---
**Author:** ![Donovan](https://avatars.discourse-cdn.com/v4/letter/d/ea666f/32.png) [@Donovan](https://boards.straightdope.com/u/Donovan)\ **Post date:** [May 11, 2000, 3:40am UTC](https://boards.straightdope.com/t/rechargable-batteries/19196/3 "2000-05-11T03:40:23Z")
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.