# Discharging battery **URL:** \ **Category:** Factual Questions\ **Created:** [October 21, 2013, 9:26am UTC](https://boards.straightdope.com/t/discharging-battery/671801 "2013-10-21T09:26:26Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [October 21, 2013, 9:26am UTC](https://boards.straightdope.com/t/discharging-battery/671801/1 "2013-10-21T09:26:26Z")
Not too long back I bought a rechargable headlight for my bicycle. The manufacturer cheapened out by not including a recharger. Instead, they just included a USB adapter cable, so I have to plug it into the back of my computer to recharge it. After I shut down my computer, I turn its power off at the surge protector. So there’s absolutely no power to it. If I leave the headlight plugged into the USB during this time, will its battery discharge? ---
**Author:** ![aNewLeaf](https://avatars.discourse-cdn.com/v4/letter/a/ac91a4/32.png) [@aNewLeaf](https://boards.straightdope.com/u/aNewLeaf)\ **Post date:** [October 21, 2013, 9:29am UTC](https://boards.straightdope.com/t/discharging-battery/671801/2 "2013-10-21T09:29:12Z")
As a general rule, you shouldn’t leave charged batteries in a charger. Hard to say what specifically will happen with a given device but a USB charger might be smart enough to get away with it. You won’t hurt anything by trying it out once. If anything gets warm to the touch, unplug it. ---
**Author:** ![Isilder](https://avatars.discourse-cdn.com/v4/letter/i/cc9497/32.png) [@Isilder](https://boards.straightdope.com/u/Isilder)\ **Post date:** [October 21, 2013, 10:36am UTC](https://boards.straightdope.com/t/discharging-battery/671801/3 "2013-10-21T10:36:07Z")
> [@dtilque](#): > > After I shut down my computer, I turn its power off at the surge protector. So there’s absolutely no power to it. If I leave the headlight plugged into the USB during this time, will its battery discharge? No. USB is 5 volts, The battery is 3.6 volts and the difference is regulated by semiconductors, which also prevent discharge via the charging circuit. (a transistor is often used as the regulator , this prevents flow just as a diode does… ) ---
**Author:** ![Telemark](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/telemark/32/372_2.png) [@Telemark](https://boards.straightdope.com/u/Telemark)\ **Post date:** [October 21, 2013, 12:33pm UTC](https://boards.straightdope.com/t/discharging-battery/671801/4 "2013-10-21T12:33:02Z")
> [@dtilque](#): > > Instead, they just included a USB adapter cable, so I have to plug it into the back of my computer to recharge it. BTW, we have a couple of wall chargers in the house and all accept USB cables so there’s no need to use your laptop to charge things. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [October 21, 2013, 12:44pm UTC](https://boards.straightdope.com/t/discharging-battery/671801/5 "2013-10-21T12:44:34Z")
Do you have any devices that charge through a USB port, and came with its own charger (AC adapter)? Smartphone, tablet, bluetooth earpiece, GPS receiver, etc.? If so, any of those AC adapters can be used to charge your light. If not, you may have a geek friend who has a drawer full of them and are happy to give you one. Or you can buy one for under $10. [Much less](http://www.amazon.com/Home-Travel-Charger-Power-Adapter/dp/B00CTTD10Q/) if you shop around. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [October 21, 2013, 12:46pm UTC](https://boards.straightdope.com/t/discharging-battery/671801/6 "2013-10-21T12:46:45Z")
> [@aNewLeaf](#): > > As a general rule, you shouldn’t leave charged batteries in a charger. > Hard to say what specifically will happen with a given device but a USB charger might be smart enough to get away with it. > You won’t hurt anything by trying it out once. If anything gets warm to the touch, unplug it. A USB power adapter isn’t a “charger”, it’s just a 5V power supply. The battery charging circuit is built into the battery or the light. And this circuit will stop charging as soon as the battery is fully charged. ---
**Author:** ![Turble](https://avatars.discourse-cdn.com/v4/letter/t/8dc957/32.png) [@Turble](https://boards.straightdope.com/u/Turble)\ **Post date:** [October 21, 2013, 3:37pm UTC](https://boards.straightdope.com/t/discharging-battery/671801/7 "2013-10-21T15:37:53Z")
Your batteries might also charge a lot faster if plugged into a wall charger rather than the computer USB port. The wall charger provides 1000 milliamps or more. On a computer, USB 2.0 provides only 500 mA and USB 3.0 provides 900 mA.