# Is it generally all right to recharge rechargable batteries before they are flat? **URL:** \ **Category:** Factual Questions\ **Created:** [November 15, 2006, 9:17am UTC](https://boards.straightdope.com/t/is-it-generally-all-right-to-recharge-rechargable-batteries-before-they-are-flat/380504 "2006-11-15T09:17:42Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![Crowbar\_of\_Irony\_3](https://avatars.discourse-cdn.com/v4/letter/c/f08c70/32.png) [@Crowbar\_of\_Irony\_3](https://boards.straightdope.com/u/Crowbar_of_Irony_3)\ **Post date:** [November 15, 2006, 9:17am UTC](https://boards.straightdope.com/t/is-it-generally-all-right-to-recharge-rechargable-batteries-before-they-are-flat/380504/1 "2006-11-15T09:17:42Z")
Hi all, I am currently using a couple of rechargable batteries (Eneloop, from Sanyo). One particular thing about rechargable batteries which I have learnt long ago (about, say 10 years ago?) was that it if you recharge them before they are flat, you are reducing the maximum battery life. At least for video camera batteries… Does this still holds true for current rechargable batteries? BTW, I have looked through Sanyo’s documentations and website on this product, but can’t find any answers. Thanks in advance! ---
**Author:** ![Malacandra](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@Malacandra](https://boards.straightdope.com/u/Malacandra)\ **Post date:** [November 15, 2006, 9:44am UTC](https://boards.straightdope.com/t/is-it-generally-all-right-to-recharge-rechargable-batteries-before-they-are-flat/380504/2 "2006-11-15T09:44:07Z")
Very quick answer: Nickel-cadmium (NiCd) batteries are subject to “memory effect” which means that they shouldn’t be recharged before they’re flat or taken off charge before they’re full, or else they will not discharge (or charge) as far in future. Nickel-metal hydride (NiMH) batteries don’t exhibit this to the same extent, and nor IIRC do lithium-ion (LiIon) or lithium-polymer (LiPoly) batteries, though these last have problems all of their own. If a lithium battery is discharged too far it goes into “deep discharge” and is unrecoverable, and if it discharges or charges too fast it can experience “thermal runaway”, more informally known as “catching fire”. Mind you, a phone or camera and its charger will be wired up to prevent this. It’s just that slapping a lithium battery on a dumb charger, or shorting it out, is a really bad idea. ---
**Author:** ![SCSimmons](https://avatars.discourse-cdn.com/v4/letter/s/e495f1/32.png) [@SCSimmons](https://boards.straightdope.com/u/SCSimmons)\ **Post date:** [November 15, 2006, 11:58am UTC](https://boards.straightdope.com/t/is-it-generally-all-right-to-recharge-rechargable-batteries-before-they-are-flat/380504/3 "2006-11-15T11:58:37Z")
[QUOTE=ExtraKun] One particular thing about rechargable batteries which I have learnt long ago (about, say 10 years ago?) was that it if you recharge them before they are flat, you are reducing the maximum battery life. At least for video camera batteries… Does this still holds true for current rechargable batteries? [/QUOTE] Luckily, Cecil [just wrote us a column](http://www.straightdope.com/columns/061103.html) on this! ---
**Author:** ![Keeve](https://avatars.discourse-cdn.com/v4/letter/k/f07891/32.png) [@Keeve](https://boards.straightdope.com/u/Keeve)\ **Post date:** [November 15, 2006, 1:41pm UTC](https://boards.straightdope.com/t/is-it-generally-all-right-to-recharge-rechargable-batteries-before-they-are-flat/380504/4 "2006-11-15T13:41:22Z")
[QUOTE=Malacandra] “thermal runaway”, more informally known as “catching fire”. [/QUOTE] I nominate the author of that euphemism for a Pulitzer! ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [November 15, 2006, 2:39pm UTC](https://boards.straightdope.com/t/is-it-generally-all-right-to-recharge-rechargable-batteries-before-they-are-flat/380504/5 "2006-11-15T14:39:01Z")
[Here’s](http://www.repairfaq.org/sam/aapsfaq.htm#adpsmem) some good info from one of the Net’s masters on all things electronics. Note that it is a bit out of date so no real info on LiIons. For any battery of type, try to avoid an actual _full_ discharge. As in down to essentially 0 volts. If it happens too fast, it will generate the aforementioned thermal problems. For rechargables, they are done for good. The key is two discharge points: when the device no longer works and what is best for the battery before doing another charge. The former is heavily device dependent. Ideally the two are nearly the same. Usually for laptops, the latter is noticably lower than the former in the case of NiMHs in particular. So, I have saved laptop NiMH packs that appeared dead/unrechargeable but doing a series of discharge/charge cycles. (Ideally using the software provided by the laptop maker.) This requires the device is set to continue to draw power from the pack for a bit in a state where it would normally not continue to run. This is the famous voltage depression/overcharge situation that above links mention. As to LiIons. When they go they go. The laptop packs I have seen have the cells in groups of 2 or 3. One battery in one group gets a bit flaky and that’s it for the whole pack. That group won’t stay charged, etc. LiIons are significantly more sensitive to a lot of things … including running too far down. Special circuitry inside the packs themselves try to keep everything hunky-dory, but the real world makes its presence known. I basically try to regularly recharge LiIons before they are significantly depleted. I do try to avoid running in “top off” mode. (Although the latter might not be the actual best practice.)