# Rechargeable flashlight batteries **URL:** \ **Category:** Factual Questions\ **Created:** [December 27, 2020, 9:00pm UTC](https://boards.straightdope.com/t/rechargeable-flashlight-batteries/929041 "2020-12-27T21:00:35Z")\ **Posts on this page:** 18\ **Page:** 1
**Author:** ![Capn\_Carl](https://avatars.discourse-cdn.com/v4/letter/c/838e76/32.png) [@Capn\_Carl](https://boards.straightdope.com/u/Capn_Carl)\ **Post date:** [December 27, 2020, 9:00pm UTC](https://boards.straightdope.com/t/rechargeable-flashlight-batteries/929041/1 "2020-12-27T21:00:35Z")
I’ve had a flashlight for a few years, and have been very satisfied with it until now. It uses a AA-sized 14500 Li-ion rechargeable battery. Two weeks ago, I left the flashlight on its recharger, as I’ve done many times before, but when I checked it afterwards, it wouldn’t light on any of the 4 power settings. I installed an identical battery and it worked fine, so I assumed the battery had failed. I used the flashlight for a week, recharged it, and the same thing happened. I checked the battery voltage. Both are charged to 5.3 volts; well above the rating of 3.6 volts. Is that the problem, too much charge? ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [December 28, 2020, 12:07am UTC](https://boards.straightdope.com/t/rechargeable-flashlight-batteries/929041/2 "2020-12-28T00:07:42Z")
I don’t know if that is _the_ problem, but it is _a_ problem. Lithium Ion batteries reach full charge at just about 4.2V/cell. 5.3V is way too high. ---
**Author:** ![Capn\_Carl](https://avatars.discourse-cdn.com/v4/letter/c/838e76/32.png) [@Capn\_Carl](https://boards.straightdope.com/u/Capn_Carl)\ **Post date:** [December 28, 2020, 12:28am UTC](https://boards.straightdope.com/t/rechargeable-flashlight-batteries/929041/3 "2020-12-28T00:28:58Z")
Good to know, thanks! I installed a 1.5v AA alkaline battery in the flashlight. It works, albeit only at the lower power settings. That tells me the flashlight is probably not the problem. I think it’s the charger. I’ll bet the flashlight is sensing the excessive voltage, and not coming on to be ‘safe’. I’m going to have to figure out a way to dissipate that overcharge and get the charge down to ~4v and see if the light works. What is a safe way to run the battery down? ---
**Author:** ![Bosda\_Di\_Chi\_of\_Tricor](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bosda_di_chi_of_tricor/32/16845_2.png) [@Bosda\_Di\_Chi\_of\_Tricor](https://boards.straightdope.com/u/Bosda_Di_Chi_of_Tricor)\ **Post date:** [December 28, 2020, 12:41am UTC](https://boards.straightdope.com/t/rechargeable-flashlight-batteries/929041/4 "2020-12-28T00:41:16Z")
Have you tried crank-powered flashlights? 30 _seconds_ of turning the crank can give you 30 _minutes_ of light. Mine work great. ---
**Author:** ![Capn\_Carl](https://avatars.discourse-cdn.com/v4/letter/c/838e76/32.png) [@Capn\_Carl](https://boards.straightdope.com/u/Capn_Carl)\ **Post date:** [December 28, 2020, 12:42pm UTC](https://boards.straightdope.com/t/rechargeable-flashlight-batteries/929041/5 "2020-12-28T12:42:13Z")
I haven’t tried one, but it’s a great idea. Never a battery issue with those. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [December 28, 2020, 1:38pm UTC](https://boards.straightdope.com/t/rechargeable-flashlight-batteries/929041/6 "2020-12-28T13:38:19Z")
> [@Capn\_Carl](#): > > Good to know, thanks! > I installed a 1.5v AA alkaline battery in the flashlight. It works, albeit only at the lower power settings. That tells me the flashlight is probably not the problem. I think it’s the charger. > I’ll bet the flashlight is sensing the excessive voltage, and not coming on to be ‘safe’. I’m going to have to figure out a way to dissipate that overcharge and get the charge down to ~4v and see if the light works. > What is a safe way to run the battery down? If that were the only problem. you could run down the charge simply by turning on the light. In fact, there is no safe way to do this, because Lithium-Ion batteries cannot be safely ovecharged. Any prolonged charging above 4.30 V will, to make a long story short, lead to spectacular explosions as metallic lithium plates on the anode and the cathode produces gas and bursts, thermal runaway, flames, metal fire… ---
**Author:** ![GaryM](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/garym/32/241_2.png) [@GaryM](https://boards.straightdope.com/u/GaryM)\ **Post date:** [December 28, 2020, 1:51pm UTC](https://boards.straightdope.com/t/rechargeable-flashlight-batteries/929041/7 "2020-12-28T13:51:36Z")
They do have batteries in them you know… ---
**Author:** ![Capn\_Carl](https://avatars.discourse-cdn.com/v4/letter/c/838e76/32.png) [@Capn\_Carl](https://boards.straightdope.com/u/Capn_Carl)\ **Post date:** [December 28, 2020, 5:36pm UTC](https://boards.straightdope.com/t/rechargeable-flashlight-batteries/929041/8 "2020-12-28T17:36:54Z")
You’ve convinced me that I need to dispose of the overcharged batteries. I’d like to discharge them to a safe level. I can’t just turn on the light; if the light worked, I’d never have started this thread. What if I dropped them in a bucket of water? ---
**Author:** ![Capn\_Carl](https://avatars.discourse-cdn.com/v4/letter/c/838e76/32.png) [@Capn\_Carl](https://boards.straightdope.com/u/Capn_Carl)\ **Post date:** [December 28, 2020, 5:38pm UTC](https://boards.straightdope.com/t/rechargeable-flashlight-batteries/929041/9 "2020-12-28T17:38:19Z")
You’re right. I should have said, ‘never an uncharged battery issue’. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [December 28, 2020, 5:42pm UTC](https://boards.straightdope.com/t/rechargeable-flashlight-batteries/929041/10 "2020-12-28T17:42:26Z")
> [@Capn\_Carl](#): > > You’ve convinced me that I need to dispose of the overcharged batteries. I’d like to discharge them to a safe level. I can’t just turn on the light; if the light worked, I’d never have started this thread. What if I dropped them in a bucket of water? Just take them to the battery recycling/disposal place, and buy a battery charger **designed for Li-ion batteries** if you are going to keep using them, along with a new battery. If the light has a built-in charging system and that has dangerously malfunctioned (it should definitely not be charging those cells beyond 4.20V), then perhaps the whole thing is electronic waste and you need a new flashlight. ---
**Author:** ![FinsToTheLeft](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/finstotheleft/32/2930_2.png) [@FinsToTheLeft](https://boards.straightdope.com/u/FinsToTheLeft)\ **Post date:** [December 28, 2020, 7:07pm UTC](https://boards.straightdope.com/t/rechargeable-flashlight-batteries/929041/11 "2020-12-28T19:07:36Z")
> [@Capn\_Carl](#): > > What if I dropped them in a bucket of water? I would have thought this was a spectacularly bad idea, but apparently there is so little metallic lithium in smal Li ion batteries that it is not an issue: > **[SOLVED: What should you do if you have a Lithium ion Battery Fire? - Battery](https://www.ifixit.com/Answers/View/557642/What+should+you+do+if+you+have+a+Lithium+ion+Battery+Fire)** > > In addition to the great contributions already here, here is something directly from Battery University which has tons of great info on Lithium batteries. If a Li-ion battery overheats, hisses or bulges, immediately move the device away from... ---
**Author:** ![Melbourne](https://avatars.discourse-cdn.com/v4/letter/m/b5e925/32.png) [@Melbourne](https://boards.straightdope.com/u/Melbourne)\ **Post date:** [December 29, 2020, 2:49am UTC](https://boards.straightdope.com/t/rechargeable-flashlight-batteries/929041/12 "2020-12-29T02:49:29Z")
> [@Capn\_Carl](#): > > I checked the battery voltage. Both are charged to 5.3 volts; well above the rating of 3.6 volts. Is that the problem, too much charge? That seems quite unlikely. Perhaps you are misreading the meter, or checking two cells in series? Two AA cells in series is a 7.2V battery, which, when flat, could well read 5.3V. 5.3V for one standard Li-ion cell is not physically realizable. When you take it off the charge, it would fall within seconds at most. ---
**Author:** ![Melbourne](https://avatars.discourse-cdn.com/v4/letter/m/b5e925/32.png) [@Melbourne](https://boards.straightdope.com/u/Melbourne)\ **Post date:** [December 29, 2020, 2:56am UTC](https://boards.straightdope.com/t/rechargeable-flashlight-batteries/929041/13 "2020-12-29T02:56:50Z")
> [@DPRK](#): > > as metallic lithium plates on the anode and the cathode Over-charging a Li-ion battery is a bad idea, as they may burst and catch fire, or catch fire and burst. Lithium batteries, aka Lithium metal batteries, are an even bigger risk than Li-ion batteries, because, unlike Li-ion batteries, lithium batteries contain metallic lithium plates. ---
**Author:** ![Capn\_Carl](https://avatars.discourse-cdn.com/v4/letter/c/838e76/32.png) [@Capn\_Carl](https://boards.straightdope.com/u/Capn_Carl)\ **Post date:** [December 29, 2020, 6:31pm UTC](https://boards.straightdope.com/t/rechargeable-flashlight-batteries/929041/14 "2020-12-29T18:31:50Z")
I checked my meter, on an alkaline AA: 1.8 volts. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [December 29, 2020, 8:40pm UTC](https://boards.straightdope.com/t/rechargeable-flashlight-batteries/929041/15 "2020-12-29T20:40:06Z")
That seems way off. 1.65 is about the maximum open-circuit voltage for Alkaline chemistry. So, if your meter is off by 1.8/1.6 = 12.5%, then it will read 4.2v as 4.75, which still doesn’t explain the super-high 5.3v reading, but maybe the error is non-linear. ---
**Author:** ![Capn\_Carl](https://avatars.discourse-cdn.com/v4/letter/c/838e76/32.png) [@Capn\_Carl](https://boards.straightdope.com/u/Capn_Carl)\ **Post date:** [December 30, 2020, 5:13pm UTC](https://boards.straightdope.com/t/rechargeable-flashlight-batteries/929041/16 "2020-12-30T17:13:27Z")
You nailed it. The battery in my voltmeter was weak. I put a new one in, and the AA had 1.5v. The 14500 has 4.0v. So whatever is causing the flashlight to fail, it’s not the battery. Thanks to all, for the advice. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [December 30, 2020, 5:53pm UTC](https://boards.straightdope.com/t/rechargeable-flashlight-batteries/929041/17 "2020-12-30T17:53:53Z")
I think it’s ironic that your battery tester needs a battery tester. ---
**Author:** ![Capn\_Carl](https://avatars.discourse-cdn.com/v4/letter/c/838e76/32.png) [@Capn\_Carl](https://boards.straightdope.com/u/Capn_Carl)\ **Post date:** [December 31, 2020, 1:55am UTC](https://boards.straightdope.com/t/rechargeable-flashlight-batteries/929041/18 "2020-12-31T01:55:50Z")
Well, I’m not going to tell you that after I pulled the 9v battery from my voltmeter, I considered using the voltmeter to check the 9v battery, then realized I couldn’t do that using a voltmeter with no power. I’m not going to tell you that, because then you’d know I’m dimmer than my dead flashlight.