Rechargeable flashlight batteries

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?

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.

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?

Have you tried crank-powered flashlights?
30 seconds of turning the crank can give you 30 minutes of light.

Mine work great.

I haven’t tried one, but it’s a great idea. Never a battery issue with those.

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…

They do have batteries in them you know…

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?

You’re right. I should have said, ‘never an uncharged battery issue’.

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.

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:

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.

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.

I checked my meter, on an alkaline AA: 1.8 volts.

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.

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.

I think it’s ironic that your battery tester needs a battery tester.

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.