How long can i leave 18650 batteries in a flashlight?

I have an unprotected 18650 battery from ebay.
it came with the flashlight.
it's fully charged and in the flashlight, which is sitting on my shelf.

how long do they last if I don't use it?
I also heard they catch on fire if over discharged???

Well if it came from ebay I personally would recycle it and buy a known trustworthy cell. I don't play around with suspect batteries.

That aside, letting one sit unused is about the safest thing you can do with one. No they will not catch fire from being over-discharged. You can ruin one that way, but its not dangerous. Overcharging one is the dangerous thing to do. And as for how long they'll last, that depends more on the light than the cell. A bare cell sitting in a box will hold a charge for a long time, many months. Cold and/or heat will accelerate the self-discharge. But some flashlights have a parasitic drain themselves, greatly accelerating the rate of discharge. If you don't intend to use the light for a while, your best bet is to lock it out by loosening the tailcap (assuming anodized threads) or using the built-in lockout if so equipped.

But then all of this assumes you've got a good cell to start with, which I personally wouldn't trust considering the source.

emarkd, pretty much so nails it. I don't trust any eBay battery stuff.

As long as you want, it catch on fire if over charge, not discharged.

Li-ions have very low self discharge so several years is not a problem.

My modified surefire fell out of my pocket while putting up one of those 4ft deep walmart wading pools, I didn't realize it until the pool was full. Needless to say, it stayed under that pool liner (leaks, storms, and all) for an entire Texas summer (May-October). After we took everything down I chipped the light out of the packed dirt like a CPF archaeologist, gave it a twist and it lit right up. The AW 18650 (protected) was absolutely fine. So at least 6 months on a shelf should be fine, if you aren't comfortable with it, set a 6 month alarm on your calendar and just recharge it twice a year. I do all my batteries once(ish) a month just in case of a freak long duration power outage.

I try to recharge all my batteries if not used every 6 months. Like said in earlier posts, low grade batteries can be can be a concern. Look to see if the battery has venting. Also, if it says it's a 4000mah or something, but the battery seems to be small for that rating, it's most likely a very cheap battery. I use Panasonic batteries personally, but I do have a few Ultrafires that I use for testing. Only one of the Ultrafires is actually a good battery. I have been using it for 4 years now, and still holds a decent charge. Sorry if I got off topic a little. Just trying to give you a better idea if you got a decent battery, or not, and would trust it sitting around.

glockboy said:

As long as you want, it catch on fire if over charge, not discharged.

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Sorry but not true. When a battery over discharges it develops metallic shunts which can cause an internal short and throw the battery into thermal run away. Also if left in the torch it is possible that there will still be a small amount of current flowing through the circuit which will discharge the battery faster than if stored out of the torch. With all batteries it is highly recommended to take them out of items you do not intend to use for more than a few days.
I know I've only been here a few days but I've been a vaper for years and know battery safety.

Thom2022 said:

Sorry but not true. When a battery over discharges it develops metallic shunts which can cause an internal short and throw the battery into thermal run away. Also if left in the torch it is possible that there will still be a small amount of current flowing through the circuit which will discharge the battery faster than if stored out of the torch. With all batteries it is highly recommended to take them out of items you do not intend to use for more than a few days.
I know I've only been here a few days but I've been a vaper for years and know battery safety.

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Is that really true? Wouldn't a deeply discharged Li-ion developing shunts also have virtually no energy with which to drive a thermal runaway?

I was also under the impression than over discharging a Li-ion posed no specific risk in itself - but rather it is just the subsequent use of an over discharged cell that causes the problems.

I must have at least a dozen over discharged Li-ion cells sitting around the house just in 10yo obsolete cellphones, cameras, and laptops and I think millions more folks do as well. Think we would have heard more about this risk in the news then.

A LI-ION battery in idle is usually not an issue, its when its being used, and over discharged by a device such as flashlight, cellphone, etc.. When the voltage gets to low, your device will shut itself down so there is no threat of damage to the battery or the device. Protected cells add extra security against this. I don't normally bother with protected batteries only because all my drivers I use in my lights have low voltage protection.

reppans said:

I must have at least a dozen over discharged Li-ion cells sitting around the house just in 10yo obsolete cellphones, cameras, and laptops and I think millions more folks do as well. Think we would have heard more about this risk in the news then.

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Charged up my original 1999 Nokia 5190 cell phone the other day just for fun! It was sitting around dead for years. It powers up and seems to hold a charge OK. If I put any load on it (made a call if it was activated) I'm guessing it would die pretty quick. Still pretty impressive for a 17 year old phone. Actually it could have been purchased in 2000, but either way it's a long time! We're talking Li-ion polymer here, I assume, if that makes any difference.

You are right in the sense that dead is dead but when you charge an over discharged battery it can go pop at any point in the charge cycle not just if its over charged. But the fault still lies with it being over discharged. Not meaning to step on any toes just wanted to point out that over discharging can still be a danger.

reppans said:

Is that really true? Wouldn't a deeply discharged Li-ion developing shunts also have virtually no energy with which to drive a thermal runaway?

I was also under the impression than over discharging a Li-ion posed no specific risk in itself - but rather it is just the subsequent use of an over discharged cell that causes the problems.

I must have at least a dozen over discharged Li-ion cells sitting around the house just in 10yo obsolete cellphones, cameras, and laptops and I think millions more folks do as well. Think we would have heard more about this risk in the news then.

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Over discharging is not the problem as you say, its then charging after this and how long its been bellow the 2.5v(or the spec of that specific cell as a variable). If for example an 18650 cell is over discharged to 2.3v or 2v, and charged up immediately, then damage will be minimal. If left for weeks.........months then in that time you get a build up which can cause issues later on. Trouble is predicting this, could be fine for 6 months then..............not so. So if left for an unknown time and bellow minimum spec, it is best practice to recycle. After all, they are cheap enough and no reason to take any risks(as you know anyway). Just a general comment.

OK, I think we're all on the same page. I just thought @Thom2022 was saying the batt could short/thermal runaway *while* it was losing charge and going into an over-discharged state.

Yes, my understanding is that, once the cell *has been* over-discharged, then all bets are off and it can blow next charge cycle, next 20th charge cycle, while in use, etc. etc.

Not much to add - but I would go so far as to say that a healthy 18650 has less discharge rate than a primary. I've had them hold for years. Shot or cheap (made shot) ones may not, but those are the signs of a cell that needs to be recycled: Inability to hold it's charge for long periods, significant loss of capacity relative to new, and if it gets warm during a proper charging sequence. Not worth the risk/hassle of keeping cells with any of those traits.

Also, if your light has a mechanical switch, not one of the electronic switches like a mouse button, it is most unlikely you will have any drain issues in the light. This type of switch opens the circuit completely. Many high quality electronic switch lights (HDS, Zebralight) have such low parasitic drain it is not an issue, but you need to know for sure before you bury your light loaded with battery in your zombie apocalypse bag.

Sellers usually advise leave battery in flashlight no more than three month when you don't use it. Don't need care it.
You just need to charge battery, because passive drain problem.

emarkd said:

No they will not catch fire from being over-discharged. You can ruin one that way, but its not dangerous.

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ven said:

Over discharging is not the problem as you say, its then charging after this and how long its been bellow the 2.5v(or the spec of that specific cell as a variable).

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This.

And pay attention to what is said about acquiring quality cells. In Chaos Theory, there is the principle of sensitive dependence on initial conditions. Everything depends on what you start with! Patronize a reputable dealer, acquire quality cells, and only use a quality charger. Avoid over-discharge religiously, but don't panic when you over-discharge. Recycle the damaged cell and replace.