18650 storage information.
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iirc if you store a fully charged 18650 without discharging for a long time you'll see noticeable capacity shrinkage thru chemistry degradation. They are best left at ~50% charged so they lose capacity at a much lower rate. Please correct me if I'm wrong.
swan
Banned
For storage keep your cells at 3.7 volts.
chillinn
Flashlight Enthusiast
No.
3.7V is a nominal charge, while 4.2V--4.35V is a full charge. Store your Li-ion cells long term between 3.7v--3.85V.
Warp
Enlightened
Man, I have so not been storing mine optimally. But then I think...why store them at a partial, not really usable, charge level, making it so I can't reach for them to use? Why have them then?
oeL
Newly Enlightened
Exactly my point of view. That's why I only have as many cells as I regularly need (+ 2 extra spares). I always store them at 4.2 V with frequent recharge. Since bare unprotected 18650 are not so expensive any more (I'm using Panasonic NCR18650GA) I replace the cells every 2 or 3 years. To reduce the chemical ageing a bit I put them in the fridge.
For long storage use cases and higher temperatures IMR cells might be an alternative, as long as the lower capacity isn't an issue. I'm keeping a flashlight with a NCG18650CG in my car, it's parked outside all year long, and I recharge the cell usually once or twice a year (~ 4.15 V -> 4.20 V).
terjee
Enlightened
I'd say that if you'd like them available for use in the next 3 months or so, just store them fully charged. For anything you have in rotation, fully charged is fine.
It's not uncommon to be rotating three batteries or packs, so that even with one in the charger, you still have one in use, and one ready for use. It's also not uncommon to have some batteries that are sitting on a shelf, ready to replace one of those three batteries/sets, when they're ready for retirement. Those shelf-batteries are the ones you might not want to fully charge. Sure, you'd have to charge it up first, but getting it from the shelf beats waiting for it to come in the mail.
Personally I like the idea of having a rotation of three batteries/sets, as well as 3-4 available for replacing those.
I've yet to come across any paper describing a staggering difference in leaving it stored fully charged, as opposed to at a lower charge. Sure, there is a difference, but it's not so huge that I'd like to recommend everyone run out and get fancy (expensive) chargers to charge/discharge their batteries to a specific state of charge.
There's also a couple of issues that are often confused with the storage-question;
One is that there's a recommendation for storing at 30% (number varies a bit) for *shipping*. A battery charged to 30% is actually safer to ship than a fully charged one. If you have an aircraft filled up with LiIons, you want that safety margin on your side.
The other is that there can be a significant difference in how many cycles a cell can take, based on how high you charge it, and how low you deplete it. If a specific cell would survive full discharge and full charge 500 times, the same cell might be able to go from 60% to 30% and back up a total of 2000 times. There's an important key point here though; With those numbers, you extract 3 times as much energy from the full discharge/charge, as compared to the other. That means three times as many charge-cycles, and that "eats up" most of the quadrupling in the number of cycles. Okay, so you get 4 times as many cycles, but that's not all that important when you'd have to use three times as many cycles for the same amount of work. There's a gain to be had sure, but it usually doesn't make too much sense to go for it. You'd also be moving wear to the light and charger, and I'd much rather wear a $5 battery that's easily replaceable, than a $50 light I might be attached to. These things are important for engineering tasks (you could increase a battery bank 300% to increase lifetime 400%), but less so for flashlights.
A lot of people take the shipping-recommendation as a storage-recommendation, and a lot of people take the quadrupling of cycles but miss the tripling of cycles needed.
The takeway key-points should IMHO be;
- Don't obsess over this. You could maybe have a small gain, but for most it's too much hassle to be worth the effort.
- Don't stress about fully discharging a battery, or charging it fully up. Charging a bit here and there is actually fine, and even good for the battery.
- There's no point (or gain) to having a routine of monthly "top ups" to stored batteries. (But there value in checking them periodically, like oeL mentions).
- If you want them fully charged for an emergency, go for it.
I'd rather store a battery with a bit too high a charge, than a bit too low. Personally I'm probably going with 70-80% somewhere, but wouldn't care too much if my charger didn't allow me to easily set a custom voltage to charge to.
edit to add; take the specific numbers with half a grain of salt, they're intended to illustrate the point, not be specific figured for any given cell.
What kind of times are we talking when we say "long time"?
Will keeping cells fully charged for a month start the damage? A week? A year?
BTW, I appreciate the info on this site. I'll bring my NCR18650GA spare cells down in voltage just to be on the safe side.
Will keeping cells fully charged for a month start the damage? A week? A year?
BTW, I appreciate the info on this site. I'll bring my NCR18650GA spare cells down in voltage just to be on the safe side.
terjee
Enlightened
There's always some degradation over time with LiIons, no matter what you do. It's not that the battery will be damaged if you charge it fully, and not at all if you don't. It's just different degrees of everything. Personally, I'd have no trouble leaving a fully charged LiIon sitting for a month, and I probably wouldn't worry too much about leaving it for a year. An over-depleted LiIon battery (even from self discharge) is worse than leaving it fully charged. That will actually ruin rather than just wear them.
I wouldn't stress about it to be honest. If you do want to take extra precaution, landing at or about 3.9V - 4V might be a decent tradeoff. That should land you at about 50-60% charge, which you can charge back up to without having much voltage-stress on the cell. If you're ever in doubt, it's better to charge a bit too much rather than a bit too little, since charging more (up to 4.2V) will just be wear, while the cell ever going under-voltage can break it.
chillinn
Flashlight Enthusiast
If you are referring to self-discharge, the benefits of fridge storage is a bit of a misnomer. Ostensibly, lower temperatures will slow a cell's self-discharge. But in practice, if the cells are not sealed in a dry container and kept dry, any reduction in self-discharge will be thwarted by moisture, which will increase the self-discharge considerably. We'll assume you're doing it correctly, but just leaving it at "stick it in the fridge" without instructions to make sure the cells stay dry misses the mark completely, will do more harm than good.
terjee
Enlightened
I have to admit, I must have unconsciously went with the assumption that sticking it in the fridge was done by advice, and probably came with instructions on how to do that correctly. Good point chillinn.
Other risks include some parts of the fridge being colder, bringing the temperature down too low for the cells. Freezing them is bad (I don't recall what temp that'd be at though).
Compared to "charge fully, leave sitting in shade on a shelf", it might be possible to extend lifetime, but it's often easy to get a worse result than the shelf. Fridge has risks of moisture and too low temp, charging to a lower voltage has risks of the voltage dropping too low by self discharge, and so on. No matter what you do, they won't last forever, and it's easy to do more harm then help, in the hopes of getting a minor gain.
I'm not advocating for shelf over fridge, or fully charging them over a lesser charge, I'm just trying to remind people to not stress too much about these things. Sure, if you're a tech geek and it peeks your interest, by all means explore your options. For most people though, it's okay to not get things 110% perfect, 98% will do just fine.
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Fully sharged Lithium ion batteries degrade quicker. The refrigerator trick still holds true for them. http://batteryuniversity.com/learn/article/how_to_store_batteries
