When and how a 18650 li-ion battery will eventually die?

When a 18650 battery is charged and discharged, this is counted as one cycle. And the cycle life is determined by the difference in capacity of your cell taken from its first-use rating to its present rating. For example, if your cell started at 3000mAh, when the battery hold less than 80% (2400mAh), we say its cycle life is over.

Moreover, the recharge cycles vary and are limited. As the battery is reused, it degrades due to oxidation and electro-chemical degradation. Generally, the 18650 batteries have a typical cycle life of 300 to 500 (charge, discharge cycles). When in high-amp or high-drain situations, this can decrease substantially to 200 cycles. If you go over the maximum discharge current limit, it can radically decrease the cycle life. If you maintain the batteries well, some good quality batteries may achieve more than 500 cycles.

So how to know your 18650 is dying? There are some ways you can tell if your 18560 is nearing the end of its life, and it's time to get new ones.
*The battery loses its charge much faster than normal. It loses its charge after a couple of days or even worse overnight.
*The battery gets overheating when charging or discharging, warmer than normal.
*Recharge time gets abnormally long.
*You have used the battery frequently over 2 to 3 years.
*The battery can hold less than 80% of its original capacity.
*Check if there is any cracking or deformation in the battery.

How about investing in a good copywriter when sending out 'forum engagement' missives like this to an American forum? I'm tired of reading bad copy translations, ie "the battery gets overheating when charging...".

I especially recycle mine if they get too hot during a charge cycle. And I never use the fastest charge rate on any lithium cells.

I guess I should deep cycle them on occasion to see how close they come to the rated capacity. Provided they came close to it when new. I'm talking about some off brands on that one.

Thank you for the clear, informative information. Using a quality charger can help users know when a battery is getting 'long in tooth'.

I find the recharge time is shorter, not longer when they start to fail. It happens on my milwaukee tool batteries.

syga said:

I find the recharge time is shorter, not longer when they start to fail.

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I`v found that too, and usually when that happens they can`t supply as much current as they used too either, the voltage is fine just not much use for anything above an amp at most.
I`v never had the overheating thing (fortunately), you can normally tell they`re end-of-life before that, then it`s time to go in the door bell until it`s discharged (may as well make us of the remaining juice).

Eventually the battery will refuse to charge or charge too quick and either self discharge or discharge quick.
They can get hot when charging. And that is actually a sign the internal resistance has increased to dangerous levels. If it gets hot it can off gas hydrogen and it could get hotter as that happens and run away venting occurs until the cell either vents enough to close the vent or relive the pressure. But ignition and potentially explosion can happen during this. Hydrogen and lithium and cobalt salts are flammable it will present as red colored fire. Cobalt salts used are very toxic so the smoke is not great to breathe.