I’m no expert but as far as i understand it, about 3v is the lowest a li-ion cell is happy to go to, any lower and damage occurs so the voltage range for these cells is 2.8-3v to 4.2v
3v= fully depleted and 4.2v= fully charged. 3.7v is the nominal, average or working voltage of the cell.
This was confusing to me at first too.
They will still be able to output some power below 3v but like i said, it’s really bad for their chemistry.
Check out my signature links for more useful charts 
Great TY CRX I think I understand. Also two more good links to bookmark.
There is a large difference in voltage, depending on when you measure it, when loaded it is significantly lower.
In some cases that difference can be more than a 0.5 volt.
In the chart below I am switching a 3A load on/off. The red line shows the voltage and it jumps up and down with load/no load:
%20-%203A-Capacity.png)
How low a LiIon cell can go depends on the chemistry and if you measure loaded or unloaded voltage. A modern Panasonic cell can usual go down to 2.5 volt when loaded, but will recover to 3.3 volt when the load is removed.
Thanks alot guys! its alot clearer now 
I think that they should still hold. I have had a good experience with this. I would say a small loss is expected after a couple of years. I love the chart that was placed above. Thanks everyone
I will toss an 18650 for ANY reason or for NO reason. Sometimes I just get tired of the wrapper colors.
When I get down to ~1 or 2 spares per flashlight, I’ll go scrounge another laptop pack & just dare any of them to so-much as annoy me.
But I charge most or all of mine more than 50 times per YEAR!! The best three or four get charged a couple-three times a week or more.
The newer ones are better than the older ones, so why keep them?
Dim
I still have some of my first 18650 batteries ...
I find they either work ok or they lose their capacity to hold a charge ( cheaper nastier batteries ) for any length of time .
Twice a year I go through my rather large 18650 collection to recharge them all , ( Takes about 3 days ) and its this time I check them for voltage ..
The ones that have lost there charge in 6 months just sitting doing nothing get discarded , but the better quality 18650 hardly lose any charge at all .
I have one panasonic that if left @ 4.2v will still be 4.2v 6 months later .
And other batteries that will be close to 3v after 6 months .. ( See the trend here )
Good batteries will hold a charge , cheap batteries will self discharge , and require more frequent charging ( maintenance )
Im also leaning more towards non protected batteries , as cheap protection circuits might put a slight drain on the battery ( I have noticed this )
Removing the protection circuit stops the battery from self discharging .
If I buy more batteries , they will not be protected .
As for degradation ( ageing ) , this is highly variable and depends very much on initial quality and how much the battery is used .
I store all my 18650 fully charged if they Are garbage in 5 years I don’t care because than we will have better and newer cells which I want anyway…
some of my first 18650s already give up last week,i got them 2010 and been cycled charger 30-35 times a year.
thank you for your services :_(
Lets say your 18650 is 10 Wh (for simplicity)
500 0-100% cycles = 5000 Wh
2000 40-80% cycles = 8000 Wh
It’s not really 4x the lifetime, it’s only 1.6x… with the addition of major inconvenience.
Kodachrome is right, 40-80% is the optimum. One cycle are always 100, no matter if you use 3 times 33.3 or two times 50%.
For example the tesla always charges his battery pack to 80% to enhance his lifetime you need to select the full charge in the cars settings and even than it only lasts for few days until it switches back to 80% max…
But you have to think of the huge amount of cells and the costs in this case, that way different with a dozen cells from a flashlight fan.
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I am sure most here recharge the cells before they are empty, I do so, if I recognize a flashlight dimming I recharge this is good for cycle punt and for fun.
Batteries are so cheap so I don’t care so much about this and I always charge them fully because I want maximum current/brightness.
Hmm, it depends what cells you are using. Personally i like high quality batteries which cost like 10$ + per cell, (panas green 3400 protected) and considering i got 2x 4 cell lights, not exactly cheap.
I only have brand batteries but a pair protected panasonics costed me below 14$…so I have bought a dozen or so, I remember times were good batteries were not available or very expensive but it’s a lot better now even ten dollar is okay if you consider to get the best protection circuit and a very good cell…
Sanyo 2600 cost around 11$ per pair…
The lgD1 4.35V batteries are also not so expensive( I ordered 3 pair for 9$ each at fasttech), I recommend them because the higher voltage is a huge benefit especially if you want to use them with linear drivers like the nanjg.
Charging them to only 4.2 gives you the benefit of high cycles with the benefit of high voltage….
I have some lipo RC packs which I store at 3.6V because these are somewhat expensive and I know that I don’t need them fully charged all the time, but even in this case I know that I can toss them in few years…
hallo.i dont want to open new thread…can you reccomend to me any protected 18650 cells-the cheap ones? i need many of them…trustfire flames?
now i need cheap ones,but protected…
You guys must not walk the dog for an hour every night or go hiking, ect.!!!
When I am done my walk, my lights[K40vn,TK75vn, TK61vn, ect.] are around 3.6/3.7v. If I want to keep them at your suggested 3.8v, I would have to shorten my walk[The Siberian would not like that!] or bring another set of batteries w/ me!
I have had my 3400mAh Orbtronic 17 months now, mostly used on turbo, and they still get 52 minutes of max run time compared to 62 when new. My batteries may last 2/3 years but I will get my moneys worth of enjoying them, which to me is not 5 or 10 minutes at a time! Then again, there are people who do that and theirs will last longer.
I see your point but it does not apply to me.
I know it is old thread, but the information is good. Thanks for the advice on this thread, summarisable as keep batteries between 4.1v to 3.8v.
I like the idea of self imposed battery life elongation — a battery charger with that facility — since my current charger is now charging some new-to-me laptop batteries freshly harvested, and does not say that they are charged though they are really bright and charged 4.1 volts. But I need to be able to rely on the little red light turning green, since I don’t want to take out my multimeter all the time, so I may have to chuck these though they seem to work okay (Sony’s from “Fukushima”). Usually they all get up to green after a day of charging. I may look into getting a charger that charges them up less.
I have started a system where I stick a little green sticker on a batter if the battery works well, a yellow sticker if it is flaky and a red sticker if it seems bad. Two red stickers and a batter gets dumped.
I work in a place where quite a lot of laptops get thrown away and I find that the batteries I harvest them are better than ultra-trust-sure-whatever-Fire (which are probably harvested batteries) but since they are unprotected, and I have so many and I some times leave lights on without noticing, I need to throw them away periodically.
They really do explode if you short them
I now keep them in cases in my bag!
But in this video they just get really hot if shorted with a piece of wire
timtak, forget about that “only down to 3.8V” rule. Even considering that being a no-load voltage, it is simply ridiculous. I usually discharge my smartphone battery “all the way down” (which is ≈3.1 - ≈3.5V loaded voltage) and after about close to 2000 usage cycles, I can only discern a small performance difference. Oh! I charge it up to just ≈4V (less than 60% fuel gauge value right now, but they're known to be inaccurate so the actual state of charge is higher). 
Samsung's datasheet specifications list a maximum 4V charge voltage for their 35E cells in UPS applications, if this is of service to you.
Cheers :-)
Depth of discharge
Discharge cycles
(NMC / LiPO4)
Table 2: Cycle life as a function of
depth of discharge. A partial discharge reduces stress and prolongs battery life, so does a partial charge. Elevated temperature and high currents also affect cycle life.
Note: 100% DoD is a full cycle; 10% is very brief. Cycling in mid-state-of-charge would have best longevity.
100% DoD ~300 / 600
80% DoD ~400 / 900
60% DoD ~600 / 1,500
40% DoD ~1,500 / 3,000
20% DoD ~1,500 / 9,000
10% DoD ~10,000 / 15,000
what i see looking at this is, you only get so many amp-hours in and out of a cell (until the last line, 10% DoD)
if you discharge 100%, and then charge back up, you get 300-600 of those cycles
50% discharging gives you 2x as many
20% gives you 5x as many
it;s still about the same amount of total energy
which makes it simple
keep it charged when you can
otherwise don;t worry about it
the other thing is, the battery does not just suddenly quit, usually
it gradually loses capacity so that if you don;t notice the top capacity being 75% of what it was new, you can keep going way longer
the numbers are intended to be ‘after X cycles, the battery has only 80% of its new capacity’
but it isn;t dead
wle
[quote=gauss163]
ok then!
(i didnt; think it was all THAT bad though)
wle
Too many variables !
I have had 18650 that only lived a few months , I may still have a 18650 or two that came with my Solarforce L2’s ( Is that like maybe 8 years ago ? or more )
Half my 18650 collection would be close to 5 years old and show no signs of serious degradation ( or noticeable degradation )
I store my 18650 fully charged … ( ? )
Where I see a lot of problems is in the protection circuit … Some have parasitic drain … I had a battery that was always showing 0 volts after 6 months of storage …
I pulled the battery apart , removed the protection circuit and I did not charged that battery in 2 years and it is till showed 4v + …
It looked nasty wrapped in packing tape , and it may have been disposed of in my last battery cull .
If you have a problem battery , it could be the protection circuit . ( If its a protected battery )