Yes I know: discharging a li-ion cell down to 2.5V is overall harmless.
Related article: Lithium Battery Failures @ Electropaedia
From HKJ's hut: How far can Li-ion be discharged? @ lygte-info.dk
Cheers :-)
Yelp, that’s why I said it might be good to test them yourself and let us know so maybe we can have some hard facts to go with what we believe or hear. As for going below the suggested rated cut off, I’d guess to say either to low or to high are probably as equal detrimental to the battery. When they developed the battery they tested the low and high quite extensively to find the sweet spot. Anything out of that range reduces the life of the cell. That’s not to say it can’t be done repeatedly and work fine. It only pertains to the number of cycles listed in the data sheet will be lower. Most people will never notice it or even care. Who counts battery cycles? 
Many people think that I am an idiot, because I never charge any Li-Ion above 3.9xV… This was the only reason to buy an MC3000… The reason for my strange behaviour is not money… but simply that I never want to count cycles and never need to test whether the batteries are still good… my strategy is: buy and forget… treat batteries well and trust that they live forever… so, deep in my heart I am lazy… even too lazy to simply replace all batteries every 1-2 years…
And that was the reason, that I raised the cut-off-voltage question here…
I've also gone quite far with regards to caring about my cells. I usually charge 'em up to just ≈3.95V, and since no usual philistine operator charging devices (LoL! :-D ) allow me to customize the output voltage this is the route I've taken in this regard:
Cheers ^:)
If you charge to 3.9v, you’re giving up about 30% capacity, but you’ll likely get well over 500 cycles until your “real” capacity drops 30%. If you charge to 4.2v, you get full capacity, and you’ll be ahead of your method for about 200 cycles.
So, if you expect to put on more than 200 cycles on your battery, your method is good, with the caveat that it’s essentially like buying a battery with 30% less capacity but 3x the cycles.
I’m not sure how many people put on more than 200 cycles on a battery. It depends greatly on the application. For electric vehicles, a 3.9v charge makes a lot of sense. I’m not so sure it’s as effective for flashlights. Personally, I tend to compromise, and charge to 4.1v for most of my cells. If I know I’m going to use them within a couple of days, I’ll give a full 4.2v charge.
Ha, ha, my Voltage (except lower storage voltage) is 3.92V… was the best number I could find from batteryuniversity: “In terms of longevity, the optimal charge voltage is 3.92V/cell. Battery experts believe that this threshold eliminates all voltage-related stresses; going lower may not gain further benefits”
OK, sorry, off-topic…
I am pretty aware of that. However, battery chemistry has improved in resilience and longevity. I even found a Samsung 35E datasheet where the absolute lowest charging voltage value quoted still was 4V even for UPS applications. Thus, even 3.95V may be a tiny bit overkill for brand name cells. On the other hand, some cheap chinese OEM cells die with just a few warm ups inside hot rod flashlights, so go figure.
Cheers :-)
Bist Du das „kirschm“? 
Hä?
Sorry, there is a member at TLF whose name is kirschm. He is well-known there for charging batteries to 3.92V only. So, I assumed you was him but I was obviously mistaken. Sorry for that.
Perhaps he got to the ‘famous 3.92V’ because it’s more or less the only ‘good and official’ number that can be found in various studies…
Anyway, let’s not talk about any charging termination voltage, this was not my topic of this thread… and to be honest - regarding the 3.92V - after 3x Gin Tonic I even allow 3.956876V… and now back to 2.5V resp. cut-off voltage according to a spec sheet and discharging…
Only a person that doesn’t understand li-ions would think your a idiot. That’s actually very smart and safer to increase the life of the cell.
I understand the consequences and I accept them. Your just being conservative and taking care of your batteries. :+1:
I watch a guy on youtube called BigClive. He’s an electrical engineer and I trust what he says pretty well. He claims that a Li-ion like an 18650 can be discharged to essentially 0V without catastrophic results. Doing it repeatedly will physically alter and damage the battery inside and effect it’s capacity over the long run. Should anyone do this? No not ideally, but the point is that if a Li-ion can survive that and bounce back, I wouldn’t at all be worried if I accidentally over discharged a cell to 2.2-2.4 or something like that. It’s not going to destroy it, and likely won’t have any appreciable effect on the lifespan of the cell that you’ll be able to notice.
BigClive LoL! Seen/spotted a few stuff from him, very funny! :-D
Cheers ^:)
Great thread!
Let me add that high current and all modern high-capacity cells also do NOT have a PTC. Like HG2, 30Q, VTC6 and GA, 35E, VC7. That is: no current limiting other than by internal and external resistance.
Right no PTC, but some have a one time working fuse.