I would think that a good rule of thumb would be that the on-board charger won’t charge a battery that is below the light’s LVP. Since voltage rises in the cell once the light turns off this is relatively conservative in my opinion. For lights without LVP then something around the 3.0v range makes sense to me.
So obviously you know your stuff when it comes to Li-ion batteries!!!
Any sign of damage? Any bulges?
Maybe they were in a bad environment before you got them, like extreme temps.
I’ve got some lights that go down to 0.1 lumen. Any light that goes below 1 lumen is not going to have a lot of voltage bounce after it cuts off at LVP. Something has to go seriously wrong for me to ever see LVP on any light but some people apparently don’t mind taking the battery all the way down there and like I said an inadvertent light turned on or left on situation could happen to anybody. So I think you do have to have it be able to charge somewhere below LVP. How much below, I’m not sure. But I am going to do some testing with a few different brands of lights with some seriously dead batteries to see what they do.
I don’t know how the batteries were stored before I got them of course but after delivery I checked their voltages and they then went directly into my battery stash. My batteries are kept in a drawer in my downstairs room so temperatures there probably range from 50F in the winter to 80F in the summer (San Francisco has mild weather 
).
No sign of bulges or other external damage so I assume they’re just defective batteries 
Given that my Sofirn S11c was willing to charge the battery that was at 0.4v I wouldn’t be surprised if built-in chargers don’t do any checking at all for low voltages. I have a few more batteries that I was going to recycle and I’ll drain them as much as I can and will also test them with different built-in chargers. It will take a few days to get them to around 0.5v’s but it’s that’s just a matter of hooking them up to resistors.
Since these batteries were presumably produced as part of batches of hundreds/thousands batteries, what would make them defective vs all the others?
Just curious why you would think they would be defective? Perhaps you don’t have a theory. 
I surmise that they’re defective because they went from around 3.7v to around 2v for the 21700 and 1.29v for the 18650 in around 3 months. If you don’t consider that defective I would be happy to learn what you consider defective batteries to be 
Perhaps I’m not interpreting your post correctly but are you saying that this is not a defect of some sort and that I’m seeing normal batteries in action?
I believe I may have a Wurkkos 21700 that is experiencing similar behavior, though not at the the same high rate as the OP’s.
It arrived in a light that I received mid-Dec '22.
When a new light arrives with a cell, I check the voltage, which is usually at the nominal rating, or a bit below, and then use the on-board charger to do a full charge to verifty that it functions properly.
This light/cell was then re-boxed, with cell physically locked out, then sat dormant until last month, and when turned on, the light’s charge warning showed red, and the voltage checked in at the lower 3.x volt range.
I have cells that I fully charged last year to prep for the winter/blackout season, and did not end up being pressed into service, that are still ~4 volts.
For whatever reason, this cell in question drained at a moderate rate, despite being stored as it shipped from the factory, with the paper isolation disc. The disc doesn’t appear to be damaged, or allowing contact to be made, so I don’t beleive it’s idle driver current consumption from an unintended connection.
Of the ten or eleven S & W cells I have, this is first to exhibit any sort of abnormality.
I recharged it and will keep a tab on it.
I’m glad some one got good use of them — I have several Efest myself – Most of them after 2 years they loose at least 20% capacity
There’s a reason why Tesla fuses every cell individually ---- If there’s a failure, the fuse takes it out of the pack
Are there cells that DON’T lose capacity after a couple of years in use?
They all loose some-- I find the Sony and Samsung cells hold up the best
Just stating from my personal experience, Efest loose the most — Even when kept at storage voltage
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Japanese made Li-Ion have long been regarded as Premium cells.
I guess they’re either defective or something happened to them before you got them. I was just wondering if you had an inkling as to why this happened twice to you.
Maybe it’s just that you got unlucky, or perhaps there are a lot of these defective batteries out there. Sofirn would probably know best, but I doubt they would admit if this is a prevalent issue with their batteries.
Hopefully Sofirn will send you a replacement battery.
I wouldn’t be surprised if there are a lot of defective batteries out there. If one was used all the time and then charged as needed the self-discharge issue wouldn’t be apparent. It’s only when a battery is stored for a period of time and then the voltage checked before use that a problem becomes apparent.
For example if I had the bad 18650 in a light that I used daily I would have thought that it was fine since I would have charged it regularly.
Similar to car batteries where they seem fine when the car is driven daily but go flat when the person is away for vacation.
I would imagine cells that are stored in Warehouses with extreme Heat/Cold are more prone to failure — I often wonder when I buy lights at a deal price, how old is the cell — I recently purchased 3 Sofirn lights with 5000mah cells – all of them tested 4800 or less ( this is with the Opus charger that test slightly high )
I didn’t have any 18650s that were suitable for testing due to rust and corrosion and or disassembly issues. I’ve got about three that likely failed due to rust and corrosion and what are called micro shorts. There’s not much distance between the positive and negative at the top so all it takes is a little bit of corrosion to form a path between positive and negative to very slowly short things out and discharge. I don’t know what percent end up shorting out from corrosion versus internal problems.
I still wanted to do some testing of built-in chargers so I took a recently retired AA alkaline battery and a little wad of aluminum foil and proceeded. Why not make it even more interesting? Although results with a low voltage 18650 will probably be different. The AA battery was at 1.10 volts. I put some electrical tape near the top to keep it centered. I used two different usb power blocks and two different USB testers. That was partly because some needed micro USB and some needed a type-C. Most all of them were done for between 30 and 90 seconds at a time. All of them were plugged in multiple times to see if the charger would respond the same way everytime. All were tested before and after with the regular 18650 to make sure the charges were working normally. I wasn’t in a rush so the battery had time to rest for a minute or three in between flashlights.
Starting with the micro USB.
Two different Boruit headlights and a wowtec A7 and wowtac A6 were charging the battery at 0.1 amps. Sofirn hp40 was 0.12 amps.
Moving on to a few with type C. Wurkkos fc11 was 0.22 amps. Wurkkos hd15 was 0.14 amps.
Then I tried 2 armytek magnetic chargers. TextThe old white one put a little tiny bit of power in at 0.01 amps. The newest armytek black magnetic charger that’s been around for a little over 2 years did not recognize or attempt to chargeText. TextThe last thing I tried was a Folomov A1 magnetic charger for loose batteries. That at different times was trying to put in 0.02 and then later 0.04 amps.
I did not measure the AA battery voltage after every light but I did measure it a few times along the way and it reached a high point of 1.25 volts. Again it started at 1.10 and a few hours later it’s sitting at 1.153v.
So like I said earlier in the thread, if lights have built-in charges I use them but I always use a USB tester to periodically watch what they are doing.
Edit. I thought I might have made some mistakes on the armytek chargers by having the tail cap too loose or not just one quarter turn loose. So I went back and retested both the old white one and the new black one and they do both recognize the AA battery and attempt to very slowly charge. The USB tester showed 0.1 amps. It appears that the charger itself probably uses 0.025 amps so it’s probably charging the battery at approximately 0.075 amps. I decided to let the newer one go for about 20 minutes to see if it would stop or what would happen and battery voltage went up to approximately 1.32 volts. After an hour battery voltage was back at 1.22v.
So is any of this useful? With a phone battery you can see over time that you’re not able to get through a full day or you can see your battery capacity diminishes over time. With a flashlight battery most people are not going to be able to see a capacity change without measuring things and most people are not doing that. That includes me. I am not capacity testing batteries. I do check voltage of any new battery I get before I use it and I check voltage of batteries that have been sitting on used for any time period before I use them that’s the extent of it. So I have found three that were self discharging very slowly and I don’t think I have any others that are but you can’t expect the average consumer to be watching and checking this stuff. A quick visual check for corrosion is good to do periodically. Any sign of rust around the positive end is a cue to take it out of service. But then what are most people going to do with a battery that shows rust that’s half or nearly fully charged. I’m not aware of any local places that want to take these batteries regardless of charge level.
Batteries are like snowflakes, no two are identical. Even in studies where theyve used the same batteries from the same spot in the same line they’ll all perform slightly differently. Do a CT scan and they’ll all have slight variations on the inside too. You never really know what you’ve got.
Even with a CT scan you don’t know exactly what you’ve got. For example, here’s a photo of an LG M50 21700 that ruptured while charging at 0.5C
Can you see what caused this catastrophic failure? Yep, jelly roll was packed a little too tight. That’ll do it. You need to give these room to expand. Everyone knows that.
Now check out these photos of a Samsung 25r that was just slightly crushed a tiny bit.
Can you see what caused these ones to fail? You can’t, because they didn’t. “During the long-term cyclic aging of aging study I, none of the deformed cells exhibited any indicators of safety-relevant issues.”
Not only that, but the 2mm and 4mm deformed cells showed “no significant increase in the aging rates” and the cells with 6mm deformation only “showed slightly accelerated aging after 300 cycles.”
Weird right?
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I sent Sofin an email last Friday mentioning the problems I had with the 21700 and 18650 batteries. They asked for pics of them but I could only send the 18650 showing it at 1.29v. I had already disposed of the 21700 so I couldn’t send a pic of that. They also asked for the order numbers so I sent them images of the Amazon flashlight orders, which included a battery.
Today they asked for my address so that they could send me a battery. I assume it’s an 18650 since I couldn’t send a pic of the 21700. Less than an hour after I sent them the info they send me back a tracking number for the package.
I’ve always been very happy with Sofirn’s customer support so it doesn’t surprise me the way they responded. I wish other vendors (and I’m not talking just Chinese ones) treated their customers as well.
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After discharging my bad 18650 I took the Sofirn wrapper off to see what was underneath it. All there was was a gray wrapper with no markings. Underneath that was just a bare can with no markings.
Is this typical of rewraps that the wrapper underneath it has no manufacturer markings? This post What's under the wrapper (Sofirn + Thorfire 18650s) - #6 by Techflow shows the manufacturer and I expected something like that rather than nothing at all.
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