I just noticed that a Sofirn 18650 3000mAh battery that I never used self-discharged itself down to 1.3v from 3.7v in a span of 3 months. I also had a Sofirn 21700 do the same thing over a short period of time. Both of these batteries were relatively new, the 18650 being from 10/2022, and other older batteries from them haven’t shown any signs of high self-discharge.
I wonder if Sofirn changed their supplier recently, and if they did, they need to change back 
Sounds like an internal short, if it wasn’t inside a light. An 18650 should stay at between 3.7-3.4v for a long time. I’d contact their customer service, and get a pair of “known good” batteries from a trustworthy seller like illumn, liionwholesale, or 18650batterystore.
I had the same issue with one of their batteries years ago.
https://budgetlightforum.com/t/review-sofirn-sp40-best-budget-headlamp/57062/26
I have a bunch of quality batteries (Samsung 30Q’s, Panasonic 18650B’s, etc) so “losing” the Sofirn batteries isn’t a show stopper for me. It’s mostly the annoyance factor of having it spontaneous go bad on me 
I think I’ll send an email to Sofirn support and hopefully they’re respond back.
If it have protection circuit remove it and see.
They might give you a discount coupon at worst, their support is pretty good from my experience. Maybe you’ll get a replacement battery, hopefully without the self-discharge issue.
Nope no protection circuit.
I’ve had good luck with their support in the past so hopefully they’ll respond positively. I also asked them if they changed suppliers since both batteries were fairly new, and I haven’t had problems with their batteries before.
They might be some Efest rewraps-- Worst cells ever for not holding up
I’m going to take the wrapper off and see if there’s any markings on it. I probably should have done that with the 21700 but that cell is long gone.
It’s one of those defects that wouldn’t be noticed if the battery was in constant use since it would be charged as it became depleted. It’s only the fact that it was stored for a few months that it became noticeable. I check all my stored batteries several times a year so it was a shock how fast this last one dropped.
I used nothing but Efest cells for years. Still use an 18500 in my EDC. Never had issues with them. Still have a great many that work fine, even 7 or more years old.
Matter of fact, 2 new 18500’s are expected here tomorrow. Great cells in my experience, top performers until a few newer chemistries usurped them.
Any idea of Sofirn lights with built in chargers would refuse to charge too low voltage batteries? With those, I wouldn’t be able to catch this situation otherwise unlike using external battery charger.
What’s interesting is that I have over 100 18650/21700 batteries from major brands (including Sofirn) and none have self-discharged to this level except for these 2 . That’s why I was wondering if they had recently switched suppliers.
When I have some time today I’ll see if this 18650 charges in a light with a built-in charger. I’ll attach a USB meter to the charging cable and if it and the light’s charging indicator come on then I will assume that the light is attempting to charge it.
5 seconds should tell me if it does or not.
I inserted the battery into a Sofirn S11c for 30 seconds and it was more than happy to charge it. I then tried an 18650 that I’m recycling and the flashlight will also charge it even though it was at 0.4 volts.
I put the second battery, which was then at 1.6 volts, into a Skilhunt H300 with magnetic charging, and it started charging it.
Based on what I see built-in chargers don’t care if the battery is at a low state, no matter what that is. So if someone always uses the built-in charger there’s no way to catch batteries with high self-discharge. So conceivably it would charge a battery even if it was at 0 volts.
Then again it’s impossible with external chargers unless the battery has been unused for some time and one knows what the voltage was when it was last used. I only knew because the 2 Sofirn’s were in storage and dropped from their storage voltage to an unsafe level in only a few months.
Interesting the Skilhunt (which is supposed to be a more premium brand) also charged overdischarged cells.
Are the stock cells with these lights protected, ie. Would require charging from “0v” to reactivate a tripped circuit?
Neither of these cells has protection circuits. The one that was at 0.4 volts only got there because I shorted it across a resister in preparation for recycling it.
I don’t know of course what charging circuits flashlights use but it wouldn’t surprise me if they all use the same one, or at most a limited number of them.
Inbuilt charging behaviour something that’s often overlooked on reviews. Makes me wary of gifting li-ion cells as I don’t trust recipients to use external cell chargers!
I never use the in-built chargers on any of my lights that have them. I do test them after I get a light with one just to see how many amps and volts are being pushed through it (I wonder if my USB voltage tester provides accurate info?) and of course what voltage the battery is at after charging terminates.
I see the chargers as something for emergencies rather than something to be used on a regular basis.
This is very interesting. I suppose if somebody leaves a light on inadvertently or it gets turned on and it’s in a drawer or wherever and it goes down to low voltage cutoff, and then it sits for a week or 6 months, you would want it to charge the battery again. So how low is too low from the onboard charging circuit point of view? I use onboard charging with any lights that have it but I always go through one of two different USB testers to watch what’s going on. And I’ve got a couple of old very dead batteries that I might do a little testing with.