Oops.
Second one terminated at 4.2 fine, testing my third now. One report on reddit of terminating at 3.9(?!?!).
1 Thank
Does it use the same battery charger IC as the other lume drivers? (BQ25616)
I don’t think so because X4 offers reverse charging as a new feature.
That one doesn’t really work though, only with OTG + A-to-C cables somehow? I have no idea how that even is possible, OTG cables do nothing differently from a C-to-C cable with a client connected (current sink on CC lines with 5.1 kOhm).
Update: It seems to be specific to the battery FFL shipped with it (apparently made by FEB, although the listing says Samsung 50S). Tested with a 50S in the same light and it terminated at 4.20V. the same battery as has the problem in that light is fine in my two other X4s, and those two are also fine with other batteries, so just this one specific light and battery combination.
2 Thanks
I still would not trust the built in charger of that light. If it overcharges im some rare conditions, I’d expect it to do that with any battery at random. Just a matter of time until it happens again.
My suggestion: return that light to FFL, tell them what happened, and ask for a replacement. I’m pretty sure they will be happy to get that light back so they can try to figure out the issue in their labs and make sure whether it is a single defective unit or a design flaw that might make other lights out there go boom.
1 Thank
Yeah, I don’t really care about built in charging that much overall, I rarely use it in most of my lights, but want to make sure other people are aware. Loneoceans already mentioned interest in doing a failure analysis on it so I might go with that, I guess.
3 Thanks
Every time I look at Firefly for a new light, I see something to put me off, goddamn.
how does this FEB behave in known-good chargers? If this FEB is the Vapcell F60 I have never had any problems with it…
1 Thank
Use a DMM. Reviews for that battery tester say that it is wildly inaccurate.
2 Thanks
I’m a touch confused. Is this a battery issue, charger issue, or both?
If it is a battery issue, can we test anyway other then inside the light?
1 Thank
Stellar X4 cut off a Vapcell T50 at 4.187 measured with a Fluke. No issues here with the light itself so far.
3 Thanks
I have to test my good batteries with X4.
I always use external charger but not rarely I use power bank because it’s practical when you don’t have power outlet. So this is a bad news from FFL
Dumb Q, but I’ve observed batteries that were discharged down to 1V or so (TS22 with switch indicator did not trigger LVP). After removal, after some time, there was some kind of voltage rebound and the voltage went up slightly.
Is there any way this circuit might actually be cutting of properly, but this battery has some weird & similar kind of rebound effect due to it’s chemistry that pushes the voltage up a little over time regardless? Like applying the breaks a little too late…
I’m not a chemist, just wondering … sorry if this is silly. I have the F60, which may be the FEB, and have not seen this issue in my Vapcell S4.
Charger. A LiXx charger must not, under any circumstances, output more than 4.2V. No matter what the battery may do wrong to trigger it, it still is a charger issue.
My impression so far is more that it is a defective unit than a design flaw though. Time will tell.
Li batteries tend to relax a bit, after both charging and discharging. Charge a battery to 4.2V, leave it idle for an hour and it will be at <4.2V. Discharge it to 3.0V, leave it for an hour, and it will be >3.0V.
The “rebound” as you call it always opposes the last change, it never “amplifies” it.
1 Thank
Correct. There are non-Li-ion chemistries that have a delayed chemical reaction that causes the voltage to rise slightly after charging, but that doesn’t happen with Li-ion batteries.
This is a charger issue and FFL blaming the battery just a lame excuse.
1 Thank
The first flashlight with rosier LEDS but white light… i choose for 4000K
219B seems to be green 
in my FW3A and they are so beautiful…
My Red Vapcell 21700 battery is 3.6V and charge it with Apple 20W USBC (first pic) and charge volt is terrible with Witrn C5.
I try Power-Z with the same charger and result is not better but powerZ seems to be a bit more professional (2nd and 3rd pics).
Witrn and PowerZ are very good products.
So I chooses another charger and cable with Baseus USBC 60W and is a little better but not the best.
At the end of charge I measure battery Volt with Fluke 179 and I see 4.195 V so with this battery, built-in charge is good.
I can’t measure body temperature in charging time but the X4 is worm so I suspect 2A are the limit for charge.
Probably the best option would be 1.8A with linear 5V in my opinion. I have to test with other power charger.
Cheers
Fine in my VC8S. Tested another F60 I have in the defective light and that was fine. Possibly something about this particular batch of batteries in combination with some specific drivers.
1 Thank