Hello fellow FL enthusiasts! I have got Convoy S2+ few months ago along with the two Liito Kala Lii-35A 18650 cells (which supposed to be 3500mAh cells). I have charged the cells and put one in the flashlight to use just as EDC. The cell was mostly sitting in the unit unused for few months, however yesterday I put it in use after a while since some juice was still present (around 50% according S2+ battery check) and it was working till the end. But when I took the cell out to get it charged, charger did not accept it at all ( LiitoKala Engineer Lii-500). I have measured the cell voltage and it was on 300mV (!) … My impression was that S2+ with the biscotti firmware has low voltage protection thus such incident should not happen on unprotected cell I was using, or am I missing something? Anyways, I have tried the simple trick with the other fully charged cell to connect it parallel to the drained one to see if the voltage rises and it did but it took some time to get it even close to 1V and I was bit afraid not to damage them both even more when keep it in the setup longer so just passed it and decided to ask professionals here for opinion.
I only have a limited expertise with the newer types. A close friend has one working in his M2 and he's happy, and I have a few other builds planned.
Did you verified that your driver signals battery voltage as it should? With a good voltmeter or a multimeter in V scale, that is.
In any case, what you say is pretty weird. Connected to phosphor converted white leds whose minimum Vf is very close to 2.5V, drivers of that class can hardly overdischarge a battery. This is pure physics. I myself did a related test once, directly connecting batteries to a white led with wires and letting it go down all the way it could. After more than 6 days of discharge the no load voltage in the cells still crept well above 2.5V.
In my opinion, and fundamentally because a cell which is properly set with another one in parallel should climb in voltage quite fast, that cell you speak of is in bad shape. My advice is, discard it in some battery recycling container or whatever is available where you live. It is recommended to discard batteries which have dwelled below 2V, namely if for an undetermined amount of time and/or much below that.
Thanks for suggestions. The firmware used in my S2+ is 7135*6 for SST20 chip. So it seems like the cell issue, hope the other remaining won’t end up the same. What would be the best 18650 replacement to be used in S2+ with good price/performance ratio?
EDIT: No lighted switch. this is the one I have ordered: “S2“:https://www.banggood.com/Gray-Convoy-S2-SST20-LED-Flashlight-18650-Flashlight-Camping-Light-Hunting-Emergency-Lantern-p-1420655.html
Convoy S2+ with SST20 LED uses Biscotti driver (at least based on my experience, it uses the original Biscotti and not the “Biscotti-clone” that comes with other Convoy flashlights that use the “12-group” driver (with SST40 or XHP70.2 LEDs).
Per my experience of S2+ with Biscotti, the flashlight will indeed shut off the light at low voltage. (The 5/3-mode group driver, used on many S2+ XM-L2, doesn’t seem to shut off the battery but will do a low-voltage-warning of blinking the LED, from what I recall).
Can you take a picture of your Lii-35A 18650 battery? Show the label, and the top/bottom.
Looks like an unprotected cell to me, so it needs replacement.
Good cells are created by a myriad of manufacturers over the world, with some good, lesser known ones being in China. The five legged top of your cell above is likely manufactured by BAK Power Battery.
Since no idea of where you live, take a look here if you aim to know where to buy:
Still, I successfully buy batteries via AliExpress often. I've said it more than once, it's how you believe. If you strictly believe right you get favourable results. If you don't you can get mixed results or straight negative ones. So know yourself and choose properly.
Does a 35A 3500 mAh hour 18650 actually exist? That claim alone sounds very questionable unless I missed something new. All of the high drain batteries I know about have lower capacity.
—edit— Nevermind, I see that’s just the model number and not a claim for discharge current.
35A 3500mAh? False advertising of course, cells with low enough internal resistance to deliver their contained energy at such discharge rate without overheating are yet to be manufactured. Maybe the Samsung 20S could pass at 35A. “Tomorrow or the day after”, just not now.
In fact I am fairly sure these cells wouldn't even meet their capacity claim in an unbiased, trustworthy 3rd party battery review. I understand this situation is changing, but still both these facts (lies) are to be there until people awakens enough, stops believing in being cheated and starts believing in trustworthiness all around.