Hm interesting one of the new FM3616CE based ones (with SS34 diodes in my case versus my previous with SS24) does have the blue led become more and more dim after the battery reached 2,5V. After that with a multimeter I saw it going down to about 2,38V when I disconnected it. By now the cell has recovered to 3,72V (still rising, this was just minutes ago…).
To say something positive, the FM3616CE might actually try to keep the cell at 2,5V, by cutting-off at 2,5V - but with a bad implementation that already begins to discharge again at 2,50…01 V so you could say, disregarding production variance and temperature dependence that maybe means something between 2,45 or 2,55V for that barrier…
Anyway, this would mean, each time the battery goes above 2,5V, it will again be discharged for at least one pulse of the FM3616CE’s internal boost converter, grinding down the battery to a certain voltage, until it won’t even recover to 2,5V anymore! Then it will probably really stop to discharge… but that might maybe be too late for the cell, I know electronics but I am not a battery specialist. Maybe someone else who knows more about batteries can even say that this is perfectly fine? But I am suspecting it is not, as the current will be quite high, for a 5V 1A discharge, this will already mean a cell discharge current of at least 2A, not counting the conversion losses.
To those who don’t know what I mean with “pulse”: The FM3616CE is very probably doing a DC/DC buck conversion for charging and a boost conversion for discharging, these kind of converters have a certain operation frequency in which they operate, for more details, see [1] below - basic electronic understanding is still required though.
In the end:
Maybe (just maybe) this is also just a lack of circuitry and the FM3616CE itself is actually quite good: Some resistors, diodes, capacitors, whatever might be missing…. When looking at the CHD-XS V1.1 circuit board, there are still multiple unpopulated parts and one 0 Ohm resistor. Maybe it would actually work if all/the correct parts would be populated……
Additional Information: I did only check for discharge cut-off doing all this, not for overcurrent protection so far!
Maybe I am experimenting more at another time. This is already over the limit of my time making sense to me (expect if I would buy 100+ and rework them, which I won’t be doing…), at least at the moment it is still fun as a short pastime
[1] DC-to-DC converter - Wikipedia
EDIT:
Ok I also verified that it is not overcurrent protected at all :-/