Thermal measurements
Because this type of flashlight does not have an easily replaceable battery, and temperature is the most important factor for battery health (excepting misuse), I measured battery temperatures during operation.
The light is an S11 from aliexpress that seems to be the same as the one Flail has added to his original post update. I added a bit of thermal paste between the brass heatsink and the PCB that holds the main LED, because there was only a tiny amount (see Flail’s pictures).
The thermocouple was placed inside the plastic shell of the fully assembled light, next to the middle of the battery and lightly touching it. The actual battery temperature next to the thermocouple could have been higher due to poor thermal contact. The battery temperature will not be uniform, so this measurement likely does not represent the hottest part of the battery (which is the part that matters). It also does not represent the hottest part of the light (not at all) which will be at different locations depending on the mode you are running.
The battery seems to be glued to the PCB with some type of foam, this is good and should help protect the battery from the hot PCB.
Front LED, medium setting (highest continuous setting without stepdown), 90% battery charge
The measurement was stopped after 25 minutes because the temperature didn’t increase any further.
Highest measured temperature was 7 °C above ambient. This is absolutely fine.
Front LED, momentary high (highest possible output), 100% battery charge
The measurement was stopped after 200 s. Temperature was still increasing at this point, but not very quickly. I doubt anybody would use momentary for much longer.
Highest measured temperature was 20°C above ambient. This is high, but acceptable and to be expected with such a high performance in a tiny package.
The stainless steel bezel was painfully hot after this run, presumably much hotter than the battery. This indicates that the heatsinking through the brass piece to the bezel works.
White side LED (“lantern mode”), 100% battery charge
The measurement was stopped after 30 minutes because the temperature didn’t increase any further.
Highest measured temperature was 11 °C above ambient. This is fine.
Red side LED, 70% battery charge
The measurement was stopped after 25 minutes because the temperature started decreasing (battery voltage dropping).
Highest measured temperature was 16 °C above ambient. This is higher than the white side LED and would have been even higher with a full battery. If you are worried about battery health, don’t run this mode continuously on a full battery, especially in a hot environment.
UV side LED, 100% battery charge
The measurement was stopped after 8 minutes because I didn’t want to cook my battery. Temperature was still rising at this point.
Highest measured temperature was 28 °C above ambient. If I had not turned the light off, Temperature at the measurement location would probably have reached 33 °C above ambient and stayed there. This is not ok and could be dangerous. Don’t run this mode continuously, even if you think it’s a nice party trick. It will ruin your battery, unless you place the light in a very cold environment. Running the UV for 2 minutes is ok.
If a manufacturer puts in a side LED with this much power, there must be a time-controlled stepdown or a temperature regulation.
Charging from ~ 50% state of charge
Flail’s measurements show that charging in this light is unhealthily fast, so I measured battery temperature during charging. Charging from empty would have been a more useful measurement (probably with a bit higher temperature), but I didn’t have enough time for that.
Highest battery temperature during charging from 50% was 13 °C above ambient. This is ok.