I agree it is possible to gauge when a light is running low on batteries, as described above.

also
the Klarus Mi1C has a battery check mode, that could be mapped for future reference

for example
on my Utorch S1 Mini, modded w sw45, the batterty check is very conservative…

4 flashes is 4+ volts on my meter
3 flashes is 3.97 volts on my meter
2 flashes is 3.87 volts on my meter
1 flash is 3.77 volts on my meter

so using an unprotected cell it is still possible to know when the battery is down to 50%, which is a good time to recharge… IF I remember to use the battery check mode (2 clicks from off)