80% a GOOD one, with no protection and no drain whatsoever, can be at 80% as long as 6months later or longer, over time though there is voltage depression when loaded from non-use.
protection curcuits can have a tiny draw, and AGED and or cheap cells start to self discharge faster, etc etc.
it would be hard to fully quantify a fact for all li-ion battery types ages and protection curcuit , and flashlight stuff, so it would be best if you do your regular maintance that you would do with anything.
(use it or lose it)
say trade it out with another one every 3 months, and use that one normally, and recharge it, then set it back into the stagnant application.
if the flashlight has "electronic switching" and does not do a Full batttery disconnect, then check it once a month untill you have a better idea of how soon the flashlight depleates a cell, or do a secondary disconnect, like put a insulator in, or seperate the battery, or unscrew the cap.
So many variables, its really going to be you who knows for the specific combination of items.
Example:
i have 3 fully disconnect car flashlights, and one of the lights i screwed up modding, and it has a tiny leakage :-( so it only stays charged for 2 months, the others dont have any leakage.
i have used good li-ions in 32*F and they acted weak, but did everything i wanted to do, sorry i did not do a chart, i just walk at night in as low as freezing and it works.
i have Deep freezed, operating li-ion lights at about -14*F with 18650 and the coin cell light, and they kept working in the deep freezer , i left them in untill they were at the temp of the freezer, after pulled out they got wet bad from condensation.
there have been reports here of freezing batteries with protection curcuits has caused a protection curcuit failure, so it should be noted that freezing isnt nessisarily a good thing, for a battery designed mostly for room temp operation. (its not like a superconductor running in liquid nitrogen
a flashlight left in the car, that reached freezing a few times (i am in california) worked everytime i needed/ tested it.
they say dont CHARGE in the cold.
you do know that Li-Ion charge state CAN be guessed mostly from the voltage it is at, at room temperature, without a load, rested, so any simple operational DMM (volte meter) can give you enough to know its general charge state. with that info your the tester
