you should be more worried that your about to have no light because the battery is dead.
Think of it this way 3.6V dead, 4.2V is 100%
This assumes your measuring it outside the light, if you have the equipment to measure it while turned on at full power then refer to HKJs graph.
Bort;
I thought that most sophisticated lights and the protection circuits on protected batteries had a cutoff voltage of about 2.8 volts seeing as how the warning for maximum battery discharge voltage on Lithium Ion batteries is 2.5 volts. Certainly that is what I have read including in the battery tests posted by HKJ as I recall. See the voltage and capacity test graph comparing LG and Keepower batteries at the below link. Reproduced from a HKJ report.
http://www.mtnelectronics.com/opencart/index.php?route=product/product&path=59&product_id=191
Most lights have low voltage warnings in the 2.7-3V range, i use mostly 105C/Qlite drives so they cut off at 3.0V. However this is voltage under load (voltage sag while current is being drawn, while the light is on) which is what you see in HKJs tests. When the battery is removed from the light it has no load on it, so the voltage reads higher, 3.6V is considered almost 0% remaining for most batteries (if you pull the battery that hit low voltage warning and check it right away on a multimeter it may read between 3.0-3.6V because it takes some rest to recover the full 3.6V
3.6V to 3.0 drop? that’s too much for PF/high drain cells…
IMO 3.6v should drop to around 3.3V under load…for Fenix lights that is, they got very efficient design battery configuration/driver (low current from cells drained)
FWIW i runed my MM15 3.5v offline voltage, turbo without it stepping down to high, which is very good considering its 4.5A to leds on stock, and cells in series. If its drop to 3.0V i think its not possible to run any high power light at turbo anymore.
Depends on where you look: http://lygte-info.dk/info/BatteryChargePercent%20UK.html
Many batteries (3100/3400mAh) has 33% of the energy left at 3.6 volt.
This is getting far more complicated then it really is or needs to be
The easiest way to do this is buy the light and cell you want, run it to low voltage warning and check battery voltage on your multimeter. That will tell you what voltage its drained at, just keep in mind to do all your reading at the same amount of time after turning off the light (one reading straight out of the light as soon as the low hits vs running one to warning and testing 30 mins later will give different readings).
My advice is find a runtime graph for the light you want if out there, buy light, several 18650s and charger, check voltage at fully charged and fully drained then light up the night.
eek, i did not know that, i will leave further advising to you since you have done the testing and are far more knowledgeable
look at the blue line…0 drop, with cells which have huge vdrop……PF are like 0.30v better under load.
http://lygte-info.dk/review/batteries2012/Common18650comparator.php
And TK 75…
Pretty sure cells are around 3.5V when it drops to high, point being, the regulation is fantastic, and you shouldn’t lose almost any output until 3.6/3.5V (offline)