Hi. To all you battery gurus out there…. 
What is the best way to store Li-Ion cells? I read somewhere that it’s not good to store them fully charged.
Is that true?
Yes it is.
For maximum life Li-ion cells should be stored at half capacity, around 3.7 resting voltage.
battery university explains it well, but the jist of it is that above 40% charge the cells die of age more quickly.
Li ion wears out due to temperature (the higher the faster they die), charge level they are stored at, and usage (drain/charge cycles).
Ideally you would store at 40, charge exactly how much you will need before you use it, use enough to bring it down to 40 and when your done store it at the 40% till the next time you need it.
This is not practical, and since they are not very expensive most people just use them like a normal battery, charge when needed, use as required and replace when they don’t hold much juice anymore.
Many thanks…. I just knew I could count on you guys 
Oh wow, I store mine fully charged and top them up regulary in case I need to use em, glad I stumbled across this thread, thanks.
i used to as well, i aim for 60-80% and fully charge before extended use
So I need a multimetre to do this accurately right? My i4 has no indication that’s accurate enough to nail about 50%.
not really, just use some mental math, for example your batteries are 2600mah and your light draws 2800ma, so you will get about 1 hour of illumination (liner driver, output will decrease as voltage falls giving you more illumination at lower brightness).
So 6 minutes of use equals about 10%
Lets say 5 minutes since its easy to work with, you charge your light, and happen to use it for 10 minutes, your down to 80%, perfectly good for storing.
If your not going to use it right away don’t worry, a few days won’t affect longevity very much, before a trip i often fully charge the battery.
I like to recharge at around 40-50% charge, so that if i need illumination for a while i’m not running around with an almost dead battery, so after about 20-30 minutes of accumulated use (keep a mental tally), recharge the battery (no need to be exact, and i often recharge a bit above my targets, gives me more wiggle room in the game that is life).
If your light draws more or less amps then adjust this formula accordingly.
Thanks for explaining that Bort, I use NCR B’s in my SRK generally
i reread my post, here is a rewrite of part of it
If you charge it up to 100% and don’t use it right away don’t worry, a few days at full charge won’t affect longevity very much, before a trip i often fully charge the battery.
also
Don’t overthink the how far have i used it so i need to recharge it soon part, it will become second nature pretty quickly
i’m fit in your illustration merely want the battery is available for use anytime at the most of its capacity. so I treat the storage like regular battery eventhough good cell is not cheap compared average income in here.