Is 4.0V on a lithium battery really 80% charge?

[Bort (BortyMcBortface)]

@jon_slider

@Bort

storage at over 80% charge ages batteries much more quickly. I want to charge to 80% to get most of the capcity but keep my batteries in good health longer.

storage is for storage

before using a battery that has been in storage, it should be brought up to a full charge, because the storage voltage is close to just 50% capacity

I have not explained well, i meant for normal everyday use i charge to 4V.
I tend to not use my high power lights much so they can be at 80% for weeks or rarely months for the rarely used lights.
My most used light is the Moon RC2 which has built in charging, it charges at 1A and has a 2.2Ah battery so i charge it for 1.5H from low voltage warning giving me about 75%ish.

so, for USING batteries, they need to be fully charged

otherwise, if you use batteries that have been at storage voltage, you increase the risk of overdischarge, And you dont get “normal” runtime of a fully charged cell.

This is incorrect.
Risk of overdischarge is mitigated by low voltage warning on most modern flashlights. Charging to less than 100% just gives reduced runtime. But storage at over 80% accelerates capacity fade.
I am using almost 10 year old 18650s that have over 95% of their original capacity.