Lithium-* batteries don’t actually have any specific useful numbers. It’s something like this (the actual numbers are pulled out of my ass and depends on battery time and test parameters and even then I’m simplifying):

• At 0 volts, the battery is dead.

• At 1 volts, the battery is practically dead.

• Discharging to 2 volts kills it after around 100 times.

• Discharging to 3 volts kills it after around 10 000 times

• Discharging to 3.5 volts kills it after 100 000 times

• Charging to 4 volts kills it after 100 000 times

• Charging to 4.2 volts kills it after 10 000 times

• Charging to 4.3 volt kills it after 1000 times

• Charging to 4.4 volts kills it after 100 times

• Charging to 4.5 has s significant chance of it catching fire

Now choose how many charge cycles you want it to survive, and you know which voltage to consider 0% and which to consider 100%. The bigger difference, the bigger capacity with the same battery.

This is why a phone with 0% battery can tell you that it’s out of battery.

You can also adjust what “killed” means. Is it when battery capacity is reduced to 80%? 50%?

I have to repeat - the numbers are not accurate, and this is strongly simplified.

It’s just an illustration of what 0% and 100% means it’s just where you are on the useful range, according to the manufacturers definition of useful.

Just to be clear: Do you want a way to save anything interesting that might happen, or do you want to save everything as automatically as possible?