Is it a matter of how strong the current is coming out of the battery? Is less power in the battery meaning “free space” in the battery and that’s what is measured?
In: Technology
It’s hard. With modern rechargeables the voltage depletes with lower charge, so like a fully charged battery is 4.2v. But it’s not really proportional – a 50% charged 4.2v battery isn’t 2.1v. And to make matters worse, the voltage at 75% is really close to the voltage at 25% – it’s kind of a ‘hockey stick’ curve where from 100% to 75% will drop the battery from 4.2v to 3.7v, but then it stays at 3.6-3.7v until it’s below 25%, getting down to maybe 3v at 0%*
So if you’re using your device a lot and often partially discharging it so it’s between 25% and 75%, you can’t rely on voltage measurement to tell you directly how much the battery has left. Instead you have to measure how much energy you put into the battery and how much energy you take out of it. This works pretty well but if you do it for a really long time, any errors can add up.
That’s why a lot of electronics have an automatic battery calibration routine – when you fully charge it, it knows that the battery is at 100% (4.2v) and then resets the estimate of how much you charged/discharged. You don’t need to fully charge it for the battery to be healthy, you just need to fully charge it so the device knows how much energy is in the battery.
*Technically fully discharging a lithium ion battery to 0v can be done, but doing so will damage the battery permanently. So electronics will cut off the battery below 2.5-3v depending on how aggressive the designers were.
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