why the last 1% of battery seems to drag on way longer than normal 1%
In: Technology
Because the percentage is an estimation based on previous performance not actually measured. (Counting how much current left since the last recharge, and comparing that to how much current could be drawn in the past)
This estimation is biased “conservatively”, wich basically means your device would rather underpromise than overpromise how much longer it will last.
A battery has a range of “charge” where it’s usable. It depends on how much electricity the battery can store. There’s a voltage on the battery that’s considered the absolute maximum where it’s holding 100% of the energy it can store safely, and another voltage where it’s holding 0% of the energy it can store. Your phone can tell where the voltage is on that range.
Because the voltage on a battery doesn’t range from 0 to 100 and instead ranges from about 3.7 to 4.2, you have to do a bit of math to convert that into a percentage that people can understand. Batteries also degrade over time. A 3 year old battery can’t store as much electricity as a brand new battery, but it still has to map that change in voltage to a percentage.
To do this, your phone guesses a little bit. It knows that when it was brand new, the change in voltage between 0 and 100 was at a certain level. It knows that it might take 12 hours for your battery to go from 4.2 volts to 3.7 volts. But as the battery ages, it might take 6 hours to go from 4.2-3.7 volts instead. It can make an estimate based on where the battery is in that voltage range to get a good idea of how much “percentage” is left.
But sometimes it gets that estimate wrong. If your phone is new, it might still need to learn exactly how quickly the battery goes through that voltage range. So it might hover around 100% for a while and then drop to 50%, then hover around 50% for a while until it drops to 20% and so on. Sometimes, your phone gets the guess wrong and it shows 0% for a while before the battery drops below the point where it can no longer safely provide more power to your phone.
Each manufacturer builds its own Power Management Integrated Circuit(PMIC), back in the day the charge counters would be less reliable so phones had them show less than the battery had.
Nowadays the counters in PMICs are more reliable, but for emergency situations they make it pack 5-10% into 1% and severely limit the phone’s performance.
With a custom ROM and root as I have on my phones, I can disable this and get the actual battery life of 1%, which is just like the other percentages.
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