Reading through the battery issue threads, I’ve seen a few arguments about whether the early shutdown problem is due to a hardware or a software problem, so I figured it might be useful to add a bit of information about how the battery system works.
In short, the phones are probably shutting down early because the performance of the battery doesn’t match the calibration of the fuel gauge software.
The idle (no-load) voltage of a li-ion cell ranges between about 4.2V when fully charged and about 3.3V when discharged. These values vary a bit from one phone model to another, but not by much. If that was all there is to it, it would be very easy to have a reliable battery gauge, but there’s another factor:
All batteries have a certain internal resistance, and the internal resistance dictates how the battery responds when you draw power from it. When you draw current from a battery, the output voltage will sag, and it’ll recover when you stop drawing current. So the phone may show a 75% charge level and the battery may be sitting at 4.0V. You start the camera app and because the camera is relatively power-hungry, the voltage drops to 3.8V. You close the camera app and the voltage recovers to 4.0V. The higher the internal resistance, the larger the voltage drop.
The internal resistance and the resulting voltage drop is a potential problem because these phones need a certain minimum voltage to function. Even if there’s still plenty of juice inside the battery, if the momentary voltage drop is too severe, the voltage will drop below the minimum threshold and the phone will shut off.
There are several factors that have an influence on the internal resistance:
State of charge. The internal resistance goes up when the cell is discharged.
Health of the battery. Old cells have a higher internal resistance than new ones.
Temperature. The colder it is, the higher the internal resistance.
This is also why some Samsung phones won’t let you use the camera when the battery is at 5% or lower. There’s still juice in the battery, but power-hungry loads like the camera would cause an excessive voltage drop at this state of charge, and the phone would shut off prematurely.
When calculating the percentage, the fuel gauge system makes certain assumptions about the internal resistance and how the battery will perform under load, and if the predictions don’t match the reality, the phone will shut down even though the fuel gauge may still be showing 25%. It expects a small voltage drop, but the battery can’t cope with the current draw, the output voltage dips below the minimum operating voltage of the device and it shuts off.
If you’re out of warranty, a new battery would probably cure the problem since it’s going to have a lower resistance. If you want to mitigate the problem a bit, keep the phone warm and turn down the brightness of the screen.