I have two batteries on my W520 ThinkPad. The main battery 55++ and the 27++ slice battery. Both are 9 cells and have a combined designed capacity of 93.24Wh.
The discharge pattern between the batteries while they're both hooked up to the W520 is that the 27++ will first discharge...all the way down to 0%, then and only then will the 55++ start to discharge.
Is there a way to configure the system to switch over and use the 55++ battery once the 27++ drops down to a particular level, say, 25%. ...without manually disconnecting the 27++?
The slice battery is protected from total discharge.The system will switch over to the internal battery when the microprocessor on the slice battery signals that it is at 0%. The microprocessor will protect the battery from further discharge when that power state is reached.
Note that the 0% here does not refer to the cell discharge level, but the 0% of power that can be safely drawn from the battery. All Li-on batteries from Lenovo have a built-in microprocessor that protects the cells.
There are no settings in the current version of Power Manager to modify this switchover point.