The bq27200 fuel gauge depends on its battery-backed memory for capacity
reporting. If the supply is off for some time, the fuel gauge's last known
capacity is reset, after which a capacity value can only be read after a
full charge cycle. Thus, most of the time, as used on the Nokia N900, the
reported percentage cannot be relied on for making power-off decisions on
battery low conditions, e.g. through UPower.
However the fuel gauge can very accurately detect and report a low battery
condition internally, even if the reported capacity is incorrect, e.g. when
it has been reset. The low battery condition is reported by the kernel via
capacity_level in sysfs, and also generates uevents.
So we can implement safe battery shutdown by requesting the kernel to more
regularly polling for status changes, then performing an immediate shutdown
when the capacity_level is reported as Low and the device is not charging.
Sicelo A. Mhlongo
ea97c08fa6