Hm, do you need separate set of properties for that? You could register either
of existing two, and read function will not allow read of anything but "present".
IMHO, this is what other modules do (/drivers/power)
One remaining trick here, you need to call unregister/register for power_supply
if you change attributes -- so please check if your patched driver survives
insertion of the battery.
Regards,
Alex.
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