There is always 0 mWh if I use /sys/class/power_state for my battery.

Probably, it has to be voltage_new or charge_now instead of energy_now in line #315 of lib/acpi.c.

$ ls /sys/class/power_supply/BAT0/
alarm               current_now   model_name     status      uevent
charge_full         cycle_count   power          subsystem   voltage_min_design
charge_full_design  device        present        technology  voltage_now
charge_now          manufacturer  serial_number  type

$ uname -a
Linux lokalhorst.lokal 2.6.35.6-48.fc14.x86_64 #1 SMP Fri Oct 22 15:36:08 UTC 2010 x86_64 x86_64 x86_64 GNU/Linux

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=547991