Comment 9 for bug 38795

Comment from the Upstream developer:
  battery.charge_level.rate = 0 (0x0) (int)
  battery.reporting.rate = 0 (0x0) (int)
  present rate: unknown

if we don't know the rate at which the battery is discharging then we can't
form an estimate about how much time is remaining.

do other operating systems know how long is left? is this an acpi
implementation bug? maybe this is a kernel bug.