#!/bin/bash

# Warn when we have about 15 mins charge left, then every 5 mins
# 5 mins ~= 0.1 Wh

c5=100000
first_warn=$((${c5} * 3))

next_warn=${first_warn}

dir=/sys/class/power_supply/BAT0

snd=/usr/local/share/sounds/BEEPKIND.WAV

# We may have been invoked in response to an ACPI event e.g.
# "mains off"; it seems that the transitions are not atomic for
# example between "mains on; battery charging" and "mains off;
# battery discharging" we pass briefly through "mains off;
# battery charging".  So we'll sleep a bit in the hope that this
# resolves itself.
sleep 2

while true
do
    if [ ! -d ${dir} ]
    then
        # The battery isn't present and presumably we're running on mains
        # power.  Since we have nothing to do, terminate.  When the
        # battery is inserted the ACPI event will cause us to be restarted.
        exit 0
    fi

    case `cat ${dir}/status` in
    Discharging)
        ;;

    *)
        # If the battery is not discharging it's probably charging or full,
        # i.e. the mains is present.  In this case we won't bother to monitor
        # it; when the mains is removed we'll be restarted by the ACPI event.
        exit 0
        ;;
    esac

    # We're running on battery power.  How much charge do we have?
    c=`cat ${dir}/charge_now`

    # If we're reached a warning level, make a warning sound and step to
    # the next warning level.
    if [ $c -lt ${next_warn} ]
    then
        play ${snd}
        next_warn=$((next_warn - ${c5}))
    fi

    # If we've got lots of charge in the battery, we can sleep
    # for a long time before checking again.  The following
    # is a guess of how long it would take us to reach the
    # warning level at a current drain of 3.6 A, which is much
    # more than we ever actually take.
    # (This doesn't allow for replacing a good battery with a
    # bad one.)
    sleep $(( ( $c - ${next_warn} ) / 1000 ))

done