An alternative to a graphical power manager is to create a simple daemon. To do that, create /usr/local/sbin/shutdown-if-low-battery with executable permissions containing this: 

interval=120
critical_level=10
while true
do
    if test "`acpi -a | grep -o off`" = "off" ; then
        battery_level=`acpi -b | sed 's/.*[dg], //g;s/\%,.*//g'`
        test $battery_level -le $critical_level && poweroff
    fi
    sleep $interval
done

Then create /etc/init.d/shutdown-if-low-battery with executable permissions containing this: 

### BEGIN INIT INFO
# Provides:          shutdown-if-low-battery
# Required-Start:    $all
# Required-Stop:     $all
# Default-Start:     2 3 4 5
# Default-Stop:      0 1 6
# Short-Description: Forces shutdown if battery is below 10%
# Description:       Forces shutdown if battery is below 10%
### END INIT INFO
NAME=shutdown-if-low-battery
DAEMON=/home/nick/.local/bin/$NAME
PIDFILE=/var/run/$NAME.pid
SCRIPTNAME=/etc/init.d/$NAME
case "$1" in
  start)
    start-stop-daemon --background --start --quiet --pidfile $PIDFILE --make-pidfile --exec $DAEMON --test > /dev/null || return 1
    start-stop-daemon --background --start --quiet --pidfile $PIDFILE --make-pidfile --exec $DAEMON || return 2
    ;;
  stop)
    start-stop-daemon --stop --quiet --retry=TERM/30/KILL/5 --pidfile $PIDFILE
    RETVAL="$?"
    [ "$RETVAL" = 2 ] && return 2
    start-stop-daemon --stop --quiet --oknodo --retry=0/30/KILL/5 --pidfile $PIDFILE
    [ "$?" = 2 ] && return 2
    # Many daemons don't delete their pidfiles when they exit.
    rm -f $PIDFILE
    return "$RETVAL"
    ;;
esac

Then add it to the default runlevel with the command:  update-rc.d shutdown-if-low-battery defaults