batt.sh

#!/bin/bash

# file: /usr/local/share/batt.sh

# Checks to see if the battery is below 
# the low battery threshold,
# if so, then put the computer to sleep

SECONDS_BETWEEN_CHECK=60
LOW_BATTERY_THRESHOLD=6
WARNING_THRESHOLD=10

HYSTERESIS_BETWEEN_SLEEP=$(($SECONDS_BETWEEN_CHECK*5))
UPOWER_BATTRY_FILEPATH="/org/freedesktop/UPower/devices/battery_BAT0"

battery_percentage_as_digits() {
	# Get the battery percentage as a whole number from 0-100
	upower_text="$1"
	echo "$upower_text" | grep -oe  'percentage: *[0-9]\+' | awk '{ print $2}'

}

get_upower_text() {
	# Long text, use quotes to preserve newlines for grep
	upower -i $UPOWER_BATTRY_FILEPATH 
}
 
get_battery_state() {
	# Battery state in human readable format
	# charging, pending-charge, fully-charged, discharging
	upower_text="$1"
	echo "$upower_text" | grep state | awk '{print $2}'
}


check_if_discharging() {
	# echo 1 if discharging
	# echo 0 is charged or indeterminate state
	upower_text="$1"
	battery_state=$(get_battery_state "$upower_text")
	if [[ "$battery_state" == @(charging|pending-charge|fully-charged) ]];
	then
		echo 0
	else
		echo 1
	fi
}

check_if_needs_to_suspend() {
	battery_level="$1"
	is_discharging="$2"
	is_below_threshold=$(( $battery_level < $LOW_BATTERY_THRESHOLD ))
	echo $(( $is_below_threshold && $is_discharging ))
}

main() {
	while true; do
		upower_text=$(get_upower_text)
		battery_level=$(battery_percentage_as_digits "$upower_text")
		is_discharging=$(check_if_discharging "$upower_text")
		needs_to_suspend=$(check_if_needs_to_suspend $battery_level $is_discharging)
		if (( $needs_to_suspend )); then
			echo needs to suspend at $(date)
			systemctl suspend
			sleep $HYSTERESIS_BETWEEN_SLEEP

		elif (( $battery_level < $WARNING_THRESHOLD && $is_discharging )); then
			echo warning at $(date)
			notify-send -u critical "Battery is at ${battery_level}%; Will suspend soon if not put on charge"
		fi

		sleep $SECONDS_BETWEEN_CHECK
	done
}

if [[ "${BASH_SOURCE[0]}" == "${0}" ]]; then
	trap "notify-send 'Low battery Script Failure'; sleep 1; exit" SIGINT SIGHUP SIGKILL
	echo Starting
	main
fi

instructions

Place files at corresponding locations (or copy by hand)

Make batt.sh executable: 

  chmod +x /usr/local/share/batt.sh
  
Allow service to be discoverable:

  systemctl --user daemon-reload
  
start with:

  systemctl --user start lowsleep.service
  
status with:

  systemctl --user status lowsleep.service
 
test by running direclty with bash

lowsleep.service

[Unit]
Description=Sleep on low battery
After=display-manager.service

# filename: /etc/xdg/systemd/user/lowsleep.service

[Service]
ExecStart=/usr/local/share/batt.sh
Restart=always

[Install]
WantedBy=default.target