Simple script to show battery info from UPS HAT for rapsberry pi, and shutdonw graceful the rasberry when battery is low level.

battery.sh

#!/bin/bash
#
#   This program is free software: you can redistribute it and/or modify
#   it under the terms of the GNU General Public License as published by
#   the Free Software Foundation, either version 3 of the License, or
#   (at your option) any later version.
#
#   This program is distributed in the hope that it will be useful,
#   but WITHOUT ANY WARRANTY; without even the implied warranty of
#   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#   GNU General Public License for more details.
#
#   You should have received a copy of the GNU General Public License
#   along with this program.  If not, see <http://www.gnu.org/licenses/>.
#
#   Copyright 2021 Daniele Basile <asterix24@gmail.com>
#
# Note: to use this script you should before enable the i2c subsystem of
# raspberry, by the rasp-config.Other than that you should install the i2c utils script package.
# Here you could find more info: https://raspberrypiwiki.com/images/6/6f/How-to-use-via-i2c.pdf.

# You can add this scritp to crontab so you could check if the power supply fall down.
# Here is a little example, we run scritp every 2min, remember to do this with root user.
# # m h  dom mon dow   command
# */2 * * * * /usr/local/bin/battery.sh

I2CGET=/usr/sbin/i2cget
DATA=$(${I2CGET} -f -y 1 0x36 2 w)
RAW=$(printf "%d\n" $(echo "${DATA}" | sed -E "s/0x([a-f0-9]{2})([a-f0-9]{2})/0x\2\1/"))
VOLT=$(echo "scale=2; ${RAW}*78.125/1000000" | bc -l)

DATA=$(${I2CGET} -f -y 1 0x36 4 w)
RAW=$(printf "%d\n" $(echo "${DATA}" | sed -E "s/0x([a-f0-9]{2})([a-f0-9]{2})/0x\2\1/"))
LEVEL=$(echo "scale=2; ${RAW}/256" | bc -l)

if [ x"$1" == "-h" ]; then
        echo "usage $0 [-p show battery level %] [-v show battery voltage V]"
fi

[ x"$1" == x"-p" ] && echo "${LEVEL}%"
[ x"$1" == x"-v" ] && echo "${VOLT}V"

if [ $# -lt 1 ]; then
        echo "Battery Voltage: ${VOLT}V"
        echo "Battery Capacity: ${LEVEL}%"
fi

# if battery level goes under 25%, shutdown the system gracefull, to avoid
# filesystem damages.
if [ $(echo "${LEVEL} < 25" | bc) == 1 ]; then
        echo "low battery level shutdown.."
        /usr/sbin/shutdown -h now
fi