Remove the loop and sleep from the script you created so it just runs and exits.

Then create a file at /etc/systemd/system/battery-alarm.service with the following:

[Unit]
Description="Sound alarm when battery is low"

[Service]
ExecStart=/usr/local/bin/battery-alarm.sh # point this to your script

Then create a file at /etc/systemd/system/battery-alarm.timer with the following:

[Unit]
Description="Run battery-alarm.service every 2 mins"

[Timer]
OnUnitActiveSec=2m
Unit=battery-alarm.service

[Install]
WantedBy=multi-user.target

Then sudo systemctl enable --now helloworld.timer to start and enable the timer on boot.

This will be a little more robust then your current script. It works without the user needing to log in. And there is nothing to get killed so will always trigger. The current script will just silently stop working if it ever gets killed or crashes.