To make the change persistent, create a file named /etc/systemd/system/battery-charge-threshold.service with the following content:
[Unit]
Description=Set the battery charge threshold
After=multi-user.target
[Service]
Type=oneshot
ExecStart=/usr/bin/bash -c 'echo 80 > /sys/class/power_supply/BAT0/charge_control_end_threshold'