How do you determine your battery has 50% before it shuts down?

Debian has a program in the battery-stats package that logs battery at a (by default) 30 second interval. That has a pretty graphing program too. I dunno if Arch packages that, but if not, not hard to roll your own.

#!/bin/sh

while true; do
    echo $(date; cat /sys/class/power_supply/BAT0/energy_now) >> ~/battery-log
    sync
    sleep 10
done

That'll get you a capacity within ten seconds of the next shutdown. If you save that log, repeat it running until it dies a couple times, you can probably tell if it's consistently at the same capacity that it goes down.

Here a picture of the screen I get before it shuts down.

I am using upower to determine the battery status. It happens once every day or so. It posts the messages in the tty as it shuts down. systemctl schedules the shutdown slightly ahead of time.