In your /sys directory, you can get the status and capacity (percentage of power left) of your device’s battery.
For the status:
$ cat /sys/class/power_supply/BAT0/status
For the capacity (percentage of power left in the battery):
$ cat /sys/class/power_supply/BAT0/capacity
This gives you a number.
You might have more than one battery on your device. You can find the list by doing this:
$ ls -l /sys/class/power_supply/ | grep -i bat
FYI the output is of symlinks.