It's not necessarily installed by default, though. If your Linux system can read the battery with acpi, it will show up in /sys. Although the 'acpi' program is

It's not necessarily installed by default, though. If your Linux system can read the battery with acpi, it will show up in /sys. Although the 'acpi' program is convenient, it adds another dependency. Also, if you run 'acpi' under strace, you'll see that it just opens up /sys/class/power_supply and parses files under that.