You can check Linux battery status by just reading from a certain file somewhere under /proc; being Linux, I'm certain there's an equivalent buried somewhere in /sys as well. You just read the file, it gives you back ASCII text, you're all set.
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.
