Display the remaining battery charge, as a percentage of maximum charge
Remember if you can't run your code on a phone, calculator, etc, you can probably install your environment on a laptop at least. If you absolutely can't run on a device that has a battery, then unfortunately your language isn't eligible since you have no useful value to print.
You must display the results on the screen. This is theoretically supposed to be a handy utility for the user to check the battery. Writing battery percent to a file is too clunky, requiring the user to then check the file. Displaying on-screen is therefore a part of the specification that can't be overridden by default IO. That said, you can assume nothing is preventing things like STDOUT from being displayed on screen in the usual way.
If the battery percent is already displayed on screen all the time by the OS, like on many phones, that does not mean you've got a zero-byte solution. You must assume that built-in display doesn't exist, and display the battery percent again somewhere else.
Edit: The output format is being opened up. You are free to display as little as just the digits of the percent, or the digits with some extra output as well.
Edit2: The digits representing the percent must appear in a known location within the output, not more than 50 lines from the end of the output, and that location shouldn't change between invocations. The digits representing the percent must be separated from other output by at least one non-digit character on each side.
For reference here is the old output format from before the change:
Remaining battery is ddd%
Remaining battery is ddd percent