Running script through launchd with no annoying prompts
Convert to Shell Script and AppleScript Pair

Convert your check for the battery status to a shell script. The shell script can then run regularly through a launchd job ticket without presenting any user interface.

When the conditions are met, your shell script can launch an AppleScript to interact with the user.

This assumes you use an ~/Library/LaunchAgent approach for the launchd job ticket. A computer wide launchd job will not be able to launch the AppleScript component in the appropriate user session.

Alternatively, you could use a third party tool to trigger on a battery event, such as Power Manager; this would avoid polling your battery and improve your battery's performance.