I never used this but read it somewhere so tell us if it works:
There does not seem to be any keyboard shortcut for the battery meter. However, Windows 7 has a super-useful Windows Mobility Center dialog that is enabled on laptops. It displays the battery meter and a few other statuses.
The keyboard shortcut to bring up this dialog is Win+X. Press Esc or Win+X to dismiss the dialog once you have noted your battery level.