You can check the battery status of your keyboard using the Logitech SetPoint mouse and keyboard software.
NOTE: If you don't already have SetPoint installed, you can download it. See answer 360023359873 for instructions.
To check the battery status of your keyboard:
Launch the Logitech SetPoint mouse and keyboard software.
Start > Programs > Logitech > Mouse and Keyboard > Mouse and Keyboard Settings
Click the My All-in-One tab at the top of the SetPoint Settings window.
Select your keyboard from the product drop-down menu at the top left.
Click the battery icon on the left toolbar to display the Keyboard Batteries screen.
NOTE: You can click the “?” icon for descriptions of the settings.
Click Apply to save your settings and then OK to exit SetPoint.