The Game Boy Advance CleanJuice will turn the built in GBAs LED to red when you have about 5% battery left (versions sold prior to August 2020 require the Low Voltage add-on board to do this).
For the Game Boy Original the CleanJuice will turn the built in GB LED to a dimmer red when you have about 5% battery left (versions sold prior to October 2020 do not do this).
Source: RetroSix Knowledge Base