What happens if the battery charge of one side of the headphones becomes empty before the other?

First, the voice guidance will alert you if the headphones battery is running low. The voice alert will come from the side of the headphones that are running low, and that side will turn off automatically. You can separately charge an earbud in the charging case, and keep using the other one if it still has power left. 

Attention:

• Stereo: 
  When you listen with only one side of the headphones, the left and right stereo channels are not combined, so you will only hear the sound associated with the active side.
   
• Google Assistant: 
  When setting up the Google Assistant feature, you can set the touch sensor on either the left or right side of the headphones to activate the Google Assistant feature. If the touch sensor assigned is the same side that is low on battery charge and in need of recharging, you will be unable to use the Google Assistant feature when using just the unassigned side of the headphones.