It is normal for the camera to pull some charge from the battery while it is powered off. That helps the camera retain your settings. The amount that it pulls from the battery to save your settings is small.
If your battery is losing most or all of its charge in one night that is a sign there is an issue with the camera or the battery. We would recommend loading a different battery in the camera to see if the same thing happens. If the issue continues we would recommend sending the camera in for repair.
Switch the camera to P mode on the top dial. Go into the menu system and reset the camera back to factory defaults. Before you do that, check to see if Wi-Fi is enabled. Always keep it disabled if you are not using the camera.
Finally, make sure the power switch is turned to OFF. When the camera goes to sleep, it is not really turned off.
12/05/2023: New firmware updates are available.
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.