
It appears that third-party batteries cannot be used in the new Canon EOS R5 Mark II, with a YouTube reviewer declaring, "if you're a third-party battery fan, this is not for you" as he unsuccessfully tries to use non-Canon batteries in the camera.
As an ultra-high power camera – between its 8K 60p video, 45MP resolution and 40fps burst speed, Neural network Image Processing and AI-powered autofocus modes – the R5 Mark II makes use of a new battery, the Canon LP-E6P. This is required to use the camera's full suite of features.