I've bought some rechargeable batteries for my Canon EOS 30, but I've noticed that their voltage when fully charged is 4.2 V, rather than the 3.5-3.7 V of a regular CR123A battery. Does anyone know if this might damage the camera?
I use rechargeable RCR123 batteries in my EOS 30V/ELAN 7NE film camera. I tried using one brand (Olight RCR123A) that had high voltage, and the camera would shut down and show an error code. Another brand I use (Soshine RCR123) also has high voltage, but the voltage is slightly lower, and the camera behaves normally.
I believe the camera is able to detect an over voltage and shuts down to protect itself.
I think you would be safe to try your rechargeable CR123A batteries in your EOS 30 based on my experience.
I have read that some rechargeable battery manufacturers use a diode inside the battery to slightly reduce the voltage, and that could be why some will work in my Canon film camera, while others don’t.