I posted about this problem a few years back. Now its happening with 2 of my 3 c100s. I am hoping maybe someone else has had this issue and managed to resolve it.

The internal battery seems to have died. After every battery change, the camera boots up and asks for you to enter the date and time etc. All other settings have reset to default.

I have followed the manual instructions to plug the cameras into the mains for 24 hours which made no difference. My batteries and chargers are all Canon originals. I have tried more than one battery \ charger to charge the batteries.

I live in Brazil where its tricky to send it to a approved Canon dealer. Assuming it does need an internal battery replacement I would prefer to attempt it myself. My cameras are out of warranty.

Thanks.

The internal battery on my C300 mk 1 died a few months ago. That battery is not user-accessible (unlike on the C300 mk 2 where it is). I couldn't find any guidance online on how to replace the battery and Canon didn't want to give me that info (I told them, truthfully, that I used to build and service equipment for recording studios, but that didn't help). So I had to send the camera to Canon's USA service center.... On the upside, the USA service center took care of the battery replacement really quickly and at a super-reasonable cost, all for a camera like yours that is out of warranty.

Perhaps your local/regional division of Canon can refer you to an authorized local repair facility; not necessarily run by Canon, but a third party that Canon approves of?

Jim