I've recently purchased a 5D at the end of the spring rebate period. I'd been noticing that my battery level indicator is not functioning correctly. It falsely reports that the battery is dead. I have seven or so batteries and 2 different bodies that I cross check batteries in and it's conclusive. Just yesterday though I noticed a new problem, which is that my 5D loses multiple internal settings as soon as it goes to sleep mode or I power it off. Logically it appears this would be a bad "internal" (system) battery. The settings are retained as long as the camera is on.
Makes me wonder if the battery level indicator could be trying to comment on the status of the internal battery somehow or if possibly the battery indicator is continually running in test mode or something, which could draw resources from the internal battery and cause it to fail prematurely. I don't even know at this point if that internal battery is easily serviceable in the 5D or if it's something that's embedded far enough that it will cause warranty complications if I change it myself. I'm sure I'll know that part tonight when I check into it further. Just wondering if anyone else has experienced this and if so, what was your solution/conclusion.