Presumably related: I was out shooting in rather damp conditions last weekend—mizzle rather than downpours—when my trusty 1D Mk III 'died'. The focus point indicators became feint through the viewfinder, it powered down after a few more shots, and wouldn't boot up again. Fearing that moisture had got to it, I stuck it in a bag with a desiccant pack overnight. Worryingly it still wouldn't boot up, meaning no more snapping for the rest of my trip.
Back at home a few days later, I've tried to work out what's what. I looked to perform a 'hard reset', removing battery and backup battery as documented elsewhere, but still nothing. I then plugged the camera into the AC adapter and it booted up just fine. I put the battery back in and again it powered up. I set the time and date again, thinking that was that and it had sorted itself out. Battery health was showing at 90% if I recall correctly, and has the final (third) led flashing when plugged into the charger.
The following day though, and I'm back to the camera not booting up from the battery, but just fine using AC adapter. Clutching at straws I've replaced the backup CR2025 (still nothing) and am currently calibrating the battery in the hope that helps. I've also tried booting from a 3rd party battery of dubious quality to no avail, so questions: Does the AC adapter use different circuitry to the battery inside the camera to power things up? Does anyone have any thoughts on what next to try?
It's at that age and value where sending away for repair isn't cost-effective but I'm reluctant to give up on it just yet. Until this incident it's been doing me proud.