Dual-battery setup with a smaller battery?
I've done the searching and reading on dual-battery setups, I think I understand the need for an isolator and a few of the various options for setup. Here's something I didn't see.
I've got a M1-ATX, so I don't have issue surviving crank or any of that. I want a second battery simply to handle the S3 powerstate overnight or for a few days. I don't need to play my PC with the engine off for hours on end, I just want a battery to handle the small drain required of the S3 (suspend to ram) state.
Does this have to be a huge battery? I'm hoping I can use something smaller (but still sealed as it'll be in the car). Can someone help me with a schematic? I don't really care if my carpc runs off the main battery or secondary battery while the engine is running (or key in accessory) but I'd like it to run off of the secondary when the key is off. I think I this means I'll switch the input of the M1-ATX from the primary electrical system to my small backup battery when I take the key out.
Has anyone done this? Is this more trouble than it's worth?