I change batteries for a living everyday.
I've tried many different ways of backing up settings.
1. Use a small 9V battery maintainer plugged into the dc socket - Many cars does not connect that socket when the ignition is off, so you have to turn it on. Some cars have optitron display which can't be turned off, and that drain the 9V battery flat, once you pull the terminal off the main battery.
2. Clip a spare battery at the terminals
- You're sure to knock it off while working the battery or the clips slide off and you lose everything.
3. Hook up a small 12V 7AH SLA battery at the dc socket
- Turn ignition to on, before changing battery. This will fix no. 1 problem, but make sure to fuse it not more than 15A.
4. Certain cars like Merc or BMW has separate jump points. You can clip a small jump starter (lithium-ion type) on these points and this will work too.