Could I add a second battery in parallel in the boot? I just want to increase the total ah of the system to be able to runt the FBH more often. Of course with the appropriate fuses on the plus side.

There are lots of 'how to do it' on here. do a search using the key word 'second battery'. I would also look in the wiki (top right of this page).

It's worth doing if you have cold temps but make sure it its done properly or it can lead to another drain on your main battery.

Good luck!

Thanks!!