Backup battery installation

Hey, guys!

I want to install a backup battery in a vehicle so that I may use one battery to run the radios and other electronics all the time and not drain the primary battery.

I’ve looked through the forums but haven’t found any information on how to achieve this.

I’d appreciate it if someone could explain how I can hook this up.

I also read some tutorials like the batteries tutorial, but I still don’t know what to do.