Dual battery systems have come a long way. I've used a middle of the road setup that cost 6-700 dollars in my Subaru, an expensive set up that cost close to $1,500 and now I'll show you one of the cheaper options that cost around $400.
I put this house battery setup in my buddies Chevy Express van, 'cause it gets used as a part time camper van on occasion and he wanted extra power to charge electronics and run a 12v.
The core of the system is a simple VSR ( Voltage Sensing Relay ) or smart isolator. This device automatically links your main battery and your auxiliary battery so that both are charged by the vehicles alternator when the engine is running. When you shut the engine off the relay disconnects and your starting battery is safe from being drained by your accessories. It's simple and effective and very useful for a camper, van or overland build.
*Shopping list:
Smart Isolator: https://amzn.to/3ROmUdc
Circuit Breaker x2: https://amzn.to/3xezUiU
Fuse Block: https://amzn.to/3IeICnD
USB Outlets: https://amzn.to/3YJ47SG
My favorite brand wire for accessories (NOT BATTERY CABLES NOT FOR ISOLATOR): https://amzn.to/3xfGsxA
Fork Connectors (Nice for Fuse Block Connections): https://amzn.to/3G9LZIF
Awesome HeatShrink Tubing: https://amzn.to/3dg43nL
For Your battery cables you will want good heavy gauge wire. Probably no smaller than 4ga. I used 2ga welding cable that I had sitting in the garage.
Jumper cables can be a more affordable way to buy heavy gauge wire.
I used these ones for the fuse block connection to the auxiliary battery: https://amzn.to/3YJcqhl
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