I am going to bring a yellowtop with me on a 1200 mile drive to Colorado. I want to keep it topped off while the car is running and when the car is off use that battery for our electric blanket while we sleep in the car and to keep the laptops powered.

I have a red top in the engine bay as the cars battery, I just don't want to use it with any of my accessories so it will happily start the truck and charge the yellow top.

I was thinking of just connecting the yellow top to the 20a cig outlet I installed in the cargo bay but It blew the fuse when I tested it.

Can anyone help me?

 

You need some bigger wire to do that. The stock wiring in the cig outlet will burn with that much power. Just get some 4 gauge wire and run your own wire from the positive terminal on your red top to your yellow top. Make sure to get a large fuse (I'd go with at least 100a) and put it as close to the red top as you can.

 

Get & install a dual battery isolator. That's the proper way to keep both batteries charged, vehicle from draining power from it & having the red top as the vehicles main battery. Have it wired out to run your accesories.

 

I figured the correct way would be via an isolator. I just wanted to use the existing run I have going to my cig because it is a 1 time use. (4ga from battery top post to distribution block fused with 30a fuse feeding 10ga dedicated run to cig outlet) I wish I could mount the 2nd battery via isolator and be done with it, I just can't find the space.

Worst case I bring my battery charger and set it to 2.5A while I'm driving to top the yellow off, only problem is the battery charger is like 10x10x10 inches of space that is quite precious in our rather small suv's housing 2 adults and a 65lb dog on a 2+ week trip.