dual batteries

I'm assuming you have a trunk mounted battery. When I was racing in the 80's I had the same issue. Instead of using a second battery, I bought a size 4DLT bus battery & installed it in the spare tire compartment, where it fit almost like it was meant to go there.

Where the best Exide orbital Marine deep cycle has a reserve capacity of 100 minutes @ 25A & weighs 41lbs, the 4DLT has a reserve capacityof 250 minutes @ 25A & weighs 87lbs. Plus it makes nice ballast for hard launches.

Pricewise 1 4DLT = 1 Exide Orbital.

Ther are even bigger batteries but weight quickly becomes an issue.

BTW I never succeeded in running that big battery down.