I found some huge batteries at a Tractor Supply Company. TSC is a large retail chain of stores that provide a lot of stuff for rural life. I don't live in a rural area, but I went to the store. Nice place, but I don't like the country music.
The huge batteries in question are some marine deep cycle batteries with about 185CA. I found one online with 160, it seems.
These batteries are $46.99 each. Could I run a couple of these in my trunk for an audio system? I'm just kind of interested in experimenting.