Make sure it's stranded.
I used it for some things under the hood and inside. I'm too cheap to buy battey cables. For a 6' #4 I think was $15 or more. I ran scrap #2 from 1 battery to a lug on the starter then back to the 2nd battery. It's a bit stiff. We used welding cable on trunk mount baterys befor. That works a bit better because it flexes more.
Depending on the car some came with some poor quality wire. I want to say that some conversion vans and RV's are done with THWN.
Does not hurt to upsize the wire. I use a lot of #12 just because thats what I have. The manufacture tends to size wires on the smaller side for cost & weight.
Tom
