Originally Posted by BiteTheBullet

I'm using a 12V/40A SPST, you could use a SPST or SPDT as long as you use the normally open contact

A relay is a simple electromechanical device, there is no bjt, Darlington pairs, or some kink of high amp transistors in there.

To save the "life" of the relay contact I use a capacitor of 40V/3000uF in parallel to the coil, this way you avoid the short-cycles that would eventually kill the contact .