This dual battery relay from Rugged Ridge fits 70-83 Jeep CJ5, 70-75 CJ6, 76-86 CJ7, 81-86 CJ8, and 87-15 Wrangler.
Don’t kill your extra battery. The Rugged Ridge Dual Battery Relay is a simple cost effective way to keep your additional battery charged when not in use. Use both simultaneously, or one at a time as you keep the other charged. The kit includes relay, switch, and wires.