Battery charging. Like, trickle charging? Not jump-starting.
Wire in a cigarette lighter connected through a fuse directly from the battery, not through the master switch. Mount it where it's convenient to power your handheld radio or usb converter or whatever else you'd normally plug into your cigarette lighter. Yeah, I know, cigarette lighters are so old school, but some stuff still plugs into them. Like the plug from a battery charger.