Mercedes sold a kit at one time, a cigarette type jack that mounted in the trunk liner and a jack with a transformer that plugged into the wall. A couple of amps should be plenty unless you have a bad battery. West Marine has a 2 amp battery maintainer, it comes with three cables, clips, lugs (wire to your battery), and lighter jack (wont work unless ignition is on) $49.99. You can unplug the charger portion and leave the cable attached to the battery.