Is there any way to charge an external deep-cycle 12v battery (not the starter battery) using one of the vehicle's 12v accessory ports and the alternator? I know an Arkpak does this, but I was hoping for a cheaper solution. The Arkpak has an inverter, etc., that I don't need -- I just need a basic charger. However, all the chargers I've found (e.g. https://www.amazon.com/gp/product/B00HWL4N08/ref=oh_aui_search_detailpage?ie=UTF8&psc=1) seem to be designed to plug into an accessory port to charge the main starter battery using 110v shore power, not to charge a separate battery via an accessory port while the car is running.