I ordered this step up convertor from china to charge my laptop from my camper battery http://www.banggood.com/DCDC-Adjustable-Digital-Converter-Step-Up-Boost-Module-USB-335V-p-931237.html and added a case with place for a standard 10A switch as well. I use it for charging my latop from my camper battey, and it seems like the cheapest solution and most energy efficient, with a switch for the display that switches between current, volt input and output. The case is made of two attached parts, so that one part is used as a station . including a bit of support