400w DC 12v to AC 110v power inverter is the battery charger for me. Now I am used to charge camera battery, shaver and laptop with this charger when camping in the countryside. The method about how to use 400w power inverter is easy to acquire. Simply connect it with car cigarette lighter charger outlet. If you are interested in knowing more details about this charger, you can search bestekmall on Google. Hope this point is useful to you. Best wishes.