hello , I am using a HT Racing starter, made by Tiawan Hobby Corporation. It has 2, 7.2volt batteries, two motors coupled through a gearbox, and starts my 90 size engines with no problem what so ever. I fly 7days a week, with an average of 6 flights a day, and i only have to charge it once every 3 months. The batteries are encased in the body of the starter, and it is surprisingly light for the power it puts out.