Many booster units sold in the car market would work in aviation too. The type that boost, or recharge, the vehicle battery are low current and simply transfer enough charge into the main battery to get a start. For instance, the Black & Decker one is claimed to transfer 7A for 15 mins, = a bit less than 2AH. This is sufficient to start the engine if the current is 200A and cranking time less than 30 secs. Of course the 200A only flows from the aircraft's battery, not the booster unit.