Been looking at various "mini start batteries" on eBay. They appear a great idea, small yet claim to start a large engine. Plenty on eBay "Emergency Battery Power: The product is designed to jump start all 12-16V battery systems on vehicles with up to 4.0L gasoline engines and on vehicles with up to 2.0T diesel engines. The unit can provide 600A peak current and can start up to 5-10 times on a single charge." Are they any good? Will they actually start an engine by themselves, or just provide a slight "helping hand" to an existing weak main battery?