You charge it up from the mains and then it will store the power and will start the car several times from full charge. Mine has lights to show how much charge is left. It seems to hold the charge for ages. I have started the car with it, left it in the boot and started it again months later without recharging it and it was fine. As I said, I haven't used it to charge the car battery, just to start the car, so I don't know if it needs to be connected to the mains to do that.