The BMW 330e actually has three batteries: a conventional lead-acid battery in the boot, just like any other 3 Series which starts the car and powers most of its electrical systems, the lithium-ion drive battery that powers the car when driving in EV mode, and the auxiliary lead-acid battery under the bonnet, which is designed to power the hybrid system itself and the switching between petrol and EV modes. The hybrid battery is charged either by plugging in the vehicle or by regeneration during braking, while the other two are charged via the alternator when the vehicle is in motion.