A lithium battery designed to replace lead-acid battery. The lithium battery has a plurality of battery cells connected in series and a battery management unit. The battery management unit includes a controller, a sensing unit connected to the plurality of battery cells and the controller, a charging control unit connected to the controller, and a discharging control unit connected to the controller. The battery management unit prevents the batteries connected in parallel from mutually charging each other and also prevents the batteries being depleted completely by outputting a low voltage in pulse mode when the battery has low charge.