An apparatus for jump starting a vehicle is disclosed. The apparatus comprises: a first and a second ignition clamp, a sampling circuit configured to sample polarity output signal from the vehicle battery when each of the first and the second ignition clamps is connected to a respective one of the first and the second terminals of the vehicle battery, a polarity determining circuit configured to determine, based on the sampled polarity output signal, a polarity of the first and the second terminal of the vehicle battery, and a clamp polarity switching circuit configured to connect the first and the second ignition clamps to a portable power supply, and enable, in response to the determined respective polarities of the first and the second terminals of the vehicle battery, a polarity on the first and on the second ignition clamps to be the same as that of the corresponding terminal connected thereto.