In this paper, an automotive onboard heater equalizer is proposed to heat low-temperature batteries and balance cell voltages without the requirement of external power supplies. The proposed integrated topology only needs one MOSFET for one cell, resulting in a compact size and low cost, which can be easily applied to electric vehicles. In particular, all MOSFETs are driven by one high-frequency pulsewidth modulation signal and the batteries can be heated internally by the ohmic and electrochemical losses and warmed externally by the switching and conduction losses of MOSFETs, leading to a high heating speed and efficiency. Further, a thermoelectric model for the internal and external combined heating is developed to provide guidance for the optimized design of the proposed heater. In addition, the proposed topology can realize passive balancing of series-connected battery strings at a higher switching frequency and a smaller duty cycle. Experimental results show that the proposed heater, by generating a periodic ramped discharge current with an rms value of 1.8 C at a switching frequency of 150 kHz, can heat the lithium-ion batteries from −20 to 0 °C within 1.9 min, consuming about 5% of the cell energy.