Battery heating system, electric vehicle, and in-vehicle system

A battery heating system includes: a temperature monitoring circuit configured to output a temperature monitoring signal; a voltage conversion circuit configured to receive a first voltage that is input by a power supply or a second voltage that is input by a to-be-heated battery; and a control circuit configured to receive the temperature monitoring signal and output a control signal. The voltage conversion circuit is configured to perform boosting processing or bucking processing on the first voltage based on the control signal or perform boosting processing or bucking processing on the second voltage based on the control signal to enable the to-be-heated battery to receive a charging current from the power supply in a first time segment using the voltage conversion circuit and the to-be-heated battery to output a discharging current to the power supply in a second time segment using the voltage conversion circuit.