The present invention discloses an overvoltage protection (OVP) circuit for use in a charger circuit system, comprising: a power transistor electrically connected between a voltage supply and a battery; an OVP circuit which turns off the transistor when a voltage supply exceeds a threshold value; and a multiplexing circuit electrically connected between an output of the OVP circuit and the gate of the transistor. The present invention also discloses a charger circuit with an OVP function, comprising: a single power transistor electrically connected between a voltage supply and a battery; an OVP control circuit which turns off the power transistor when a voltage supply exceeds a threshold value; and a charger control circuit which controls the gate of the power transistor to determine a charge current to the battery when the voltage supply does not reach the threshold value.