Charging method and charging circuit

The present invention provides a charging method, which is suitable for charging a first rechargeable battery and a second rechargeable battery connected in series with each other. The method first enables a charging circuit to detect the voltages of the rechargeable batteries. If the sum of voltages of the rechargeable batteries is less than a first default voltage, the charging circuit will generate a constant current pulse signal for charging the rechargeable batteries; if the sum of voltages of the rechargeable batteries is greater than the first default voltage, the charging circuit will generate a constant voltage pulse signal for charging the rechargeable batteries until the sum of voltages of the rechargeable batteries is greater than a second default voltage; then, the charging circuit will generate the constant voltage pulse signal to sequentially charge the first and second rechargeable batteries until the voltages of the first and second rechargeable batteries are greater than a third default volt