Electric vehicle pre charging circuit

When the power battery supplies power to the motor inverter circuit, there is a capacitor C1 in the process from the DC bus to the inverter circuit.

As shown in Figure C1, when the controller controls the main positive relay to close its connected contactor, high-voltage DC is introduced from this circuit to C1. Due to the capacitance characteristics, at the moment of charging, it is still 0V. Therefore, the circuit current that returns to the negative pole of the battery pack after passing through the capacitance is infinite, which is prone to short circuits. Therefore, a pre charging circuit is needed.

Before the formal discharge, the pre charging relay is opened through the controller. At this time, the corresponding contactor will pull in, causing high-voltage DC to pass through this circuit through the pre charging resistor. Assuming the pre charging resistor is 100 ohms and the battery pack voltage is 600V, the maximum charging current at the moment of power on is 6A. Assuming C1 is 400 microf, the charging time constant RC=0.4S. When the time reaches RC, the capacitance voltage at this time is 0.63 times the battery pack voltage, After about 3 RC cycles, the end of charging will be reached, and as the capacitor voltage increases, the current decreases until the end of charging, with a current of 0.

Similarly, when the charged capacitor reaches the RC time constant during discharge, the capacitor voltage is 0.27 times the battery pack voltage. The discharge starting current decreases with the increase of the pre charge resistance, and the time constant also increases with the increase of the resistance. Conversely, the discharge time constant also increases with the increase of the capacitance value.