Pre-Charge and Discharge

Definition

Pre-Charge: Occurs prior to closing main contactor between battery and motor controller, prevents arcing since the motor controller can act as a capacitor, and the battery has no limit for how much instantaneous power it can put out. 

Discharge: Occurs after opening main contactor, allows the energy within the motor controller to be discharged safely to ground. This prevents the motor controller from storing energy after the vehicle is powered off. 

Implementation: 

Pre-Charge: 

• Implemented with a separate relay and a 75Ohm Power resistor. By opening the pre-charge relay first, small amounts of current (~1.6A) is allowed to flow between the motor controller and the battery pack. This allows the voltage differential between the two to slowly reduce to 0. Only when the voltage difference across this resistor is 0, will the main contactor open. 

Discharge: 

• Uses another relay connected between the Motor controller and ground. There is a 750 Ohm resistor between the two. This relay should close whenever the main contactor is open, allowing any residual energy to safely be dissipated through the resistor.