Precharge Circuit

The pre-charge circuit monitors the high-voltage (HV) system, ensuring the pre-charge and discharge processes (like turning the car on/off) happen at safe, controlled rates, preventing rapid current spikes that could damage the tractive system (TS). The pre-chargers job is to measure the tractive system voltage and accumulator voltages to ensure that the precharge is adequate before closing the accumalator isolation relays (AIRs). The TS voltage should be within 95% of the accumulator voltage when the AIRs close to ensure there isn't a insane amount of inrush current which can damage the tractive system.

This is the example pre-charger that we are currently using, based on the example by Michael Ruppe (Link: https://michaelruppe.com/2020/10/09/a-plug-n-play-precharger-fsae-electric/). The linked resource and video is very useful with lots of information about the design process and a detailed breakdown of how it functions. 

The pre-charge process

When pre-charging occurs, the main contactor is open and the pre-charge contactor gets closed, meaning that all the current must flow through the pre-charge resistor, limiting the inrush current. This means that the TS voltage is bought up to the accumulator voltage at a safe rate. Once the pre-charge is complete and the TS voltage is at 95% of the accumulator voltage the main contactor can be closed and the pre-charge contactor can be opened. This is the normal operation state where the pre-charge resistor is bypassed, meaning that now the current has a low resistance path to flow and allows for large ammounts of current to be drawn from the accumulator. During this process, the pre-charger will constantly be monitoring the charge proccess, ensuring that it stays within a designed bounds. Too slow can still result in a large in rush current and too fast would mean that TS isn't connected or there isn't a pre-charge resistor. Due to the TS, the main passive property of the TS system is the capacitance at start up. Due to the size of the capacitors for the inverter, it acts like a short circuit pulling lots of current. The pre-charge resistor will limit the rate of this charge up and we will be able to see a nice RC curve. 

The above image is a example of the pre-charge sequence (NOTE: This is a random image, but the concept remains the same). As the voltage increases, the current will fall, where the main peak current is at the start where the voltage rises the fastest. 

Without the pre-charger the only thing limiting the current flow to charge the capacitors is the internal resistance of the TS wires and the internal resistance of the capacitors, however this is always too small of a value, resulting in a larger peak in the initial current draw.

During this process, the pre-charger monitors the accumulator voltage, before and after the pre-charge resistor. 

Timer-based pre-charge systems are extremely effective but unable to diagnose and respond to wiring faults, which can still result in faults. A possible example is if the precharge resistor isn't connected to TS. This would mean after the time delay eg. 3 seconds, there will still be a huge inrush current when the main contactor closes. This is one of the negatives with a timing only based system. It operates blindly and assumes that all the system are functional, however this isn't always the case. An active system will always be better than a passive system, however adding complexity.