The PWM motor controllers common in EVs have a sizable bank of capacitors on their input. When you apply a Voltage across a capacitor it initially appears to be a short-circuit, that is, the Voltage across the capacitor is zero. If there is very little resistance in the circuit, e.g. a closing contactor with no precharge, then the current will be very high. Nearly all of the traction pack voltage will be across the closing contacts. The large Voltage difference and sudden high current (known as an inrush current) can cause damage to, and in extreme cases, welding of the relay contacts. Also of concern to some is the stress on the controllers electrical components caused by the inrush current.
![Image]()
{see Contactor with no precharge.}
This can all be prevented by the use of a precharge resistor across the contacts of the main power relay. The precharge resistor allows the capacitors in the controller to slowly charge BEFORE the contactor closes. This means that there is less voltage across the closing contacts and little or no inrush current.
![Image]()
{see Contactor with precharge}
The problem with having a precharge resistor across the contactor is, there is high Voltage on the controller terminals even when the car is turned off. This is because the capacitors remain charged all of the time.
I've heard it argued that keeping the caps charged all of the time keeps them 'fully formed' and thus, extends their life. While this is technically true, it is not really an issue with modern capacitors. Unless you plan on putting your controller in storage for years, the capacitors will likely outlast their associated active components (transistors and diodes) whether you keep them fully formed or not.
Many DIY'ers add some sort of power switch, circuit breaker or disconnect to remove the high Voltage from the controller when the car is parked.
![Image]()
{see WithPowerSwitch}
This solves the 'high Voltage on the controller' problem BUT introduces a new wrinkle. You must now turn things on in the correct order or you will defeat the purpose of the precharge resistor.
For example, if you first turn on the contactor and then close the power switch there will be no precharge. You will have reintroduced the high Voltage/large inrush current problem.
In this case, you must first close the power switch, wait an appropriate precharge delay period, then close the contactor.
If a precharge switch is added in series with the precharge resistor it can be used to turn the high Voltage on without switching a large current flow, as is done with the contactor or power switch.
![Image]()
{see WithPrechargeSwitch}
In this configuration the power switch becomes an emergency disconnect that is normally left on. The precharge switch is turned on first and then, after a delay, the contactor closes.
This is different than the previous design because now the "on switch" (the precharge switch) can be a relatively small relay and the turn-on sequence can be easily automated to avoid closing the contactor before precharge.
Here is how I did it. I have a Step-Start device that turns on the precharge relay when the start signal is received (the ignition key is turned to the START position). After a time delay the contactor is turned on.
{see StepStart}
![Image]()
There are additional safety and convenience features of the Step-Start Device, but the basic function is to make sure that the precharge relay is always turned on BEFORE the contactor and that at least some minimum amount of time passes between the two events.
This can all be prevented by the use of a precharge resistor across the contacts of the main power relay. The precharge resistor allows the capacitors in the controller to slowly charge BEFORE the contactor closes. This means that there is less voltage across the closing contacts and little or no inrush current.
The problem with having a precharge resistor across the contactor is, there is high Voltage on the controller terminals even when the car is turned off. This is because the capacitors remain charged all of the time.
I've heard it argued that keeping the caps charged all of the time keeps them 'fully formed' and thus, extends their life. While this is technically true, it is not really an issue with modern capacitors. Unless you plan on putting your controller in storage for years, the capacitors will likely outlast their associated active components (transistors and diodes) whether you keep them fully formed or not.
Many DIY'ers add some sort of power switch, circuit breaker or disconnect to remove the high Voltage from the controller when the car is parked.
This solves the 'high Voltage on the controller' problem BUT introduces a new wrinkle. You must now turn things on in the correct order or you will defeat the purpose of the precharge resistor.
For example, if you first turn on the contactor and then close the power switch there will be no precharge. You will have reintroduced the high Voltage/large inrush current problem.
In this case, you must first close the power switch, wait an appropriate precharge delay period, then close the contactor.
If a precharge switch is added in series with the precharge resistor it can be used to turn the high Voltage on without switching a large current flow, as is done with the contactor or power switch.
In this configuration the power switch becomes an emergency disconnect that is normally left on. The precharge switch is turned on first and then, after a delay, the contactor closes.
This is different than the previous design because now the "on switch" (the precharge switch) can be a relatively small relay and the turn-on sequence can be easily automated to avoid closing the contactor before precharge.
Here is how I did it. I have a Step-Start device that turns on the precharge relay when the start signal is received (the ignition key is turned to the START position). After a time delay the contactor is turned on.
{see StepStart}
There are additional safety and convenience features of the Step-Start Device, but the basic function is to make sure that the precharge relay is always turned on BEFORE the contactor and that at least some minimum amount of time passes between the two events.
