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Precharge, what is it, why do I need it, how do I do it.

138286 Views 59 Replies 29 Participants Last post by  Rayco
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.
{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.
{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.
{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.
{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}

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.


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Fair enough. Could someone please explain in detail how you would wire the pre-charge resistor? is it just another power cable going to the controller with a resistor connected in-line?

The manual for your controller will have a wiring diagram. And you can go to controller company websites and download manuals which will show you. And you can use the search feature on this web site to find threads and posts about it. And this thread, a sticky, I think talks about it.

But I do think some basic diagrams should be here in the EV Information section.


I'm trying to find a relay for the precharge circuit that can "block" the pack voltage (144) but work (close) off of the 12V system. I was hoping to just use an automotive relay for this but from what I can find these are only rating up to about 50 VDC or so. I don't want to have to pay to get another ev200 just to switch 5 amps at 144volts. What is everyone else using for parts?
Hi Cap,

I don't have a part for you, but a suggestion. Put a contactor like the EV200 in the negative (or mid pack) battery line turned on by the keyswitch. Then when it is turned on, it precharges thru the resistor across the contacts of the positive line contactor. After a few seconds, or when a precharge indicator lamp goes out, have a switch (like on the drive position of the gearshifter) which closes the positive contactor. That way the precharge is off when the keyswitch is off. And the high voltage accessories (DC/DC) can come on ahead of the precharge.

Different controllers can require different precharge circuits. There is no one-size-fits-all diagram.

@rfengineers. Is it right to connect pre-charge relay to "start position"? According to this design when driver press throttle pedal with maximum capacity instantly inrush current would appear and it may damage the load. Should pre-charge relay work for once only starting(1)? Or should it work depend to throttle pedal everytime(2) against to inrush current. If case (2) is true pre-charge relay shouldn't wire to "start position". There is a high voltage circuit in the link below. In this circuit pre-charge relay works depends to "run position". Please would you enlighten me about pre-charge relay function? Best regards...
The pre-charge circuit should just operate when the system is first turned on, not every time the throttle is depressed. The motor controller stays on during periods when the throttle is not depressed during the drive cycle and therefore stays charged. It is only when the key is turned off that the motor controller will bleed down the capacitor voltage and then need to be re-pre-charged when restarted with the keyswitch.

The pre-charge function is to limit inrush on initial energizing of the motor controller. The controller itself will limit current in the normal operation of controlling the motor including rapid depression of the throttle pedal.

Number (1) is correct method.

Here is a good paper on it:
Does controller's fully capacitors discharge over itself after turned off? I thought Big load accumulate in capacitors after switch off. May it damage controller's circuit when it discharges?
There is no surge when a capacitor is disconnected however it may remain charged for a long period, perhaps even after the device has been disconnected. So, such devices as controllers which have large capacitors are designed with bleed down resistors to discharge those internal capacitors in a reasonable period, usually a minute or so. A few watts of power is sacrificed during operation for safety's sake.

Note: When working with such devices it is prudent to check the terminal voltage before applying tools. Most equipment is properly designed with bleed down but there is no law for it in the DIY parts market.
So I guess that the output of this formula would be the resistance and from there you could calculate the power dissipation at full load. But then the next question arises: the time to pre-charge is quite short. If you get 1000W maximum input current, do your really need a 1000w resistor or would it be overkill ? (as it won't be a continuous current).
This may help.
Davide, only one problem, the current does not flow into the positive terminal of the relay, the Electrons do. but current flows the other direction (Especially for those of us educated in "Classic Electric Engineering" I could treat you to the 30 minute lecture on current flow but.....and the fee is $50 and you won't have to feel ignorant again. 8^)
I assume you refer to Davide's quote here:
In that case, the contacts of the contactors are polarized (one terminal is labeled '+'); connect the contactor so that normally (that is, while discharging) the current flows into the '+' terminal.
He is correct and consistent with EE convention with regards to current flow. You should follow his advice if you wish for your contactor to work properly.

You can teach how you wish, but I, and I suspect Davide, do not feel ignorant about it ;)

For easy reference see:
Major, and Davide, The discrepancy comes from engineers speaking to technicians.
As Engineers tend to use, Conventional Current Flow (From Positive to Negative).
Alternatively most Technicians use Electron Current Flow (From Negative to Positive) thus the simple statement, "the current flows into the terminal marked (+)" is inadequate to describe the direction of current flow. The relay is not a source, it is marked similarly to a "Load" and the (+) of the pack is connected to the (+) of the contactor.
Davide stated it correctly. Why do you choose to insult him (and me) and confuse the issue for the reader?
The Curtis AC controllers handle the precharge internally meaning there is no need for an external precharge resistor.
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