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

138283 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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The "Pre-charge" is to charge up the capacitors in the controller input circuit to full pack voltage (144 V.?) At a controlled rate and not as an "Inrush" Surge. that is what the resistor or household 120 V. light bulb id there to do. It limits inrush or initial charge-up of the input capacitors as otherwise the initial surge will shorten the life of the power contactor it is connected across. To start charging with only the 12 volt is unnecessary, and potentially a problem because the 12 v. auxiliary supply,(Battery and/or dc/dc) is preferably ISOLATED from the high voltage of the 144 v. traction battery pack. If you have more questions ask them (I just LIVE to give answers.)
For your set-up I would expect a One thousand ohm at five watts (1.0 K Ohm 5 W.) resistor not a ten thousand ohm and it needs to supply a limited current to the power input connection on the motor controller for 10 seconds or so before the main power Relay provides full power to that controller connection so that the input filtering capacitors have a "Pre-Charge" (similar to "Inrush" limiting In other applications.) before a large surge charges the capacitors like a bolt of lightning...
Perhaps two 12 volt automotive light bulbs in series for 24 volts similar to side marker light bulbs (Less than one amp) and they would glow brightly as you apply the power thru them, and when they appear to have dimmed out bypass them for full operating power. That gives a visual indicator of the Pre-Charge taking place. Around here the truck stops sell replacement side marker assemblies with two lamps for reliability. rewire them into series configuration and buy the yellow or amber colored unit and put it where you can see it but not in your line of sight when driving.
neuweiler, The formula for "Pre-charge " is the R C Time constant Formula. That is Capacitance of the input capacitor bank totalized in Farads multiplied by the resistance in ohms is the time in seconds to reach 63.7% of the full applied voltage across the capacitors. We want the full voltage so the time needed is at least three times that Time calculated up to five times that Time calculated. Then the full pack voltage without the resistance is connected. But, there is no input surge because the capacitors have been slowly Pre-charged eliminating a major voltage difference when the main contactor is closed. Unless the vehicle is switched off for 10 minutes or longer there is generally no reason to Pre-charge again during a drive. The use of sufficiently rated tungsten filament light bulbs (Preferably heavy duty vibration resistant like "Garage closer, Ceiling fan, or oven and appliance rated bulbs)which offer a regulation of current magnitude during Pre-charge and therefore Pre-charge more quickly, but, at a lower peak current rate, are preferable and also less expensive than a large physical size resistor. You cannot use the "Start" position on the automotive ignition switch because the start is after "ON" and pre-charge will already have been missed. (In quality modern equipment like the controllers from EVnetics, the pre-charge is internal...) Good luck if you need more explanation I am always available for advice thru EVTI.ORG contact page.
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EC nut, thanks a lot! Do you happen to have the formula at hand?
To be honest, the light bulb approach would be a bit too geeky for my taste. I'd prefer a resistor in a cool looking case.
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).
Neuweiler, I gave you the formula but you didn't recognize it in text. "the R C Time constant Formula. That is Capacitance of the input capacitor bank totalized in Farads multiplied by the resistance in ohms is the time in seconds to reach 63.7% of the full applied voltage across the capacitors. We want the full voltage so the time needed is at least three times that Time calculated up to five times that Time calculated." In algebraic form that is RC=Tc or resistance in Ohms times the Sum of the input capacitors (Because there are usually several you just add them together.) equals the "Time Constant for that amount of resistance and capacitance. and the "TC" in seconds is the time to charge the capacitors to 63.7% of the applied voltage then in the same number of additional seconds it charges up 63.7% of the remaining voltage and in five times the charge reaches 99.5% so we consider the capacitors fully charged. As a rule of "Thumb" we usually use about 470 to 750 ohms. Also the actual average dissipated power is only a fraction of a watt however we use a physically substantially larger resister for physical strength and vibration resistance from driving on cobblestone roads and such. Typical is a five watt resistor supported with a nylon clamp and bolted to a strong surface. For the same reason I usually connect it up with (American Wire Gauge) # AWG-12 stranded wire. The voltage does not affect the resistance for a particular time ... nor does the amperage capability of the system as that current does not pass thru the resistor. we just allow the capacitors to charge up slowly for five to ten seconds to minimize the surge of charging up discharged capacitors when first applying the full battery pack voltage. (It saves the relay some wear and tear upon switching on...) I'm here if you have more questions.
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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^)
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.
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