# Precharge set up



## dougingraham (Jul 26, 2011)

Lets first explain why you need to precharge.

The motor controller (and other devices too) have a somewhat large bank of filter capacitors on their input. These capacitors will if allowed draw thousands of amps for a fraction of a second from the traction battery if all you do is close a switch. It is similar to and would in fact sound like a lightening bolt and at those current levels the contacts of the switch (contactor) will weld shut or at the very least suffer surface damage that will prevent them from conduction of current under normal conditions. So the reason to precharge is to equalize the voltage of the battery pack and the voltage across the capacitors in a controlled manner. For safety reasons you do want a way to disconnect the motor controller from the battery when the vehicle is not powered up so this precharge needs to be done everytime you power up the car, not just the first time you connect the controller to the battery.

The precharge itself is simple. A resistor is connected across the terminals of the contactor which limits the current flow. The device that controls the precharge watches the voltage on both sides of the resistor and when it is nearly equalized the contactor is closed. This can be also be done with a simple timer but watching the voltage difference is probably the easiest and certainly the most reliable.

The Soliton 1 controller does all of this internal to itself so you don't have to add any outside components. Other controllers like the Zilla and at least some of the Curtis controllers tell you exactly what outside components are needed and handle the sequencing of the precharge so there really isn't anything you have to do other than connect up the parts.

I hope that clarifies things.


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## major (Apr 4, 2008)

Davide explains pre-charge very well here: http://liionbms.com/php/precharge.php


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## epyon (Mar 20, 2008)

Thanks for that , it helped both of us understand it better . But , I then went to EV WEST and bought there electronic precharger . An then I read to put some kind of spike protection on the motor and controller an close to the precharge contactor , but my precharger is electronic . What would be good or best ? And to make things weird , I use 4 contactors to reverse my motor (bought from EV WEST too) .


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## major (Apr 4, 2008)

Reversing contactors go between the controller and the motor. The precharge is only required on the circuit between the battery and the controller.


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## PStechPaul (May 1, 2012)

I have been looking into electronic precharge circuits that do not waste energy and power in a resistor, although the total energy is not really very significant. Davide's analysis is very helpful and shows that a 10 ohm resistor will work well with 300 volts and 10,000 uF. The total energy is:


```
E = 0.5 * C * V^2 = 450J (w-sec)
```
And initial current and power are:


```
I = 300/10 = 30 amps
P = 300 * 30 = 9000 W
```
A wire-wound power resistor can usually withstand short time surge of at least 10 times nominal rating, based on I^2t, and based on t=10 seconds for continuous rating. So a 100W 10 ohm resistor has a continuous current rating of 3.2A an I^2t of 3.2^2*10 = 100. At 30 amps the allowed time would be:


```
t = 100/30^2 = 0.11 sec
```
The time constant of 10 ohms and 10,000 uF is 100 mSec, at which point about 2/3 of the total energy has been transferred to the capacitors, and the current will have dropped to about 10 amps, or 1000W. After another 100 mSec, the current drops to about 3 amps and the resistor will be within its continuous rating. The surge rating of a resistor depends on how it is constructed. High current can produce powerful mechanical forces that can cause the windings to expand radially and contract axially, which may cause the encapsulation material to crack, and the thermal properties determine how hot the resistive element can get, which can cause it to melt and "fuse".

One disadvantage of a resistor and contactor is that, in case of a very leaky or shorted capacitor, there will always be a high current in (and voltage across) the resistor and it will eventually burn up, or the contactor will be damaged if it closes after a preset time. The only safeguard in this case is another contactor which can handle the full current and DC voltage, and such a device can be large and expensive.

An electronic precharge circuit can use a MOSFET or IGBT and a series inductor and rectifier in a current mode buck configuration where PWM can be used to limit the current, and it will protect against an overload or short circuit. The solid state device can turn off completely if the capacitors do not reach full voltage charge in the expected time frame. And once the input and output voltages have equalized, an inexpensive low voltage automotive type relay can close across the MOSFET or IGBT to eliminate losses due to its resistance and/or diode drop.

Another way to provide a "soft start" or precharge with an AC supply is to use SCRs for part of the diode bridge, and use phase firing to bring up the applied voltage slowly while monitoring the current. An inductor is also helpful in this case to eliminate the high current spike that would be present whenever the peak input voltage exceeds the capacitor voltage.


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