# Solid-State precharge - no more pre-charge logic



## charliehorse55 (Sep 23, 2011)

What if a high power MOSFET was used as a precharge switch? 

The TV200 contactor has a typical closing time of 15 ms. If you want to charge the capacitors to at least 63% before the contactor closes, you should aim to charge to 63% in 10 ms. Depending on the capacity of your controller's caps you will need between 5 and 30 amps to flow. A MOSFET rated for this current flow costs less than $1, and can be operated off the same +12V source as the contactor. 

As the MOSFET will turn on basically instantly (around 10us with a 470 ohm gate resistor), the contactor and MOSFET can simply be wired in parallel (with a large 10-100 ohm power resistor in line with the MOSFET). When you start the car, the MOSFET will open first and complete the precharge before the contactor's arm has time to move into position. This would effectively remove all logic surrounding pre-charge and replace it with a single TO-220 size part.


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## Ziggythewiz (May 16, 2010)

Pretty sure the purpose of the precharge isn't just to inconvenience you. For mine, curtis recommends 750 ohm. If they wanted it precharged nearly instantly, they would have specified closer to the 1 ohm range.

Sure your math is good? I don't think it would charge as fast as you think, especially going through a 10-100 ohm in addition to the fet.


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## charliehorse55 (Sep 23, 2011)

I'm pretty sure the point of precharge is just to prevent any component from being exposed to currents over their maximum rating. If the controller has a maximum rated input of 500A, precharge at anything under that should be fine. It's mainly trying to avoid the 4000A+ that would occur if you had no precharge. 

As for calcuations, with an 800uF capacitance (For the 450V Tritium AC controller):

R * C = T (for 63% charge)

T / C = R

10 ms / 800uF = 12.5 ohm

Peak current is 36A with 450V pack voltage. (450V/12.5 ohm = 36A)

To calculate the required power rating of the resistor, a rough estimate can be made by dividing current by 2 and multiplying by the pack voltage and the time duration:

36A/2 = 9A * 450V * 10 milliseconds = 81 joules

Find a resistor that can absorb 81 joules without over heating. A 25W resistor should work pretty well for this purpose. The FET has a comparatively negligible resistance and should not be an issue (usually less than 10 milliohms).


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## EVfun (Mar 14, 2010)

I don't recommend accepting a precharge of only 63%. That's 1 RC time constant, not a precharge target. I would shoot for less than 10 volts difference when the contactor closes.


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## charliehorse55 (Sep 23, 2011)

EVfun said:


> I don't recommend accepting a precharge of only 63%. That's 1 RC time constant, not a precharge target. I would shoot for less than 10 volts difference when the contactor closes.


A 10V difference is a 2.2% (10V/450V). 

solve y = 0.632^x for y = 0.022

x = 8.30 

I think this can still be handled:

10 milliseconds / (800uF * 8.30) = 1.5 ohms

Peak current is 300A, which while high, is still under the controller's maximum rated battery input current. There are many mosfets with pulsed drain currents well above 300A. 

This may work for better for DC, as they may have smaller capacitors, lower voltages and larger peak input current ratings.


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## Ziggythewiz (May 16, 2010)

I guess I just assume the manufacturer would have specified a 12 ohm precharge if they thought that was safe, instead of the 750.

Also, mine must be different as it never charges more than about 70%


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## charliehorse55 (Sep 23, 2011)

If you are using a physical relay, the initial 300A burst would most likely arc-weld the contacts together (unless you used a large contactor of similar size to your main one). I'm going to write to a few controller manufacturers about this and see what I get back. 

Really, the capacitors are usually connected directly to battery in. 

Again, I really think precharge is meant to stop two things:

a) Current over the maximum battery draw
b) Arc-welding the main contactor closed

If you close the contactor when there is a large current, an arc will form just before the contacts close. As they close, they will weld together. With the solid state solution, the first is fine (hopefully your pack can handle 300A burst....) and the second is also handled, as the relatively small remaining charge will not be enough to weld the contactor shut.


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## Ziggythewiz (May 16, 2010)

Yeah, understand the concepts, just don't know if it's good for the caps to fill at the rated current for the controller. FWIW my controller seems to have 7040uF so that would change things a bit.

I wonder what the Soliton guys use for their precharge values. I don't know who else integrates the precharge with the controller, so they'd probably have the best answer as to what works.


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## GizmoEV (Nov 28, 2009)

Ziggythewiz said:


> I wonder what the Soliton guys use for their precharge values. I don't know who else integrates the precharge with the controller, so they'd probably have the best answer as to what works.


The Zilla and Synkromotive have a built in precharge circuit. I'm assuming that the WarpDrive would too.


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