# Contactors, Pre-charging et al...



## CrunchTime (Feb 13, 2009)

As suggested by the docs I acquired with my gear and research here and elsewhere, I have two contactors - one comes on with the key, and stays on until the key goes off.

The other is turned on and off by the potbox (PB-6) switch as I accelerate or not.

(1) Is this still considered a good design?

I have recently added a pre-charge resistor across the latter contactor, having been convinced it's a good thing by the discussions here.

(2) Now that the second contactor no longer isolates the controller/breaks the circuit the way it used to, what benefit is there in having it continually switching?

Would I be better to simply allow the pre-charge to do its thing, then close the second contactor permanently until key-off? It has been pointed out that if the controller tries to accelerate before the contactor closes, there'll still be inrush happening (this is after the initial switch on, while driving stop/start for example). This assumes that the potbox switch is slower than the resistance change to the controller, I suppose...

What do other people do?

(If it matters - controller is a Curtis 1231C; 144V LiFePO4 100Ah pack; car is not mobile yet)


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## Woodsmith (Jun 5, 2008)

On my tractor I have a small contactor the engages on the 'engine running' position of the key. It connects the 12V circuits and connects the precharge resistor across the main contactor. When the precharge time is up I click the key to the momentary starter position it activates and latches the main contactor in the on position with the aid of a small relay.

If I have to shut off the key, or if there is a 12V failure in any of the safety circuits, the main contactor is unlatched and the key has to turned off and back on again to re energise it, much in the same way as having to turn the ignition off before being able to restart an ICE.


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## CrunchTime (Feb 13, 2009)

Woodsmith said:


> On my tractor I have a small contactor the engages on the 'engine running' position of the key. It connects the 12V circuits and connects the precharge resistor across the main contactor. When the precharge time is up I click the key to the momentary starter position it activates and latches the main contactor in the on position with the aid of a small relay.


Thanks Woody - that's what I was thinking I might do. Nice to have a use for the "Start" position of the key 

Anyone else got an opinion?


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## Lee Hart (Oct 16, 2009)

The main contactor is controlled by the potbox switch for safety. If the controller fails "full on" (a common failure mode), the first thing you naturally do is release the throttle. The potbox switch will then turn the main contactor off, and you stop.

If releasing the throttle does *not* turn off the contactor, it takes time for the driver to decide "What do I do now!?!" It might take him a second to think of turning the key off. A car at full throttle can go a long way in one second!

The contactor is being switched at zero current, so wear on it is negligible. The rated mechanical life of a contactor is on the order of one million cycles.

If the noise bothers you, consider how your contactor is mounted. Is it on a large piece of metal that is acting as a sounding board? If so, add some rubber bushings. Is it out in the open, easy to hear (and touch)? if so, put it in a box (which also keeps dirt and bugs out, and is safer anyway).

I prefer to have contactors or switches that break both the positive and negative sides of the back. That way, I can safely work on other parts of the car with no danger of high voltage. As you described, I turn the negative contactor on with the key and leave it on, and switch the positive one with the potbox. But I also have a small relay to switch the precharge resistor out when the key is off. This keeps the pack totally isolated with the key off.


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## CrunchTime (Feb 13, 2009)

Lee Hart said:


> The main contactor is controlled by the potbox switch for safety. If the controller fails "full on" (a common failure mode), the first thing you naturally do is release the throttle. The potbox switch will then turn the main contactor off, and you stop.


Makes sense...



> The contactor is being switched at zero current, so wear on it is negligible. The rated mechanical life of a contactor is on the order of one million cycles.


Is it possible for the throttle to be depressed fast enough to request power from the controller (which then starts draining the pre-charge) before the contactor closes? If so this would mean that there is the possibility of arcing as the contactor closes. If it is possible, is the drain going to be fast enough to cause a problem?



> But I also have a small relay to switch the precharge resistor out when the key is off. This keeps the pack totally isolated with the key off.


Well doh! can't believe I didn't think of that. It was having the one pole still connected that I didn't like. So I'm back to the current set-up (no pun intended) with an extra relay...

Also, the NZ EV rules mandate the ability to disconnect both poles manually when working near either battery pack. I figure the contactors and a breaker will suffice for the front, and a grunty Anderson connector at the back...


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## Lee Hart (Oct 16, 2009)

CrunchTime said:


> Is it possible for the throttle to be depressed fast enough to request power from the controller (which then starts draining the pre-charge) before the contactor closes?


In general, no. A contactor takes on the order of 10 msec to close (0.01 second). Most controllers have a slight delay from when you press the throttle until it takes action. Also, the inductance of the motor limits the rate of rise in the current. The controller's capacitors will be supplying current during this ramp-up time. All this means that the contactor will close before the capacitors have discharged by any significant amount.


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