# Shunt and precharge question



## Lionstrike (Oct 29, 2011)

Hi all,

I'd posted here some time ago with plans of building an Electric Ford Ranger. Well, things took a turn for the weird when I went to a local EV competition with my Vectrix VX-1, and someone was selling one nearly identical to the one I was going to build, for less than the cost of me finishing mine.

SCORE!

So I spent the Summer upgrading and modifying it. He wasn't the first owner either, it was passed down and modified by every owner. I'm just the latest. Cool little truck, I love it.

I've got a question on the wiring though. First of all, unless I am mistaken, I don't see a precharge on it. It uses a Curtis 144v controller, and it runs on 120v flooded batteries (20, 6 volt cells). Everyone's driven this thing for years, no mishaps... should I be concerned?

Second of all, I am reading about 350 amps continuous on the ammeter. I talked to a buddy of mine who has a similar truck and he says that number doesn't sound right to him. I agree. I'd expect all kinds of melted wires, terminals, and general havoc there... but nothing. (And no, the parking brake isn't on LOL... but I did do that by accident before).

Here's the thing... the shunt is wired in a weird place. I put the shunt off of B- normally... this thing is coming from M-. (Pic attached).

Battery pack voltage reads about 124 volts right now, but at the shunt I have a cycle analyst under the hood reading about 113v. Clearly there's a voltage drop somewhere and I think that's messing up my ammeter readings. Could this wiring do that?

Thanks for any help all.


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## major (Apr 4, 2008)

You damage the main contractor every time you close it without a precharge. And the current shunt is in the wrong place. It must be in the most negative battery lead for proper CA operation. 

BTW, if you are actually drawing 350 motor Amps, which is possible while only using 100 to 200 battery Amps, you need to shift gears.


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## Lionstrike (Oct 29, 2011)

major said:


> You damage the main contractor every time you close it without a precharge. And the current shunt is in the wrong place. It must be in the most negative battery lead for proper CA operation.
> 
> BTW, if you are actually drawing 350 motor Amps, which is possible while only using 100 to 200 battery Amps, you need to shift gears.


Thanks for the response Major, I ordered two precharge resistors from EVsource.com. I caught your response from the other thread with the other guy who has two contactors... is it necessarily wrong to put a precharge on both contactors? I think that I would prefer that setup since I have a main pack disable switch that I am going to use when parked and charging. 

That shunt is going on B- the next time I open the hood.


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## major (Apr 4, 2008)

Lionstrike said:


> I caught your response from the other thread with the other guy who has two contactors... is it necessarily wrong to put a precharge on both contactors?


I'd say yes, fundamentally wrong. It actually eliminates the _precharge_ and replaces it with a _stay charged_. A resistor across both contactors does not allow normal isolation and bleed down of the caps in the controller. This presents a hazard as one might expect the controller and high voltage circuit to be off when the car is shut off. It also can shorten the life of the caps and other components which sit there forever at high voltage. If left for a long time, especially if there are other high voltage loads, it can drain the battery.

Also, if the resistor value is chosen per the manual, and you use one on each contactor, you actually have twice the resistance and may not be charging the caps quite high enough thereby causing an arc on the contacts when you close the first contactor.

major


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## Lionstrike (Oct 29, 2011)

Major, thanks a ton. Your advice has been extremely helpful.

I've done some re-wiring tonight. The attached image represents my current understanding of the system with the exception that the precharge for the main contactor isn't in yet... I ordered it from EVsource.com and it'll take a few days to get here. The system stays off until it comes in.

I don't know what energizes the B- contactor, but since it's a 12v coil, I am going to say that it's the 12v battery pack system and both the B+ and B- contactor engage simultaneously. 

I hope that this is a little more correct. I really want to spend some time driving this truck and less time working on it soon.

Look about right?


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## major (Apr 4, 2008)

Lionstrike said:


> Major, thanks a ton. Your advice has been extremely helpful.
> 
> I've done some re-wiring tonight. The attached image represents my current understanding of the system with the exception that the precharge for the main contactor isn't in yet... I ordered it from EVsource.com and it'll take a few days to get here. The system stays off until it comes in.
> 
> ...


A quick look indicates OK. Just be sure to close the negative contactor first and allow time for the precharge to occur before closing the postitve contactor. You should see the voltage to the controller rise to like 80 or 90% of the battery voltage before closing. Typically 5 to 10 seconds for most systems, I think. I use very low Ohmic resistors taking like 1 to 2 seconds.


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## Lionstrike (Oct 29, 2011)

major said:


> A quick look indicates OK. Just be sure to close the negative contactor first and allow time for the precharge to occur before closing the postitve contactor. You should see the voltage to the controller rise to like 80 or 90% of the battery voltage before closing. Typically 5 to 10 seconds for most systems, I think. I use very low Ohmic resistors taking like 1 to 2 seconds.


Got it, thanks.

I am pretty sure that the microswitch on the potentiometer closes the main B+ contactor. 

Now that I think of it... the original wiring probably opened and closed the contactor every time I hit the throttle. That poor contactor is probably hurt. I am glad that it still works.

So I'll keyswitch on... this will activate the 12v system which should close the B- contactor. The B+ contactor should not be energized until the throttle is hit.

Therefore, if I understand correctly... I'll wait about 10-15 seconds... and then I should be good to ride. I'll try that.

Thanks again for the advice.


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## mharvey (Oct 30, 2009)

That was me that asked about a resistor on both contactors. So I can confirm the behavior first hand that Major said. 

Also the 25W, 750ohm resistor from EVSource will take @10secs to get to 80% of pack voltage. I'm running 144v Curtis 1231C with the exact same spec resistor and this is what I'm seeing. I have a 1-10 sec adjustable timer relay maxxed out at 10 secs to get to 120ish volts before it releases which is probably fine but Id prefer closer to pack voltage so Im dropping to 500ohm 25w which would give me some breathing room I'm guessing by the formula @ 7 secs. 

Also the is this from Zeva that monitors the precharge voltage and flips the contactor on when precharge is complete. Rated at 200 ohm. I haven't used it but if it works as advertised, it's an answer in a little black box.

Has anyone used a rheostat for testing and sizing of the precharge? Seems if you had a beefy enough one say 1kohm @25w, you could dial in just the right timing and eliminate the guesswork.


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## Lionstrike (Oct 29, 2011)

mharvey said:


> That was me that asked about a resistor on both contactors. So I can confirm the behavior first hand that Major said.
> 
> Also the 25W, 750ohm resistor from EVSource will take @10secs to get to 80% of pack voltage. I'm running 144v Curtis 1231C with the exact same spec resistor and this is what I'm seeing. I have a 1-10 sec adjustable timer relay maxxed out at 10 secs to get to 120ish volts before it releases which is probably fine but Id prefer closer to pack voltage so Im dropping to 500ohm 25w which would give me some breathing room I'm guessing by the formula @ 7 secs.
> 
> ...


Cool little precharge box. I like it. 

I think that the way that mine is wired works off of the throttle though, so I am not sure that something like that would work for my particular design specs. The throttle has a microswitch that closes the contactor when it's depressed. 

The Cougar Revolt controller works something like that from what I understand. It takes 12v to power up the controller, and it outputs 12v output when it's precharged which should close a contactor. 

So I think that I'll just do what you say and wait about 10 seconds before driving off. I think that keyswitch on closes the contactor on B-, and B+ is the "main" contactor. That's where I plan on putting the precharge.

It hasn't come in yet though. I am in Pennsylvania and it ships from Utah. I am waiting anxiously.


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## mora (Nov 11, 2009)

Lionstrike said:


> The Cougar Revolt controller works something like that from what I understand. It takes 12v to power up the controller, and it outputs 12v output when it's precharged which should close a contactor.


Revolt controller doesn't have automated precharge unless one uses custom control board. There is only a programmable +12V output with timer. I think it was designed to operate a contactor that has precharge resistor wired in parallel. Precharge should happen within programmed time and then controller will close the main contactor. Controller doesn't know if precharge actually happened or not.

I'd ditch the microswitch and operate the main contactor using precharge circuit (ZEVA one will do) that keeps the contactor closed until you switch the system off. This way you can use both contactors to fully isolate the battery pack when the key is off. Precharge happens when you turn the key and system stays on when it has actually happened.


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## Lionstrike (Oct 29, 2011)

mora said:


> Revolt controller doesn't have automated precharge unless one uses custom control board. There is only a programmable +12V output with timer. I think it was designed to operate a contactor that has precharge resistor wired in parallel. Precharge should happen within programmed time and then controller will close the main contactor. Controller doesn't know if precharge actually happened or not.
> 
> I'd ditch the microswitch and operate the main contactor using precharge circuit (ZEVA one will do) that keeps the contactor closed until you switch the system off. This way you can use both contactors to fully isolate the battery pack when the key is off. Precharge happens when you turn the key and system stays on when it has actually happened.


Decent idea. I'll have a look at it. I am kind of done working on it for right now though, I have no garage and Winter is coming. It's getting too cold to work outside.


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