# Dual Contactors and Precharge Resistors Question



## mharvey (Oct 30, 2009)

I was hoping someone could explain this to me. 

I have a dual contactor (SB-200 one on each the neg and pos) 144v setup with a Curtis 1231c controller on which I added a 750ohm 25w precharge resistor (according to Curtis specs) to each as I figured if one is good, two is better. Everything works fine except that now the traction pack volt meter never zeros out when the vehicle is off. When I disconnect one of the resistors, it goes to zero. In retrospect, I think having one on each doesn't really do anything for me and if fact are preventing cap discharge in the controller. But I wanted to ask. My questions are:
1. Do I gain anything by having a precharge resistor on both contactors?
2. Just for my understanding, why is the meter behaving this way with 2 resistors? Are the pair not allowing the controller caps to drain when the vehicle is off? The voltage drops to @120v but stays there indefinitely.

Any help or insight would be great.


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## kennybobby (Aug 10, 2012)

mharvey said:


> ... as I figured if one is good, two is better.


Sounds like my kind of good ole ******* engineering--hey y'all watch this.

Without a schematic it's difficult to know what you have done, but i would guess that one of the resistors (the low side?) has burned out and gone open circuit. Without a complete circuit path no current can flow to drain off the caps. Or you may have switched the contactors under load and damaged the contacts and/or coils (do you have freewheeling diodes on the coil drivers).

i'm just curious about why you have a contactor on the low side? The OEMs put them on both sides but they also have computer control to sequence the turn on and turn off events such that they are never switched under load with current flowing, but most diy don't have that level of detailed control.


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## mharvey (Oct 30, 2009)

kennybobby said:


> Sounds like my kind of good ole ******* engineering--hey y'all watch this.
> 
> Without a schematic it's difficult to know what you have done, but i would guess that one of the resistors (the low side?) has burned out and gone open circuit. Without a complete circuit path no current can flow to drain off the caps. Or you may have switched the contactors under load and damaged the contacts and/or coils (do you have freewheeling diodes on the coil drivers).
> 
> i'm just curious about why you have a contactor on the low side? The OEMs put them on both sides but they also have computer control to sequence the turn on and turn off events such that they are never switched under load with current flowing, but most diy don't have that level of detailed control.


Attached is a supplemental wiring diagram which doesn't have everything but does show the particulars in question. And most "******* engineers" scribble wiring diagrams (if they do them at all) on a bar napkin.  So, not sure if I qualify.

Actually the dual contactor setup was already installed without any resistors. I added them as I usually do (but when I design a system I have a single contactor and use a dc breaker disconnect). The 2 contactors are typically used for safety in case one goes "plasma event" you can key off to disable the other and disable the pack. I prefer a manual cable on a DC breaker but this is what was installed and it does work. 

The contactors do not switch under load. They both turn on with the ignition and the controller is enabled by the pack voltage lead to the controller via the throttle microswitch (pretty standard setup). 

Neither of the resistors is "blown" and the vehicle runs fine. The only issue is the voltmeter showing voltage with the ignition in the off position. Since the meter is connected to the load side of the contactors this tells me the voltage is coming from the controller caps. I just wanted to verify as I don't usually use a dual contactor setup. And "yes" there are diodes on the contactor coils.


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## kennybobby (Aug 10, 2012)

Oh you got them fancy cocktail napkins to draw on...

It looks on the drawing that the HV volt meter will never go to zero because it is shown connected to the pack side of the contactors, not the controller side. Or is there another meter?

Don't see a fuse from the traction pack to the KSI and CIR relays.

Looks like a stray fat red line drawn from the zivan to the 12V line by the 50A switch--closest to the 'k' in 'traction pack'.


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## major (Apr 4, 2008)

mharvey said:


> I was hoping someone could explain this to me.
> 
> I have a dual contactor (SB-200 one on each the neg and pos) 144v setup with a Curtis 1231c controller on which I added a 750ohm 25w precharge resistor (according to Curtis specs) to each as I figured if one is good, two is better. Everything works fine except that now the traction pack volt meter never zeros out when the vehicle is off. When I disconnect one of the resistors, it goes to zero. In retrospect, I think having one on each doesn't really do anything for me and if fact are preventing cap discharge in the controller. But I wanted to ask. My questions are:
> 1. Do I gain anything by having a precharge resistor on both contactors?
> ...


Having a resistor across both contactors allows the high voltage to the controller all the time, so it is always precharged, never de-energized. When the controller is charged (precharged), very little current flows through the resistors, so little voltage drop, so you read close to pack voltage at the controller input.

I often use a contactor on each positive and negative battery lead for full isolation when off. I turn on the negative contactor with the keyswitch. The positive contactor is the only one with the precharge. I use an enable switch or the crank position on the keyswitch to a latch circuit to start the precharge and positive contactor closure.

You need the charge resistor only on one of the contactors. But you must turn on the other contactor first.

Reference for precharge: http://liionbms.com/php/precharge.php


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## mharvey (Oct 30, 2009)

kennybobby said:


> Oh you got them fancy cocktail napkins to draw on...
> 
> It looks on the drawing that the HV volt meter will never go to zero because it is shown connected to the pack side of the contactors, not the controller side. Or is there another meter?


Actually it is on the load side of the contactor. Corrected the drawing.



kennybobby said:


> Don't see a fuse from the traction pack to the KSI and CIR relays.


It's there. Just let it off drawing. 



kennybobby said:


> Looks like a stray fat red line drawn from the zivan to the 12V line by the 50A switch--closest to the 'k' in 'traction pack'.


Yeah must have happened dragging stuff around. Also corrected. 

Thanks for QA'ing my napkin


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## mharvey (Oct 30, 2009)

major said:


> Having a resistor across both contactors allows the high voltage to the controller all the time, so it is always precharged, never de-energized. When the controller is charged (precharged), very little current flows through the resistors, so little voltage drop, so you read close to pack voltage at the controller input.
> 
> I often use a contactor on each positive and negative battery lead for full isolation when off. I turn on the negative contactor with the keyswitch. The positive contactor is the only one with the precharge. I use an enable switch or the crank position on the keyswitch to a latch circuit to start the precharge and positive contactor closure.
> 
> ...


Thanks Major. Makes sense. Quick question on your latching circuit looks like. According to the elithion article, K1 could be an AC relay as long as its rated for the voltage. Do you just hold the key in "crank" position until the relay engages and then release? Or what else is timing the precharge stage?


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## major (Apr 4, 2008)

mharvey said:


> Thanks Major. Makes sense. Quick question on your latching circuit looks like. According to the elithion article, K1 could be an AC relay as long as its rated for the voltage. Do you just hold the key in "crank" position until the relay engages and then release? Or what else is timing the precharge stage?


I've done it a number of different ways. Always seems like something special. Like a racing technical regulation or something. And often the controller actually takes care of the precharge on the positive side once the negative is closed. One of the first circuits I did used a time delay relay to drop out the resistor after a short period. Needed isolation in case the main contactor opened.

I have used the _hold keyswitch in start position_ for precharge with a voltmeter showing the controller voltage. When it was high enough, switch on the enable. I'll try to paste in the typical latching relay circuit which can be used.


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