# Why would a pre-charge resistor/relay kill a digital voltmeter?



## CFreeman54 (Jan 14, 2009)

I installed a Pre-charge resister/relay to balance the voltage of the Li Ion and AGM pack prior to engaging the main contactors, because before when I engaged the Li Ion pack it Killed my Paktrakr (which is only monitoring the AGMs); apparently the Paktrakr did not like the 3-5 volt jump when the Li pack engaged. 

I did make a mistake, and put a 1k-Ohm resister on the line (between the main contactor + terminals) first. This is when the voltmeter died. I corrected my mistake and replaced the 1K-Ohm resister with the correct 1 Ohm resistor, but the digital voltmeter is still dead; it reads 1 V. 

Fortunately, I did not have the much more expensive digital ammeter connected yet. If I replace the voltmeter it means another two days work of tearing the dash apart and putting it back again though...Ahhhhhh!

Why did the resistor kill it though? And more importantly, will the 1 Ohm resistor kill the digital ammeter if I connect it? ...or a new digital voltmeter if I replace the damaged one? Can one of you electrical engineer gurus here explain it to me please? See my control wiring diagram:


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## peggus (Feb 18, 2008)

Yeah.. It appears your car was attacked by the flying spaghetti monster and he ate your volt meter....

Seriously though. I can't even make out what your volt meter is connected to in that spaghetti mess of a wiring diagram. Perhaps if you cleaned it up a little it would become clear to you.


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## CFreeman54 (Jan 14, 2009)

What is clear to me is that Peggus has no intelligent or helpful advice to offer. Yes, I can "clean up" the diagram....by eliminating the details and complexity present in the car. 

"When ones words are no better than silence, one should keep silent".


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## Gavin1977 (Sep 2, 2008)

CFreeman54 said:


> What is clear to me is that Peggus has no intelligent or helpful advice to offer. Yes, I can "clean up" the diagram....by eliminating the details and complexity present in the car.
> 
> "When ones words are no better than silence, one should keep silent".


Peggus's advice is extremely helpful. Im an ee with many years experience and your diagram is a nightmare to trace through. I suggest you download a free schematic editor and draw it in that.

This will help us understand what is going on, it may well also tell you what went wrong, and it will be of great help to you when something goes wrong in the future.


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## Qer (May 7, 2008)

CFreeman54 said:


> Yes, I can "clean up" the diagram....by eliminating the details and complexity present in the car.


You can also do it right and keep the complexity while still making it cleaner and more readable. Like this:










This is readable and makes it easier to follow and find problems. I have to say that the Flying Spaghetti Monster comment was rather fitting.


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## etischer (Jun 16, 2008)

Is your volt meter designed to read high voltage? Does it require a voltage divider? Is it powered by the high voltage pack, or does it require a separate 12v power connection for power? Have you bench tested this volt meter before connecting it? Can you measure the voltage with a hand held dvm?


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## CFreeman54 (Jan 14, 2009)

Thanks guys. Now I am getting some useful feedback. I will look for a schematic editor...I did not know there was such a thing. 

Etischer; Yes, the voltmeter can read 0-200 V. It is the same type meter from Asia Engineers that Stuntdriver has in his car (but higher voltage capacity). It requires an isolated 12V power supply. The directions show that the high voltage input should also not come off of their ammeters shunt high voltage line...which is a resistor I guess...so maybe that has something to do with the problem? (Fortunately, I had not connected the ammeter yet...because I was thinking about how to give it an isolated power supply) 

The volt meter had just been installed the day before and was working fine prior to me installing the pre-charge resistor/relay between the positive terminals of the main contactors to balance the voltage between the lithion ion and the AGM pack. It would toggle between the lithium ion pack and AGM pack voltage with no problem. (There is a digital speedometer and tachometer being powered by the same 12V supply, but that did not seem to be bothering the voltmeter; I tested it off the line before installation.) When I installed the pre-charge resistor/relay and activated the relay, then closed the contactors, the voltage meter went to 1_ _ ._ _ V . That is when I knew I had a problem. 

After two days of work tearing apart the dash, installing the meters, reinstalling the dash in the car, only to have the voltmeter die, I was not happy....and I can not replace it or power up the digital ammeter until I understand what happened. I don’t want to damage any more components.

The pre-charge resistor/relay to balance the two parallel LiFePO4/AGM packs is an attempt to protect my Paktrakr from the voltage jump that occurs when the Li pack kicks in. There is only a few volts difference between the packs, but previously my Paktrakr died when I turned on the Li ion pack. (The Paktrakr is only monitoring the AGM pack). The Paktrakr rep said there was nothing wrong with the way I have it wired, and maybe it just didn’t like the voltage jump. Someone suggested balancing the pack voltages before engaging the contactors to reduce voltage jump influence on the Paktrakr. The pre-charge resistor/relay balances the voltages but created a new dilemma.


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## etischer (Jun 16, 2008)

Is your traction pack isolated from the 12v? You might have slight leakage to chassis ground from your traction pack. Maybe your precharge contactor could be leaking voltage?

You might measure with a volt meter to be sure.


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## CFreeman54 (Jan 14, 2009)

As far as I know the traction pack is isolated from the 12 V. I don't think the new resistor/relay could be leaking high V. The contactors the wiring comes off of and the resistor/relay are all within a composite box. What two points would be the best to use to check the chassis for ground fault?


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## iruraz (Sep 4, 2012)

@Qer which editor did you use for diagram? I am seeking a editor like yours. Thanks in advance.


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## Ziggythewiz (May 16, 2010)

Freeman,

Did you ever get this worked out? I'm interested in your mixed pack setup. Do you have a thread detailing it?


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