# Vehicle Ground Problem



## Ziggythewiz (May 16, 2010)

You already disconnected any gauges?


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## J-Ro (Aug 9, 2012)

Yep -- Everything down to the heavy gauge cabling has been disconnected.


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## Ziggythewiz (May 16, 2010)

What kind of battery pack and motor?

FLA can get acid trails that conduct and brushed motors often have carbon dust.

What's the chassis-> pack resistance?


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## kennybobby (Aug 10, 2012)

*Where are you measuring?*



J-Ro said:


> Yep -- Everything down to the heavy gauge cabling has been disconnected.


How are you measuring chassis ground voltage--with respect to what reference?


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## J-Ro (Aug 9, 2012)

Hi Guys,

I have AGM Batteries, fairly new and good condition. The motor is a FB1-4001, brand new.

Cabling is not grounding out on the chassis in its run from the batteries to the motor. It is all nicely mounted on standoffs & convolute tubing, I went under and verified just for fun that there is a clear air gap between both pack wires and the chassis the entire run forward.

I am unable to get my meter to give me a resistance from the pack to the chassis..I assume this is becasue the difference in potential?

I measured chassis ground voltage by putting my negative meter lead on the pack most negative and then my positive meter lead on the vehicle chassis.

Does anyone know if the Open Revolt controller's aluminum heatsink is supposed to be isolated from chassis ground? Its in contact with chassis ground and i think this is the closest the battery pack gets to the chassis aside from the motor itself.

Thanks for the input, please keep the suggestions coming!


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## Ziggythewiz (May 16, 2010)

If the resistance was too high you might not get any reading. Hook a light bulb up same as your meter and see what you get. Bright = bad.


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## J-Ro (Aug 9, 2012)

I unbolted the motor controller and jammed a peice of plywood in between it at the chassis ground its touching. Becasue of the bad angle and quick job i cant say for certian that I completely eliminated contact but at any rate, doing this I was able to reduce the voltage between pack neg and chass neg down to about 3 volts with 140ish volts in the Open Revolt caps.

Is this helping the issue make sense?


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## mora (Nov 11, 2009)

Controllers aluminium heatsink should be isolated. Did you put controller together yourself? There should be some isolation material between mosfets and heat spreader. I used very thin mica pads. If you disconnect your controller and drain caps empty you could measure resistance from copper bars to aluminium heatsink. If you see some resistance you've found the cause.


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## Ace_bridger (Nov 22, 2011)

I have an Open Revolt in my Golf and am also running with 144V of Lead (Trojan T-1275s) which run at 141.6 empty and 160V ish full.

I too see that my pack is not isolated from the vehicle chassis as I can measure a potential difference between the pack -ve and the chassis.

Apart from biting me when touch a pack terminal and the chassis I have no issues with the lack of isolation. I'm not running a DC-DC yet but plan to in the very near future...perhaps I might have issues.

My Open Revolt is mounted on plywood so should be isoltated from the car body/chassis.

Sorry not to offer any advice but I figured the similarity of my situation and yours might offer you some consilation!


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## J-Ro (Aug 9, 2012)

Thats interesting that you see the same thing. Did you personally assemble your controller as well?

I found out the issue while my sweaty forearm was resting on chassis ground material while tighting a nut mid way through the pack. I got tickled by about 50ish volts. I'm glad that I didnt get it from more volts.

i Drained the caps and measured the resistances:

B+ to Heat sink is 6ohms and seemed to be slowly falling the longer I held the multimeter there but may have been more of a hover.

M- to heat sink is 20 ohms steady

B- to heat sink started at 41ohms and quickly rose up to 427 ohms and stayed there. After going B- to heat sink I now had 0.296v stored in the caps. I assume the voltage and the rising resistance had to do with the multimeter pumping power in to measure the resistance?

So what does all of this mean? I feel like I knocked the assembly of the controller out of the park so I'm not instantly ready to say the issue is from shotty workmanship. Do these readings point to a specific area where the problem may be?

Thanks for the help!


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## GerhardRP (Nov 17, 2009)

You seem to have a high resistance connection. I would take my volt meter and probe around various points in the the system for voltage vs the chassis as reference. Some points will be positive, some negative. Somewhere you will find zero volts. That is the location of the short [or if high resistance, is it a 'long'?]
Gerhard


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## J-Ro (Aug 9, 2012)

If I follow this method of probing until I get zero volts I end up in the middle of the battery pack. This battery doesnt get within3 inches of the nearest chassis ground through wood on its base and the perminals are about 3 feet away from the nearest chassis ground.

Does this still tell me something? Is it possible that the connection on that battery is a high resistance connection?


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## GerhardRP (Nov 17, 2009)

J-Ro said:


> If I follow this method of probing until I get zero volts I end up in the middle of the battery pack. This battery doesnt get within3 inches of the nearest chassis ground through wood on its base and the perminals are about 3 feet away from the nearest chassis ground.
> 
> Does this still tell me something? Is it possible that the connection on that battery is a high resistance connection?


So, now you know where the short is, but you don't know its resistance. I presume you are using a modern DMM with 10 MegOhm impedance. If the resistance of the short is on this order, you can estimate the resistance by measuring the plus and minus terminals relative to the chassis. If the sum is less than the battery voltage, we can calculate the resistance. Otherwise another test is needed. [Someone previously suggested a light bulb from a terminal to ground to detect very low impedance shorts.]
Gerhard


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## J-Ro (Aug 9, 2012)

So if I isolate the controller from the chassis I no longer have a problem.

I guess this is the solution?


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## GerhardRP (Nov 17, 2009)

J-Ro said:


> So if I isolate the controller from the chassis I no longer have a problem.
> 
> I guess this is the solution?


 Put some voltage back on the caps and probe around for zero volts vs. the heat sink inside the controller to find the short. [The spot you found on the batteries was only a mirror of the controller short.]


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## kennybobby (Aug 10, 2012)

This advice makes no sense whatsoever--who cares what the resistance of a short is, it's a fricking short circuit that shouldn't be there!

You could have a blown fet that is shorted to the heatsink. You need to remove and check all heatsink components until you find out why your B+ is shorted to the heatsink (and hence your chassis).


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## TigerNut (Dec 18, 2009)

J-Ro said:


> So if I isolate the controller from the chassis I no longer have a problem.
> 
> I guess this is the solution?


Mora is right... many MOSFETs have the heatsink terminal at the drain potential and require electrical isolation to the heatsink. Heat sink compound is not a sufficient insulator; in fact, depending on what is in it it might even be conductive. You need to have mica or some similar material between the MOSFET tabs and the heatsink. This also requires insulators between the mounting screws and the MOSFET tabs.
Kennybobby is also right - if you somehow blew a MOSFET then you need to know that, for the sake of the remainder of your controller as well as your own safety.


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