voltage present between Battery Positive and Chassis (metal) Ground

Thread Starter

Chaitanya96

Joined Mar 15, 2022
4
Hello, I am performing one experiment in which I am controlling the switching of Batteries via a contactor (12V) and the supply from the battery is provided to the circuit inside. During some tests due to some wire harness failure there was some short circuit in the system and the contactor got impacted but still its working (showing complete 12V). Now again when the contactor is switched the battery turns on suceessfully but due to some reasons there is some voltage with respect to the chassis (metal body) ground inside which the contactor and the circuit is present and the positive of the battery. Can anyone tell the possible root cause for this and how do I overcome this issue. Thanks
 

Ian0

Joined Aug 7, 2020
5,163
What happens if you measure the voltage from Battery negative to the chassis? Is the voltage constant or does it rise or fall slowly with time? What happens if you put a shunt resistor (say 10k) across the probes of your meter?
 

ThePanMan

Joined Mar 13, 2020
368
How about a schematic?
Yes, definitely.

We need to know how you're testing. Normally if you check from battery positive to chassis ground, assuming the battery negative is tied directly to chassis ground - you're going to see the battery voltage.

If you're looking for a short circuit - we can't see or test your circuit from here. It could be one or more of a myriad of issues. A chafed wire. A screw contacting a metal housing. A wire touching another wire. Insulation worn through somewhere. A shorted component somewhere. Dirt or other contaminants have gotten into the circuitry and is providing a pathway to ground.

Some people call every electrical issue a "Short". A "Short" circuit is one where power finds its way back to the source without following the full circuit path. In other words - an unintended shortcut for electrons to follow. Another common issue can be an "Open" circuit. One where the intended path is interrupted. Like pulling the plug on the vacuum cleaner accidentally and pulling the plug out of the socket. That's an open circuit. One easily fixed by pushing the plug back in. But in a circuit where the intended path is through a light bulb (for instance), either the power or the return line is broken. When one tests using a meter they may find no voltage at the lamp OR they may find voltage at the lamp but it doesn't light because the return line is broken.

I only say this because far too often people don't fully understand electrical terms. I'm not suggesting you don't know - I don't know what you know or don't know. So I say this in general for all readers to potentially learn something. But truth be told, we can't troubleshoot your circuit without wiring diagrams, a schematic or pictures - "CLEAR" pictures. Can't tell you how many times someone has provided a picture we just can't tell anything about the board other than "It's a board".
 

Thread Starter

Chaitanya96

Joined Mar 15, 2022
4
Hello Sorry for the Delay. Attaching the Schematic for reference. So here ideally I should get the voltage just with Positive and negative of the battery which is near the casing but when I am checking on the metal Casing (considering here as Chassis) I am seeing the voltage on multi-meter when i probe the negative terminal anywhere on the chassis . Kindly Feel free to ask if any doubt. And please pardon if the schematic is not up to the mark. Thanks. Schematic_1.jpg
 

Tonyr1084

Joined Sep 24, 2015
6,894
I'm sure I don't understand your drawing, but if you're finding voltages from Battery + (B+) to the chassis then somewhere your B- is still shorted to the housing. Otherwise your housing should be completely isolated.

You said your contactor got impacted. First, I don't see how that could happen and still function. To me the coil of the contactor may have been burned out. Or just burned severely, but still functional. But under no circumstances should that provide a pathway to ground (the housing).

What's controlling the contactor? You're calling it a contactor, which is just a very very big relay. Contactors are used in things like Air Conditioners and other heavy machinery. From what I can tell, you have a 96V battery running to a buck converter that drops the voltage down to 12 volts. That 12V runs the PCB, and I'm assuming the PCB controls the contactor. Something your diagram doesn't show is how the contactor is powering the load. There should be some voltage source that the contactor switches on and off. I only see one line, which to me represents a single wire. That would imply the thing you're calling a contactor is actually providing a voltage to the load. Like I said, I don't understand your drawing. It's not the typical schematic drawing we are accustom to.
 
Hello Sorry for the Delay. Attaching the Schematic for reference. So here ideally I should get the voltage just with Positive and negative of the battery which is near the casing but when I am checking on the metal Casing (considering here as Chassis) I am seeing the voltage on multi-meter when i probe the negative terminal anywhere on the chassis . Kindly Feel free to ask if any doubt. And please pardon if the schematic is not up to the mark. Thanks. View attachment 266606
 
Hello I think in this your circuit the PCB ground is connected with you chassis ground. So May be It directly connected with chassis ground or Metal case.

Thank you !!
 

Dodgydave

Joined Jun 22, 2012
10,316
That sounds like the B+ is shorted to the chassis, remove the battery and measure the resistance from each battery terminal to chassis and see what is happening, .
 
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