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Discussion Starter · #1 ·
This seems like an obvious question that I should know by now, but why are some negatives in a 12v system connected to the car chassis? What's the point?


I'm installing a Battery Monitory Kit (Magnum BMK) and it is meant to be wired into the system adjacent to the house/aux battery's negative terminal, with one side connecting to the battery, and the other side to basically everything else. Will it be an issue if some of the 12v accessories are grounded to the chassis?
 

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if the house battery is to be charged from the alternator in parallel with the engine battery it must be grounded to the chassis at some point with a heavy cable.
 

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'16 2500 159 HT Granite Window - OH(io)
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This seems like an obvious question that I should know by now, but why are some negatives in a 12v system connected to the car chassis? What's the point?
The easy answer is that it saves on wire. The entire chassis serves as the negative termination, so you only have to run the wire for the positive. The starter and alternator, for example, require lots of current to operate properly. If the chassis wasn't grounded, you would have to run a heavy negative wire to them along with the heavy positive wire. As it is, the negative is provided by bolting them (the starter and alternator) to the chassis.
 

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Discussion Starter · #4 ·
I'm putting in a magnum Battery Monitory Kit (BMK) and the "shunt" where the electrical readings are taken is meant to be wired to the house battery's negative terminal.

Will it matter if some of my electrical load and various items use the chassis ground for their "-" (Negative) while others are hard wired directly back to a negative bus bar which is connected to the shunt with 0/2 AWG cable?
 

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No it will work perfectly because that single negative wire carries all the current that the chassis grounds collect. Think of the battery as a source of electricity that has to flow (current) from the positive terminal through the appliances and somehow return to the negative terminal. The path doesn’t matter to your monitoring equipment as long as the shunt in the negative wire carries it back.
 

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2014, 138WB, High Roof, Gas, SW MT
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Hi,
There is nothing wrong with using the chassis as the negative return path for 12 volt circuits, but I never do as its hard and time consuming to get good chassis grounds and over time they tend to corrode and make for electrical problems that are hard to find (just as they do in cars).
Seems to me its faster and more reliable to just run two conductor wire to the 12 volt loads -- it costs a few dollars in extra wire, but worth it.

Gary
 

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Discussion Starter · #7 ·
What's the reasoning behind some 12v accessories or appliances, like the inverter, also have a small "ground wire" in addition to their positive and negative power connections?

Is there any interference is the chassis (ground) is being used as a 12v negative return?
 

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'16 2500 159 HT Granite Window - OH(io)
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The use of the additional ground wire is known as "bonding". It is used for the prevention of ground loop currents, electrical noise, etc. There are volumes and volumes of information out there concerning "bonding", and it will probably make your head explode if you try to absorb it all.:)
 

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2017 2500 HiTop 159 Cargo Van white.
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Ground back to the battery whenever possible.
 

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Will it matter if some of my electrical load and various items use the chassis ground for their "-" (Negative) while others are hard wired directly back to a negative bus bar which is connected to the shunt with 0/2 AWG cable?
No it will work perfectly because that single negative wire carries all the current that the chassis grounds collect.
We're not certain we understand RD's answer or maybe we don't understand your system/BMK. So, adding to the comments, our answer is that your BMK will only 'register' current flowing through its shunt. The normal practice is to connect NOTHING to the negative terminal of your House battery EXCEPT for the shunt (which, in turn, is connected to ground/chassis/negative return) to insure that the shunt/BMK 'sees' all current entering and leaving your House battery.

In this manner you will be 'measuring' everything that is going-on with your House battery and nothing relating to the vehicle battery, starter, alternator etc.
 

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2015 Promaster 3500 159 Ext gas silver
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Hi,
There is nothing wrong with using the chassis as the negative return path for 12 volt circuits, but I never do as its hard and time consuming to get good chassis grounds and over time they tend to corrode and make for electrical problems that are hard to find (just as they do in cars).
Seems to me its faster and more reliable to just run two conductor wire to the 12 volt loads -- it costs a few dollars in extra wire, but worth it.

Gary
It is more than a trivial burden to run separate ground wires rather than utilizing chassis ground for negative 12v runs. For instance it would entail about 15' of 2/0 cable just to join my chassis and house battery grounds. The important thing is to have clean metal and use a compound such as Thomas & Betts Kopr-shield CK CP8TB. For any connection exposed to weather I follow up with a coating of Dicor or similar sealant. I even used this process to improve some of the OEM ground connections. The Kopr-shield is also excellent to use with crimped lug terminals on heavy cable.
 

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What's the reasoning behind some 12v accessories or appliances, like the inverter, also have a small "ground wire" in addition to their positive and negative power connections?

Is there any interference is the chassis (ground) is being used as a 12v negative return?
Regarding using the little ground wire. I am about to connect my Inverter and it also has this little ground wire. Should I connect this wire as well? Should I continue having my ground from the house batteries? Since the Inverter and house batteries are adjacent to one another can I use the same bolt to share the ground?
 
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