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Old 05-31-2017, 08:00 PM   #29
NRR
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Everything in a car or RV is referenced to chassis ground as far as I know or remember. Well I am old enough to remember positive grounds also. That does not apply here. Adding any kind of switch between the negative side of the battery and the chassis possess a problem if the switch has any resistance. I mean ANY when closed. If there is .0001 ohms of resistance in the switch. And the current is just 20ADC with a voltage of 12V. That is a .002 volt difference between the battery negative terminal and the chassis ground.


V = I x R


Chassis ground voltage to the negative battery terminal is supposed to be ZERO volts. That is why we clean battery terminals and connections to the battery or chassis when things get funky.


Adding a switch to the negative lead of the battery is a dumb idea per the above.
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Old 06-11-2017, 08:56 AM   #30
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UPDATE
I decider to switch the positive side.
Works well,all seams fine.
Thanks everyone for the input.
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Old 06-11-2017, 02:11 PM   #31
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Originally Posted by NRR View Post
Everything in a car or RV is referenced to chassis ground as far as I know or remember. Well I am old enough to remember positive grounds also. That does not apply here. Adding any kind of switch between the negative side of the battery and the chassis possess a problem if the switch has any resistance. I mean ANY when closed. If there is .0001 ohms of resistance in the switch. And the current is just 20ADC with a voltage of 12V. That is a .002 volt difference between the battery negative terminal and the chassis ground.


V = I x R


Chassis ground voltage to the negative battery terminal is supposed to be ZERO volts. That is why we clean battery terminals and connections to the battery or chassis when things get funky.


Adding a switch to the negative lead of the battery is a dumb idea per the above.
I suppose resistance on the positive side is okay, just not on the negative side.
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Old 06-11-2017, 07:56 PM   #32
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Resistance in series is resistance. Pure and simple. It does not matter where in line you put the resistance. The voltage drop will add up to the full potential.

Cat put shut off switches on the negative side of many heavy equipment pieces.

Electrically it does not matter.
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Old 06-12-2017, 06:01 PM   #33
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Resistance in series is resistance. Pure and simple. It does not matter where in line you put the resistance. The voltage drop will add up to the full potential.

Cat put shut off switches on the negative side of many heavy equipment pieces.

Electrically it does not matter.
In my reply, just before yours, I was going for sarcasm. Guess I failed. 38 years in the computer industry, much of it repairing mainframes. I did learn a little about electricity and electronics. Resistance in a circuit, at the positive end, the negative end or anywhere in between yields the same result. There are reasons to switch one end or the other, but not in this case. This is a simple 12v circuit.

Disconnect switches in most motorhomes disconnect the positive side. I'm sure that has to do with the layout, configuration of the wiring. Negative cables off the battery typically go directly to the chassis. Adding a disconnect would require extending the cables to the switch and back again. Positive leads already extend to various locations so it's easier to throw a disconnect in at a convenient location. You also don't want to switch the negative off the engine battery. Some switches can handle the current of a starter, some can't. Most motor homes switch the positive off the engine battery but that only disconnects power to the dash and other electronics. Power from the battery to the starter itself is usually a direct line with no disconnect.

Bottom line is you can break this simple 12v circuit anywhere you want before the breaker/fuse panel or between the frame and connection to the battery.
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