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Old 05-12-2021, 06:04 PM   #1
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Where to Place a Battery Monitor Shunt

I want to install a battery monitor shunt into the negative line from our TT batteries. Our DIY aftermarket battery cutoff switch sits in a junction box between the two separate battery boxes (attached photo).
Now I want to add an after market battery monitor more accurate than the OEM display. The monitor I purchased (did not come with an instruction book) includes a 350Ah shunt to be installed on the negative cable as well. Since both the cutoff switch and the shunt are to be installed on the negative line, does the shunt get placed between the battery terminal and the cutoff switch, or does the cutoff switch go in between the battery and the shunt?
If the shunt comes first I would be able to check the battery levels during warmer weather between outings without removing the batteries, but I would think this configuration allows a parasitic draw from the monitor screen.
If the switch comes first in line there is no parasitic draw when the switch is off, but I also can't see if the battery power is good without turning the cutoff switch back on.
I can live with either wiring order, but which order is best practice? Shunt before switch? Or switch before shunt?

Either method, I will likely install a third battery box between the existing two batteries, and rewire both the switch and the shunt within the third box in the order O settle on. Thanks for your input in advance.Click image for larger version

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Old 05-12-2021, 06:52 PM   #2
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I wouldn’t worry about the parasitic draw a bit. I’d go for the better monitoring access.

But that’s easy for me to say—my rv stays plugged in at home.

Edit— I’d have to go check to see how I wired mine. But I never really use the disconnect switch. I guess I’m hardly the one to be answering questions about best practices!
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Old 05-12-2021, 07:07 PM   #3
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Between the cutoff and the switch.

Reason you can look at the charge without turning on the switch. Also a shunt must have power at all times it measures the amps pulled from the battery and deducts that from the amp hours you say you have.

Without the 24/7 connection the draw could be calculated incorrectly.
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Old 05-12-2021, 07:33 PM   #4
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The shunt will only measure current flowing across it when there is a complete circuit. It should be the first thing connected to the battery. Wouldn't matter which side of the switch it's on, as it won't read anything if the switch is in the 'off' position.

Any reason you're switching the negative side of the system instead of the positive?
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Old 05-14-2021, 06:56 AM   #5
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Thanks for the feedback. So I will install the shunt first.

Richard you ask a good question. Before I installed the cutoff I poked around the internet for that answer and then decided on using the negative side -- keeping with the automotive practice of pulling the negative first before servicing the tow vic. Most of what I read at the time that was in favor of using the positive side was from MH owners, and it had to do with some feature that had to be running at all times.

After posting my original question here the other day, I visited some other brand-specific TT forums which I had not seen before, which mostly advocate for the positive side. Same with pulling up diagrams using my browser's image search. So it seems the majority of post-ers say positive, although both pos and neg advocates argue electrical flow logic that make them sound correct. >sigh< Is this Deere vs Case argument? I don't know.

What I DO know from the diagrams I saw is that if I put the shunt on the negative side and relocate the cutoff to the positive side, I can spread things out a bit . Will probably use an empty battery box between the two batteries, to house both the shunt and the cutoff, on opposite sides of the box
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Old 05-14-2021, 09:25 AM   #6
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The problem with switching the negative side is that you still have potential current flowing through all the positive conductors. Should one of those conductors short to ground (by rubbing through the insulation etc) you have a problem.

If you switch the positive side, a wire shorting to ground won't be a problem.

The shunt should be the first thing (and only thing) connected to the negative terminal of the battery bank. Everything else gets connected to the other side of the shunt.
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Old 05-14-2021, 09:40 AM   #7
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Quote:
Originally Posted by richard5933 View Post
The problem with switching the negative side is that you still have potential current flowing through all the positive conductors. Should one of those conductors short to ground (by rubbing through the insulation etc) you have a problem.

.
That's not true !

There is no circuit with the negetive switched off.
In a DC battery circuit, there is no potential to ground. You could short the positive battery terminal to the trailer frame and nothing will happen, with the negetive off.
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Old 05-14-2021, 09:51 AM   #8
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That's not true !

There is no circuit with the negetive switched off.
In a DC battery circuit, there is no potential to ground. You could short the positive battery terminal to the trailer frame and nothing will happen, with the negetive off.
Generally speaking you are correct. I suppose my thinking was more towards all the 'alternative' paths to ground I've seen tied to batteries over the years.
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