Battery Monitor/Shunt grounding question.

Converting Cargo Trailers into TTTs

Battery Monitor/Shunt grounding question.

Postby onlyridepark » Fri Feb 09, 2018 12:40 am

This may be a dumb question but I've searched here, plus YouTube and Google. Plenty of shunt wiring instructions and videos but none really answer this for me.

Everything I have seen shows the negative loads coming through the shunt and back to the battery negative. Problem is, not one place have I seen the mention of grounding any of the system to the chassis. I know all loads must go through shunt but shouldn't/couldn't they terminate at a chassis ground? And if they do go to the battery negative, shouldn't the battery at least be grounded to chassis? Is the chassis ground purposely avoided for some reason?
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Re: Battery Monitor/Shunt grounding question.

Postby John61CT » Fri Feb 09, 2018 12:57 am

The engine block / chassis connection(s) can be made anywhere.

A main return buss can go right after the shunt, and run both to loads, distribution blocks etc as well as to "ground" from there.

All DC circuits when separated by VSRs etc should have the return sides tied to the same "grounds".
Last edited by John61CT on Fri Feb 09, 2018 1:01 am, edited 1 time in total.
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Re: Battery Monitor/Shunt grounding question.

Postby John61CT » Fri Feb 09, 2018 12:59 am

And nothing here has anything to do with grounding AC circuits, much more dangerous and complex topic.
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Re: Battery Monitor/Shunt grounding question.

Postby Iconfabul8 » Fri Feb 09, 2018 3:51 am

It is done both ways. I ran all my grounds back to a common buss. Battery neg to shunt, shunt to neg buss. My DC board build
Others like Don/Flboy went Batt neg to shunt, shunt to frame ground. Here is a link to his very nice schematic.
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Re: Battery Monitor/Shunt grounding question.

Postby John61CT » Fri Feb 09, 2018 6:00 am

As long as the shunt is the only / last connection to the bank negative

the rest is not an issue.
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Re: Battery Monitor/Shunt grounding question.

Postby John61CT » Fri Feb 09, 2018 6:02 am

Of course, other AH counters or ammeters can be placed to measure only specific loads or charge sources, the above is for when you want to count everything going to/from the bank.
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Re: Battery Monitor/Shunt grounding question.

Postby flboy » Fri Feb 09, 2018 6:08 am

If the intention is to measure all of the battery's load and you are using chassis ground , then the shunt needs to be between the chassis and the battery ground connection.

If you are not, for whatever reason, using a chassis ground, as in the case of home running every return, then you don't need to put the battery ground on chassis. In the case of measuring full load, just make sure the shunt is the first connection to the battery negative and all other returns go through it.

The reason I like chassis ground is that it simplifies and uses less wiring.

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Current Build CTC:
viewtopic.php?f=42&t=65498
Old CTC Build (sold)
viewtopic.php?f=42&t=50647
YouTube Videos of Finished Trailer:
https://www.youtube.com/watch?v=FASF-x9ofMY - Interior
https://www.youtube.com/watch?v=wb-pvBbyyBs - Exterior
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Re: Battery Monitor/Shunt grounding question.

Postby NotJammer » Fri Feb 09, 2018 8:50 am

Yes, chassis grounds are normal for automotive, meaning TV.

Do you think an aluminum chassis is good for use for grounding? I ask as my trailer in nearly all aluminum.

I recall old trailers that relied on the trailer ball connector for grounding the trailer lighting system. A very bad idea!

Don't anybody do that!

What about an inductive amp meter? No physical shunt required?
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Re: Battery Monitor/Shunt grounding question.

Postby flboy » Fri Feb 09, 2018 9:10 am

Aluminum chassis is fine for a common ground from a conductive perspective. Just make sure to use an anti-corrosion, conductive type lubricant wherever you attach so that you do not get the dissimilar metals galvanic corrosion going. on. If I were to homerun... this would be more a reason to. I use a copper based paste used in marine applications when I am building antennas with aluminum elements.

Aluminum wiring was used for many years. Copper is just better for wiring and can handle more current for same size gage due to its molecular structure but has issues when used with dissimilar metals that need to be taken into account. If I had an aluminum chassis trailer, I would have probably homerun returns for this reason alone. The ground connections would not be as reliable and could be a source of trouble over time. On steel that is interior and/or painted over.. ussually no issues.


You can measure DC current without a shunt and just a clamp on sensor, but not with an inductive pickup which requires a changing field like with AC. You have to get a Hall Effect type sensor. So, yes it can be done, but the good old shunt is cheap and works well. In any case, you are still putting a sensor over the return wire and taking that to a meter for readout. No advantage for a permanent install. Yes for a mutimeter/clamp on application for troubleshooting.



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Don (Flboy)

Current Build CTC:
viewtopic.php?f=42&t=65498
Old CTC Build (sold)
viewtopic.php?f=42&t=50647
YouTube Videos of Finished Trailer:
https://www.youtube.com/watch?v=FASF-x9ofMY - Interior
https://www.youtube.com/watch?v=wb-pvBbyyBs - Exterior
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Re: Battery Monitor/Shunt grounding question.

Postby NotJammer » Fri Feb 09, 2018 9:58 am

I agree I may home run all CTC wiring including all grounds.

What gauge positive wire from TV engine compartment 12 vc isolator to run to a QR at the rear of my TV? 25 ft run.

I will use the 2017 F150 steel frame as a ground for that circuit. The wiring on F150 was downsized to just enough...

On long driving days, my TV alternator should contribute.

Are you a Ham? I'm a non-operating one. I just listen on my cheapo HT.
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Re: Battery Monitor/Shunt grounding question.

Postby flboy » Fri Feb 09, 2018 5:42 pm

NotJammer wrote:I agree I may home run all CTC wiring including all grounds.

What gauge positive wire from TV engine compartment 12 vc isolator to run to a QR at the rear of my TV? 25 ft run.

I will use the 2017 F150 steel frame as a ground for that circuit. The wiring on F150 was downsized to just enough...

On long driving days, my TV alternator should contribute.

Are you a Ham? I'm a non-operating one. I just listen on my cheapo HT.



I am not sure what "QR" stands for? How many Amps do you need to put through that 25ft run? That will determine the Gage wire you need.

Yes, I am an Extra Class operator and also hold a Radio Station Authorization from the FCC for GMRS. Just invested in some equipment over the last few years for my base station and mobile since I am not paying for my kids to go to college anymore and they are all off the "payroll" and on their own. :-) I enjoy the hobby, but mainly for the technical challenges and it takes me back to my Army days when I was trained in Electronic Warfare Intercept Systems and worked repairing this type of equipment.

I do not care so much just to talk over the air. I imagine I will enjoy the conversation more as time goes on and I have more time for that kind of thing. I first got my Technician class license in 1993 when I had a friend at work bet me lunch I couldn't walk in and take the test and pass it any time. They had a test the next morning (he thought he was setting me up), so I took it, passed, and had a nice lunch on him.

I was the same as you with only an HT until late 2014 when I decided to upgrade to Extra Class and get full FCC privileges and full power on HF/VHF/UHF and beyond. The purpose was for Emergency work and to have a mode of communication when I am out in the middle of BLM land or etc. which is where I will be next year this time in my CTC.

The HF phone and digital modes came in handy last year as I was of some service to Puerto Rico after the hurricanes (health and welfare messages). Only thing coming out of there was HAM radio for weeks and Amateur Radio set up communication centers with the Red Cross. Those guys were using car batteries and wire antennas to contact HAM Operators all over the US. I also participate in the Local Emergency nets which start up every time we are threatened by a hurricane.

I have a digital setup where I can pass digital message thousands and thousands of miles via HF or relay via VHF to VHF "Packets" kinda like a node on a wireless internet... RTTY (Teletype), PSK31, Olivia, etc. are all digital modes the computer sound card can render these days to modulate my radio microphone inputs. Same with the decode end from speaker jack to sound card microphone input and processed by the PC software. I just added a Digital Signal Processor for the audio outputs so I can filter out a voice or data at the noise floor. That type of technology is getting very affordable. 10 years ago it was not affordable. It still amazes me that I can bounce a UHF signal off a satellite passing over with a hand held antenna I made and reach the other side of the world with only a few watts. So cool! :thumbsup:
Last edited by flboy on Fri Feb 09, 2018 8:21 pm, edited 1 time in total.
Don (Flboy)

Current Build CTC:
viewtopic.php?f=42&t=65498
Old CTC Build (sold)
viewtopic.php?f=42&t=50647
YouTube Videos of Finished Trailer:
https://www.youtube.com/watch?v=FASF-x9ofMY - Interior
https://www.youtube.com/watch?v=wb-pvBbyyBs - Exterior
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Re: Battery Monitor/Shunt grounding question.

Postby onlyridepark » Fri Feb 09, 2018 6:42 pm

Thanks for all the replies. If I am hearing this right, it is normal and acceptable for all grounds to run back to the battery negative without a chassis ground on the DC circuit. My plan will be for all loads to come back to a common bus bar, then through the shunt and to the Battery negative. It just seemed weird not grounding the battery to chassis. I assume since all loads will come back to battery negative this is why it isn't necessary.
Or did I completely misunderstand everything you guys just said?
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Re: Battery Monitor/Shunt grounding question.

Postby flboy » Fri Feb 09, 2018 7:23 pm

onlyridepark wrote:Thanks for all the replies. If I am hearing this right, it is normal and acceptable for all grounds to run back to the battery negative without a chassis ground on the DC circuit. My plan will be for all loads to come back to a common bus bar, then through the shunt and to the Battery negative. It just seemed weird not grounding the battery to chassis. I assume since all loads will come back to battery negative this is why it isn't necessary.
Or did I completely misunderstand everything you guys just said?


You understood it well and have the right idea. The home run of each negative to a bus bar, then through a shunt to the negative terminal of the battery completes the circuit. On a chassis ground system... put simply, the chassis is just the bus bar.... a very large bus bar where you connect wires closest to the point of use. It just saves a lot of wire. I have a steel frame so not the issues with aluminum and galvanic corrosion.
Don (Flboy)

Current Build CTC:
viewtopic.php?f=42&t=65498
Old CTC Build (sold)
viewtopic.php?f=42&t=50647
YouTube Videos of Finished Trailer:
https://www.youtube.com/watch?v=FASF-x9ofMY - Interior
https://www.youtube.com/watch?v=wb-pvBbyyBs - Exterior
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Re: Battery Monitor/Shunt grounding question.

Postby onlyridepark » Fri Feb 09, 2018 7:36 pm

flboy wrote:You understood it well and have the right idea. The home run of each negative to a bus bar, then through a shunt to the negative terminal of the battery completes the circuit. On a chassis ground system... put simply, the chassis is just the bus bar.... a very large bus bar where you connect wires closest to the point of use. It just saves a lot of wire. I have a steel frame so not the issues with aluminum and galvanic corrosion.


It also seems like using the chassis ground system then having your shunt between the chassis and battery negative, you would lose some accuracy on the battery monitor. As surely not all loads would pass through it and there would be some losses, like through the tow vehicle itself and any time something was actually earthed. Probably overthinking that part and it really doesn't apply to mine since it will all be home run. Would be nice to only run half the wiring but in reality, I don't have a lot of wire to run anyway.
Thanks again for the help.
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Re: Battery Monitor/Shunt grounding question.

Postby John61CT » Fri Feb 09, 2018 7:37 pm

onlyridepark wrote:it is normal and acceptable for all grounds to run back to the battery negative without a chassis ground on the DC circuit. My plan will be for all loads to come back to a common bus bar, then through the shunt and to the Battery negative.

Yes bringing full return to the negative buss is far far better than relying on any chassis, especially long term.

Just didn't feel like arguing all the ways modern construction techniques make that risky.

You can also "ground" to chassis anywhere you like if you want, no harm.
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