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Old 04-10-2021, 12:23 PM   #1
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Grounding question on 12V system

So my 200AH AGM is just about 60% now and I decided to add in a lithium setup and keep the AGM system as well. While researching equipment and a DIY Lithium battery I ran into a something that kind of made me scratch my head a bit. I will be using a Victron Orion Isolated ground charger to charge my lithium from the alternator. That got me thinking about the ground.


So in my original setup which has been hassle free except for the dying AGM, everything runs to a common ground which is grounded to the van chasis. I only ran the positive cable from the alternator/starter back to my house electrical and then used the chassis ground for the entire system. Right now everything utilizes that ground or is tied into it, chargers, loads, and batteries. When I add the Lithium and it's charging components, it too will use that same chassis ground.



Is using this ground for everything bad??



When I start the DIY lithium battery I will post some info on it. I am sure others have done it here, but it's kind of a cool thing and you can save a good amount of money if you diy it.
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Old 04-11-2021, 08:53 AM   #2
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That's all correct, but if you have any high-load devices grounded to the body (like any 12v appliances), you may need to upgrade the engine-body ground strap under the hood or run a redundant ground wire to the chassis.
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Old 04-11-2021, 12:49 PM   #3
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I'd run a separate, heavy ground wire from the alternator / start battery. The steel frame is not nearly as good a conductor as a copper wire, so you will be suffering a bit of a voltage drop compered to copper when charging from the alternator. Lithium batteries can draw a lot of current from the alternator and the higher the draw, the larger the voltage drop through the ground. Running a larger ground wire will only take an hour or so, you will improve your charging circuit and it's an inexpensive upgrade.
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Old 04-11-2021, 04:53 PM   #4
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I'd run a separate, heavy ground wire from the alternator / start battery. The steel frame is not nearly as good a conductor as a copper wire, so you will be suffering a bit of a voltage drop compered to copper when charging from the alternator.
Are you sure about this? I agree that copper is a better conductor and a copper chassis would be a much better conductor than a steel one, but voltage drop is also a function of the conductor's cross section. The effective cross section of a steel frame is pretty big. The ground straps at both ends do need to be of sufficient capacity. (I'm also a fellow Ham, KJ6SVX).

In any case, here's a link to some info on chassis grounding:

https://epicenergy.ca/chassis_grounding/
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Old 04-12-2021, 07:51 AM   #5
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Is using this ground for everything bad??

I get the feeling this conversation is about to get too technical for me but I'll throw this very simple answer out for the time being.

Lots of grounds, good.
Not enough grounds, bad.

I know nothing about lithium and really don't even care to but I've never seen any 12v system have too many ground connections.
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Old 04-12-2021, 08:25 AM   #6
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I have seen the failure of the ground strap from the engine block to the chassis, due to corrosion, cause problems.
The companies that tour computer controlled automated scenery for big live shows found they needed big ground planes in their racks to deal with all the third harmonic noise from the electronics.
Ground it all real well!
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Old 04-12-2021, 08:44 AM   #7
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Thanks all. I grounded to the body right now, but remembering that I was never truly satisfied with that ground. I have not hand any issues, but I definitely want to rethink it.


For some reason my brain got twisted around on this looking at a couple wiring diagrams.



120V systems all have the neutrals going back to the same bus and then to the earth ground of the system and tied in with all the safety grounds as well.


If anyone is interested in going more in depth on Lithium and the DIY aspects of it here are a couple amazing links:


https://diysolarforum.com/


Good forum and a lot like this one in that there are folks who are always willing to help out with questions and show you their work.


https://www.mobile-solarpower.com/


This guy is great and has some very good youtube videos and how to's.



I kind of came to a crossroads when the 4D AGM lost capacity. Seeing some of the DIY kits, I decided to build a 120AH system and keep the AGM system in there too as mainly a starter backup.
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Old 04-12-2021, 01:11 PM   #8
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Are you sure about this?
Well, I was, but thanks to your great link, not so much anymore. I based my additional ground wire on the voltage drop I saw between the start battery and my house battery's in the back of the van and adding a second ground wire was a small improvement. I also felt it overcame any issues of additional resistance in the factory grounding straps, and resistance due to probable corrosion at connection points on the frame. The wire is not connected directly to a frame ground in the rear, but rather to the house battery's and replaces the original wire grounded to the frame. I do think that my house battery charging seemed to improve with the additional ground. I'll have to give this further thought though, based on the info in the link. I feel a little smarter today, thanks..........
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Old 04-12-2021, 03:51 PM   #9
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So you don't need the isolated version of the Victron Orion, the non isolated version would be just fine. The difference being the internal ground planes are isolated at the unit, being that the ground is the same, no real need for that. It might be useful if you were measuring current with shunts on the ground lines, and possibly noise from one system to the other. It wouldn't hurt if you used it but it is not needed.

On the KISAE DMT1250 DC to DC charger that I have installed, they are non-isolated all ground connections are tied in the box. I was trying to use a shunt to measure and record the current coming out of the unit. The problem was that the ground cables were common, I wasn't getting the full current on the output cable. I tried moving input cable direct to the start battery, to see if I could get it more accurate. In the end it seamed better at lower current, but at heavy loads it was still splitting across both paths to ground. I abandoned the shunt and used a hall effect sensor on the positive line.

Building your own lithium bank sounds fun, I would still be reluctant to do so in a vehicle that is built for off road capabilities. I also don't understand why you are keeping your existing house AGM setup, doesn't that just add complexity, additional load on the alternator , etc. Seems like it is easier to just dump it.

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