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Old 03-11-2021, 10:55 AM   #1
Mini-Skoolie
 
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Battery Wiring and Negatives? To Share or Not to Share?

My system seems to be working fine however my battery monitor (Victron 712) Doesn't seem to update as my battery charges, instead choosing sometime in the middle of the night to change its displayed SOC%. My question about my wiring is below and I can't seem to find a resource that says my logic is right or wrong.

My question is: is my wiring logic correct? My components are:

3x 12v AGM batteries in parallel
Victron MPPT 110/30 Solar Charger
60A Sterling B2B Charger connected to my vehicle battery

The Solar Charger and B2B charger are wired to the positive and negative terminals of one battery in the chain and then the battery at the other end of the chain has a positive outlet for power to my inverter and 12v circuit board. The negative from there then comes through the shunt and back into the battery.

Now, I've heard of shared negatives and I get it but my logic says that I should have the negative flow back through the battery before going back to the components (B2B and Solar Charger) which supply the power. Rather than just connecting the negatives of the B2B and Solar Charger to the shunt and having only 1 negative for my entire circuit.

Can anyone advise me if I'm right or wrong and if the single shared negative would be better? and why that would be so (specifically why the negative doesn't need to flow back through the battery)

Attached is a diagram (no B2B drawn in) as a visual reference to what I just described above.


Thanks in advance!
Attached Images
File Type: jpg Battery Diagram.jpg (75.1 KB, 20 views)

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Old 03-11-2021, 11:10 AM   #2
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How about an outside wide angle camera and a flat screen monitor near the floor?
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Old 03-11-2021, 03:48 PM   #3
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I think it’s messed up.

The shunt needs to measure everything in and everything out to function correctly.

IMO you should connect the shunt to negative ‘battery 3’ and every other negative to the other side of the shunt.

Next, connect all of the positive wires to the positive on ‘battery 1’

Keep all the parallel wires the same.

This way the current will flow through all the batteries instead of killing one on the load side and charging from the other. You’re making an unbalanced mess kinda now the way it is.
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Old 03-12-2021, 01:49 AM   #4
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Normally all DC circuits should be connected to a chassis "common reference ground".

Not to be confused with true Earth ground.

Nor Negative Return path.

_______
All of the circuits you want to measure, negative returns must pass through the shunt, usually very close to the relevant source battery bank terminal.

Any circuits bypassing that "choke point" obviously don't get measured.

The shunt should be rated, like a fuse or wiring, for an ampacity at least 20-30% more than the highest total current it will see.

Larger size shunts will be less precise for low current rates.
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Old 03-19-2021, 01:12 PM   #5
Mini-Skoolie
 
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Quote:
Originally Posted by rffffffff View Post
I think it’s messed up.

The shunt needs to measure everything in and everything out to function correctly.

IMO you should connect the shunt to negative ‘battery 3’ and every other negative to the other side of the shunt.

Next, connect all of the positive wires to the positive on ‘battery 1’

Keep all the parallel wires the same.

This way the current will flow through all the batteries instead of killing one on the load side and charging from the other. You’re making an unbalanced mess kinda now the way it is.
So just have one positive terminal connected in the whole setup is what you're saying. My logic was that in DC the current can only flow in 1 direction, hence the in and out way I set it up. In that sense, would my devices use the instantaneous charge being supplied by the input before drawing from the battery storage?

I can understand the common negative, the common positive for input and output is what I'm struggling to wrap my head around.

Unbalanced mess is also what I was afraid someone would say in one way or another.
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Old 03-20-2021, 06:38 PM   #6
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Not sure what you mean by current only flowing in one direction.

Say you have a charge source supplying 50A. If you have a concurrent load of 60A, the remaining 10A gets pulled from the battery and no charging is going on.

When the load drops to only 20A then 30A is available to go into the battery.

With a dual buss design a relay can isolate the battery from the charge source when it reaches Full, but still allow loads to be fed.

And another relay can cut off loads when the bank goes too low, but still allow charging when the source goes on.

If you don't need that level of control, ten a single buss design is much simpler.
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Old 03-20-2021, 09:07 PM   #7
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With all due respect to the amazing amount of knowledge on this site, when it comes to "dangerous" or "expensive" or the worst kind "dangerous and expensive" things, I tend to be more conservative.

For me, when it comes to electrical / solar, I think a hybrid approach is great for the DIY with just enough knowledge to get hurt or break something that's expensive.

I figure tapping into someone that designs solar/inverter/charger systems to create a good design and educate me on the why and how of it will be part of the cost of my system. This way, I get something I know is correct, will work and I understand how to build, maintain and fix it.

Just my 2 cents.

Best of luck.
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Old 03-22-2021, 11:21 AM   #8
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It’s kinda non intuitive but don’t overthink things on the electron level.

When you’re charging and drawing electricity things are going opposite directions, but ultimately all that matters is that the wire lengths to each battery is about the same, or as same as possible. If you draw from one end of a string of batteries it’s easier to pull energy from the side where the wires are, so battery 3 is getting pulled down harder than battery 1. When you charge, battery 1 is taking most of the energy, and battery 3 is suffering, exasperating the problem.

Wiring the way I suggested will keep the middle battery slightly different than the outside ones but way closer to balanced.
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Old 03-24-2021, 10:30 AM   #9
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Thank you rffffff for all your information, I'm going to work on switching all this round when I get back to the old girl later this week!
Can't really believe that after the big learning curve I climbed learning all the electrical stuff that I just took my idea of how I should wire the batteries as gospel and didn't look it up
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