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anders-1 avatar image
anders-1 asked

Where would I put the shunt when 2 batteries in parallel are some distance apart?

I'm trying to figure out where to locate the shunt in my setup which has a battery under the drivers seat (1) and another in the cupboard at the back of the van (2) where all the electrics are located. The battery under the drivers seat is earthed so that the load is balanced across the bank.

My question is do I need to locate the shunt near battery (1) as that is where the earth is, and run another 4m cable back from the shunt to where all the electrical equip is located at the back of the van? This would mean that the +ve supply cable would be say 50cm and the -ve over 4m. This to my mind would unbalance the load, but I'm not sure? Whats the best setup in this situation? Thanks!

BMV Battery Monitor
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2 Answers
JohnC avatar image
JohnC answered ·

Hi @Anders

Your batts aren't balanced properly set up like that anyway. And you don't show the 'earth return' of your loads/charger. If they all come back together into your "All electrical equip" box, then you could do it like this...

Note I've deleted the second -ve link from Batt 2.



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anders-1 avatar image
anders-1 answered ·

Hi @JohnC

Thanks for your answer and help, I appreciate it. So this would mean effectively earthing both batteries ? (as i'm guessing in your drawing shunt also has to be connected to -ve of Battery 2?) With both earthed at different points on the chassis would that balance the load properly across both batteries?

I've drawn what was my proposal to make it a bit clearer:

But just doesn't feel right with that new long cable


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JohnC avatar image JohnC ♦ commented ·

@Anders

The only real difference there is that I've used the chassis earth return instead of a new -ve wire. You could go either way.

You wouldn't need to earth at all in your scenario, and if you needed to for meeting any chassis load earths, then you'd need to earth off the Load side of the shunt, not the Batt side.

Bear in mind too that the shunt needs a data cable, and you'll have a preference no doubt for a shorter cable there too, and at which end you prefer to mount the shunt.

If you have loads using a chassis-earth-return instead of -ve wires, then the shunt in my pic would need to be relocated to Batt1. In yours it could be at either end, but then definitely no earth at Batt1.

Hope this makes sense.. :)

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anders-1 avatar image anders-1 JohnC ♦ commented ·

@JohnC

If I don't need to run the 'proposed cable' and can go with your option I would much prefer that as it would mean less cabling. I did however leave something (B2B charger) off the drawing as I wanted to simplify it, but now realise its probably important, as I think I'm using the earth like you describe, as a "chassis-earth-return" (although I don't really fully understand what that means and how it impacts! sorry I'm still learning alot with electrics). Sorry for confusing the situation a bit by leaving that off.

So I've added that to the picture, and gone with your option, would it still work?

I thought that the shunt always needs to be connected to the negative terminal of the battery, at least I thought I read that in the instructions.

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JohnC avatar image JohnC ♦ anders-1 commented ·

@Anders

Yeh, in this new scenario you'd need to relocate the shunt directly off the -ve terminal of Batt#1, before it hits 'earth'. As you've read, and thankfully absorbed.

At least the data wire(s) from the shunt only need to be something smaller, if you want that to go to the rear of the vehicle.

Think of your 'earths' as just a convenient/lazy way to replace cables, by using a heavy chassis conductor. It's different to an AC 'earth/ground', which serves a different purpose entirely.

And just for fun, you may spot the Victron Orion dc/dc converters come in 'Isolated' models, which means no -ve continuity. If your B2B was the same it wouldn't be of concern.

I hope I'm not confusing you, hey..


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anders-1 avatar image anders-1 JohnC ♦ commented ·

@JohnC

Thanks again, its really helping and I think I'm getting the hang of it! For some reason I was stuck on thinking the earth had to be connected at the shunt, but I think this maybe isn't the case?

So hopefully this will work, what do you think?

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JohnC avatar image JohnC ♦ anders-1 commented ·

@Anders

No, that won't do.

Sorry mate, I made a new pic but the site won't accept it right now.

If you go back to your pic #6, just move the shunt from where you have it, to the Batt#1 > Earth wire. All will work then


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anders-1 avatar image anders-1 JohnC ♦ commented ·

@JohnC

Oh no, I thought I'd cracked it! so I do need to run that long 'proposed' cable from Batt1 back to where the electrical equipment is. Is this not the same as I've put in pic#2 (apart from the B2B omission)?

My question is that this cable to the electrical equipment would be 4.5m long and the positive cable only 0.5m. Wouldn't this make things unbalanced / inefficient? Would the 4.5m 'proposed' cable need to be much thicker than the positive 0.5m?

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JohnC avatar image JohnC ♦ anders-1 commented ·

@Anders

The cable lengths don't matter. To balance your batts you need to have the +ve wire at one batt and the -ve at the other. Both for loads and charging.

The chassis earths replace the -ve wires, but when you do that you need to be careful where you place the shunt. It must go between the batt -ve terminal and the rest of the -ve wires in your system (in your case the chassis 'earth').


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