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Victron SmartShunt question

tosimplify

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Nov 28, 2020
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I have two 24 volt lifepo4 batteries with Anderson connectors that I want to wire in parallel. I was planning to do this by getting a set of cables for each battery with Anderson connectors for the battery end of each, and copper lugs on the other end so I can connect each battery to a positive and negative busbar. What would be the best way to install a Victron SmartShunt in this type of parallel setup? Would I just connect both of the battery negative copper lugs directly to the shunt's "Battery Minus" stud, and then connect the "System Minus" stud to the negative busbar? Or is there a better way?
 
Excellent. In this setup, how important is it that all wires going from each battery to their corresponding busbars be the same length? I ask because if the two negative battery cables have to go thru the shunt first, the resulting total length to the busbar will be more than that of the two positive cables. Does this matter?
 
I'm doing the same thing with a modular/portable setup. I have (2) physical battery banks in milk crates (2x 12v Lion UT-1300's in series to make 24v) - each with an SB175 2/0 cable going to my "control" box (Growatt 3000 inverter, breakers, outlets etc). The control box has a pair of SB175's mounted for the batteries to connect to.

From those SB175 receptacles -the two negative leads sum to a shunt, and then on to the inverter. Positive leads from the SB175 sum into a 200a Bussman breaker, into a BlueSea switch (wired w/ resistor for a pre-charge/inrush circuit), and then to the inverter.

I do also have 300a MRBF fuses on each of the battery bank positive leads as well - at the battery terminals. 300a is overkill - but my 2/0 cable is the Class M ultra flexible welding cable (rated to 300a). All cable lengths the same for each battery bank.

Feedback welcome. I've thought about moving to a Class T fuse (rather than the breaker and MRBF's), not sure if it's really needed though.
 
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So is it the total length from the battery terminal to the busbar that matters? Meaning for the negative then, it would be battery to shunt + whatever length the shunt itself adds (couple inches?) + shunt to busbar, but in the positive, it would only be the single length from battery to busbar. Both would need to match?
 
Correct - the reason for having same length cables is to not introduce different levels of resistance (from cables) that makes one set of batteries have a slightly different dis/charge behavior than the other - which can lead to some batteries working harder than others, and having premature wear. So, this really just means - same cable length to each bus bar - what happens on the other "side" of that bus bar shouldn't affect this. Though of course, no reason to make cables longer than they need to be (cost and resistance).
 
Wouldn't it only be important that the two negative lengths (i.e. 1 wire from battery + shunt + 1 wire to busbar) match each other, and then the two positives match each other separately? Meaning the total negative length can be different from it's corresponding single positive cable? If not, then getting the resistance to be the same across all wires would involve more than getting the lengths to match, since on the negative side, the shunt is presumably introducing resistance as a result of the two connections and the shunt itself. None of that exists on the positive cables.
 
Just to be sure we're on the same page, I made the following diagram to illustrate what I'm thinking. Each cable is marked with a unique letter, so here is my understanding:

A = B

C+E = D+E

But the real question I have is whether C+E and D+E must also equal A or B. And also, does the length of the shunt factor into this too?

battery_setup.png
 
I think you want A=B=C=D
I probably wouldn't go to this effort, but if we're being picky, you only need A=B and C=D. The two groups don't need to be equal. The point is to equalize the resistance of the two loops (bus->bat->bus) for each battery so really the only requirement is A+C = B+D since resistance on one lead is indistinguishable from that on the opposite polarity lead. But what are you going to do, measure a milliohm of resistance? Not easy to do. Matching lengths is all you can practically do.
 
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