Correct size battery cables

[teddysx3]

Hello all, I have a set up where the cables between the batteries are equal, and the cables from the positive and negative to the bus bar are equal, but they are longer than the ones that the are between the batteries. I think it's called split feed.

The other way is common point connection, where each and every cables is the same length and is connected to the Lynx, I know this is best practice for more even charge/discharge of the batteries.

My question is, is it that big of a deal how my setup is wired? Should I consider spending big bucks on cables? The longest cable is 3-4 feet, so I need to get another 4 new cables and a new bus bar..

Thank you!

 

[12VoltInstalls]

longer than the ones that the are between the batteries

That’s fine if they are sized for the full load and fused correctly.

is it that big of a deal how my setup is wired

IMHO- not a big deal

 

[Humblebumble7]

Hello all, I have a set up where the cables between the batteries are equal, and the cables from the positive and negative to the bus bar are equal, but they are longer than the ones that the are between the batteries. I think it's called split feed.

The other way is common point connection, where each and every cables is the same length and is connected to the Lynx, I know this is best practice for more even charge/discharge of the batteries.

My question is, is it that big of a deal how my setup is wired? Should I consider spending big bucks on cables? The longest cable is 3-4 feet, so I need to get another 4 new cables and a new bus bar..

Thank you!

You should be fine with the setup as long as the cables are the proper gauge (125% of maximum amps). 3-4 feet is not a big deal unless it gets unruly for neatness.

 

[teddysx3]

Yes all cables are over sized a bit. The issue I was concerned was uneven charge/discharge. I see it's not that big of a deal.

 

[Oldwave]

https://www.victronenergy.com/upload/documents/Wiring-Unlimited-EN.pdf

 

[12VoltInstalls]

Yes all cables are over sized a bit. The issue I was concerned was uneven charge/discharge. I see it's not that big of a deal.

It is a big deal

Your cabling, however, is fine.

 

[teddysx3]

It is a big deal

Your cabling, however, is fine.

Yes ofc it's a big deal, I meant the difference between my setup and a setup with all equal lenght cables. Thanks for the input.

 

[FilterGuy]

Before I say anything else: I agree that care should be taken to try to balance the load/charge on the batteries. In most cases, it is relatively easy to do so why not do it? I certainly always recommend it to people I help.

However, I will be a bit of a contrarian on how big of a deal it is. Most LiFePO4 batteries have a cycle life of ~4000 (or more). If the batteries do a full cycle every day, that works out to ~11 years. It is the rare solar system that is fully cycled every day, so the actual cycle life is probably closer to 15 or 20 years (or more). If one of the batteries wears 20% faster, that is still 12 to 16 years (or more)....and even then, it has 80% of its capacity. The battery is still usable, just not at it's full capacity. Consequently, even with uneven wear, the calendar life of the battery is likely to get you before the cycle life.

Another factor I have wondered about is what really happens when one of the parallel batteries gets weak. I suspect, but certainly can't prove, that as the bank discharges, the weak battery will just not contribute as much as the others, so the wear will shift to the other batteries... Perhaps over time, it all equalizes out.

 

[OffGridInTheCity]

If I could also attempt a bit of contrariness....

I'm hooked up so the current flows from top left edge to the bottom right edge of a 9x parallel 48v batteries in my solar battery bank - encouraging current flow thru the set of batteries. However, my 20ft main negative 4/0 AWG wire and 10ft main positive 4/0 AWG wire are clearly different lengths but I don't have any issues.

I attribute this a great deal to my situation where max charge/discharge is <0.1C / cell. Often below the 0.05C /cell range. I believe the C level contributes to how pronounced one sees the affects of batter bank wiring. Very low C is more forgiving whereas higher C exacerbates any wiring issues. In my case, I call <0.1C low... not sure what the curve of xC vs issues looks like but I'm thinking >0.5C get's more in this range where things matter more.

 

[Mattb4]

In my opinion the place that gives the most problems is the terminals, fasteners (bolts, washers or screws) and not the wire length. The difference in resistance of 6 inches to a few feet of copper wire is so slight as to be negligible. However a bad terminal crimp, some corrosion or a loose connection can cause unequal charging.

 

[12VoltInstalls]

will be a bit of a contrarian on how big of a deal it is. Most LiFePO4 batteries have a cycle life of ~4000 (or more). If the batteries do a full cycle every day, that works out to ~11 years. It is the rare solar system that is fully cycled every day, so the actual cycle life is probably closer to 15 or 20 years (or more). If one of the batteries wears 20% faster, that is still 12 to 16 years (or more)....and even then, it has 80% of its capacity. The battery is still usable, just not at it's full capacity. Consequently, even with uneven wear, the calendar life of the battery is likely to get you before the cycle life

I understand what you are saying; makes sense.

agree that care should be taken to try to balance the load/charge on the batteries. In most cases, it is relatively easy to do so why not do it?

That is the overriding factor imho

That’s fine if they are sized for the full load and fused correctly…..
IMHO- not a big deal

It is a big deal
Your cabling, however, is fine.

Yes ofc it's a big deal, I meant the difference between my setup and a setup with all equal lenght cables. Thanks for the input.

Filterguy’s comments fully withstanding, the length difference you described is not a literal battery balance concern anyway, and offgridinthecity’s situation is not either; the ‘system’ resistance pos(+) to neg(-) to his busbar/inverter load is indiscernible by his batteries because in practical terms it is ‘one’ load.

Yes all cables are over sized a bit. The issue I was concerned was uneven charge/discharge. I see it's not that big of a deal.

With oversized cables (I do that, too) - unless one has insanely long lower-voltage DC runs on one battery and super-short on another- the oversized cables shrink any resistance factors in nominal/practical use.

I appreciate @FilterGuy ‘s comments because I can get ocd on little things and that isn’t necessarily beneficial.

 

[DIYrich]

"Equal length" is more "equal path". What I mean is that the positive and negative cables don't have to be the same length, but the sum of the two should be.

Say battery A has a positive cable length of 10 inches, and a negative length of 20 inches, and battery B has a positive cable length of 20 inches, and a negative length of 10 inches, they both have the same path length, and should be good.

Look up cabeling parallel batteries in a rack system. Battery 1 is the first battery on positive, and last battery on negative.