unequal length battery wire

[1201]

say I have 3 280ah 1s packs and the positive cables from each one to the main bus bar are different lengths, eg 1ft, 2ft and 3ft - is this bad for a battery system? and what will happen if this is done? thanks

 

[Texas-Mark]

As long as the cable gauge is sufficient for the current, it won't matter for those lengths.

 

[1201]

As long as the cable gauge is sufficient for the current, it won't matter for those lengths.

Hey thanks I appreciate that!

 

[BentleyJ]

As long as the cable gauge is sufficient for the current, it won't matter for those lengths.

I would substitute the term "sufficient" for "oversized" such that variation in cable resistance is so small it becomes negligible.

 

[Tulex]

I've actually wondered the same, as I've read so many times about keeping the lengths the same.
I've got 2 batteries connected that have equal cables, decided to add a third, and this will require the cables to be about a foot longer.

 

[Tulex]

I would substitute the term "sufficient" for "oversized" such that variation in cable resistance is so small it becomes negligible.

How does it affect the outcome if they aren't sufficient? Not an issue for me, I'm oversized with my cables, but just want to understand.

 

[Texas-Mark]

How does it affect the outcome if they aren't sufficient?

Too small and you get a larger voltage drop across the cables (the longer the cable the higher the drop). Thus you can get a slightly out of balance situation. Unless they are seriously undersized, it still won't make that much of a difference. More likely to have bigger drops inside of the batteries themselves (i.e. slightly different voltage drops across the BMS).

 

[BentleyJ]

The total resistance of an electrical circuit is the sum of all the components. Internal battery resistance (which is very low for Li chemisty), wires, crimp lugs, terminals, bus bars, connection at the inverter and internal resistance.

The point being to oversize the cables such that the contribution of voltage drop in the different length cables is a very small percentage of the total circuit losses.

 

[1201]

I would substitute the term "sufficient" for "oversized" such that variation in cable resistance is so small it becomes negligible.

Hi. If I want to use a 125a breaker, what size wire do you recommend? Longest run would be 4 ft? Would 1/0 be sufficient?

 

[BentleyJ]

1/0 THWN building wire is rated for 150 Amps @ 75C. so would be a good match up with a 125A breaker.
I would recommend 1/0 welding cable because its so much more flexible and easier to work with due to the fine stranding.

 

[1201]

1/0 THWN building wire is rated for 150 Amps @ 75C. so would be a good match up with a 125A breaker.
I would recommend 1/0 welding cable because its so much more flexible and easier to work with due to the fine stranding.

Thanks very much for that! welding wire is definitely a lot easier to work with. thanks!

 

[BentleyJ]

If you are going to use compression lugs on the battery cables, keep in mind welding cable expands more when stripped so the lugs need a slightly larger ID. Sometimes these oversized lugs are easier to find if you use the term DLO cable in the search.