Equal Length Battery Cables?

[kevolones]

Regarding equal length battery to Inverter cables:

How should I go about this when I "interrupt" the cable's length with a Shunt Monitor (at negative) and Fuse+Switch (on positive)?

I understand the reason behind having same length wire is due to the resistance of the wires, but Im not sure if anything needs to be done to take in consideration the extra terminal lugs required for the shunt, switch and fuses and also if the devices themselves add any resistance/voltage drop. Are those negligible?

In that case, just sum up the wires length on the positive side and make sure its equal to the sum of the wires on the negative side?



Thanks in advance!!!

 

[kevolones]

Extra info in case it matters:

• Distance from Inverter to battery terminals is around 3'
• Conductors: 4/0 welding cables

 

[timselectric]

Only parallel cables need to be same length.
Two batteries
Two inverters
Ect.....

 

[circus]

I understand the reason behind having .... the wires length on the positive side and make sure its equal to the sum of the wires on the negative side?

I don't

 

[timselectric]

I don't

They don't

 

[kevolones]

Only parallel cables need to be same length.
Two batteries
Two inverters
Ect.....

Good to know! The battery manual mentioned the home run cables to be the same length but it could be they referred to parallel stuff.

Thanks!

 

[rmaddy]

It makes no sense for negative wires to be the same length as positive wires.

If you have a battery bank made of 6 batteries in 2S3P, for example, then the following must be true:

- The three series connections should be the same length as each other
- The two or three positive parallel connections should be the same length as each other
- The two or three negative parallel connections should be the same length as each other

None of those three sets of wires need to be the same length as each other.

If you have multiple inverters in parallel on the same battery bank then the wires to the inverters should be the same length as each other. But again, the positives do not need to match the negatives.

The main goal is equal resistance across parallel paths.

 

[timselectric]

Yes
That's misleading, if it's not in the paralleling section.

 

[TorC]

If you have an odd setup and can't get identical runs on your paralleled batteries, it's suboptimal, but not fatal to shift wire a little within a series as long as the total length is the same. Something like, for battery one: 4'-batt-1'-batt-3'=8' cable, and on battery two: 4'4½"-batt-3"-batt-3'4½"=8' cable. An oversized cable will exhibit less degradation in performance in this case than one just barely within spec.

 

[kevolones]

Thanks to everyone for the clarification!

 

[gelmjw]

Thanks to everyone for the clarification!

The lengths of the round trips matter, not the length of individual segments of circuit.

E.g, a parallel circuit with 2ft NEG and 1ft POS can be matched with another with both POS and NEG of 1.5ft. Both paths equal 3ft.

 

[jameshowison]

The lengths of the round trips matter, not the length of individual segments of circuit.

E.g, a parallel circuit with 2ft NEG and 1ft POS can be matched with another with both POS and NEG of 1.5ft. Both paths equal 3ft.

In my case I have a set of 9" red/black and a set of 12" red/black. I have two 48v LFP server rack batteries wired in parallel. I used a black 9" and a red 12" to one battery, and a black 12" and a red 9" to the other. So that seemed to mean that I have equal length round trip (ie both round trips in the parallel section are 21" long.)

Yes I'm seeing quite unbalanced charge and discharge. One battery hit zero while the other showed 40% ... Could it be related to those unequal lengths, or is it more likely that the internal resistance of one battery is different than the other (perhaps due to loose cell terminal connections?)

 

[gelmjw]

In my case I have a set of 9" red/black and a set of 12" red/black. I have two 48v LFP server rack batteries wired in parallel. I used a black 9" and a red 12" to one battery, and a black 12" and a red 9" to the other. So that seemed to mean that I have equal length round trip (ie both round trips in the parallel section are 21" long.)

Yes I'm seeing quite unbalanced charge and discharge. One battery hit zero while the other showed 40% ... Could it be related to those unequal lengths, or is it more likely that the internal resistance of one battery is different than the other (perhaps due to loose cell terminal connections?)

1. Swap the cables to see if the problem persists or, follows the cables. If the problem persists, the cables are not the cause.

2. Ensure that the batteries are fully charged by charging each one separately, not in parallel. Reconnect in parallel. If the problem returns, it was always the batteries. I don't have enough experience with LFP to add anything more.

 

[Zil]

In your diagram. The battery interconnecting cables, the light grey, must each be the same.
The positive and the negative do not need be the same. But each should be as short as reasonable.
The positive fuse should be at the battery then the switch then the "inverter/charger".

 

[Just John]

In my case I have a set of 9" red/black and a set of 12" red/black. I have two 48v LFP server rack batteries wired in parallel. I used a black 9" and a red 12" to one battery, and a black 12" and a red 9" to the other. So that seemed to mean that I have equal length round trip (ie both round trips in the parallel section are 21" long.)

Yes I'm seeing quite unbalanced charge and discharge. One battery hit zero while the other showed 40% ... Could it be related to those unequal lengths, or is it more likely that the internal resistance of one battery is different than the other (perhaps due to loose cell terminal connections?)

I have found a cheap thermal camera helps identify the problem.
It saves a lot of time finding bad connections.