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Battery cable length - from Lynx distributor

SenileOldGit

SenileOldGit

Solar Enthusiast
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Oct 15, 2022
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My electrician is installing my Voltacon 5kW inverters in a new shed I built for them - does it matter that the 70mm cables shown are different lengths? Or are they so thick that the resistance difference won't affect the voltage that each inverter sees from the three Pylontech US5000 batteries I have? (Which will be fitted to the Lynx Power In on the right, next week.)

DSCN80831920.jpg
 
FilterGuy said:
Please explain why.
Click to expand...
Voltage drop is affected by size of conductor, length of conductor, and amperage flowing through the conductor.
When the two inverters are connected together (acting as one system).
If in parallel, the unit seeing the higher battery voltage will carry more of the load.
If in series (split-phase), when the voltage difference is enough. The units will error and shutdown.
 
How much of a voltage drop will there be through a 70mm cable, that short distance? What will the difference between the two sets of cables be? I imagine it will be miniscule.
 
timselectric said:
It does matter.
How much it matters, depends on how much load you have running.
Click to expand...
Saying "It does matter" and then "How much, it depends" is ludicrous. It might matter a tiny, tiny amount, so little that it doesn't affect the inverters at all... And I even gave you a clear photo of the length difference of the cables, and the (massive, 70mm) size of them...

Do you think my inverters will be able to detect 0.00027245V difference on the rare occasions (probably never) that I'm taking 10kW from my three batteries? (I set my inverters to charge at a max of 80A, so even on a sunny day, the voltage drop will be even lower than what's shown in the screenshot below.)

Voltage drop in 70mm cable.PNG

EDIT - THE ABOVE SCREENSHOT IS INCORRECT, see screenshot in my post below.
 
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SenileOldGit said:
How much of a voltage drop will there be through a 70mm cable, that short distance? What will the difference between the two sets of cables be? I imagine it will be miniscule.
Click to expand...
According to the picture. The longer conductors appear to be twice the length of the shorter ones.
So , quite a bit of voltage drop at full load.
 
SenileOldGit said:
My electrician is installing my Voltacon 5kW inverters in a new shed I built for them - does it matter that the 70mm cables shown are different lengths? Or are they so thick that the resistance difference won't affect the voltage that each inverter sees from the three Pylontech US5000 batteries I have? (Which will be fitted to the Lynx Power In on the right, next week.)

View attachment 188897
Click to expand...
You should ask the manufacturer.
 
timselectric said:
According to the picture. The longer conductors appear to be twice the length of the shorter ones.
So , quite a bit of voltage drop at full load.
Click to expand...
What does "quite a bit" mean? It's there in my screenshot - one foot of cable has a voltage drop of 0.00027245V... The fact that the longer cables are twice the length of the already short shorter ones is not as important as how much longer they are, which is about a foot. So can any inverter notice a voltage difference of 0.00027245V between battery cables?
 
SenileOldGit said:
Saying "It does matter" and then "How much, it depends" is ludicrous. It might matter a tiny, tiny amount, so little that it doesn't affect the inverters at all... And I even gave you a clear photo of the length difference of the cables, and the (massive, 70mm) size of them...

Do you think my inverters will be able to detect 0.00027245V difference on the rare occasions (probably never) that I'm taking 10kW from my three batteries? (I set my inverters to charge at a max of 80A, so even on a sunny day, the voltage drop will be even lower than what's shown in the screenshot below.)

View attachment 188939
Click to expand...
I’m sure the cable diameter size is not 70mm, that is 2.75 inches.
 
timselectric said:
If in series (split-phase), when the voltage difference is enough. The units will error and shutdown.
Click to expand...
If the cables are properly sized, I would think that would be very unlikely. For the OPs set-up the difference due to the wire lengths will probably be millivolts. In fact, I would not be surprised if the variances in fuses, lugs, connectors, etc would be greater than the resistance difference of a couple feet of 70mm cable.
timselectric said:
If in parallel, the unit seeing the higher battery voltage will carry more of the load.
Click to expand...
Will it? There is a lot of dynamic circuitry between the battery cable and the AC lines. Each inverter will draw whatever current it needs to supply the load it sees. If one inverter is seeing a slightly lower voltage, it will draw slightly more current to compensate.

Everyone is different, but if I were the OP, I would not worry about it.
 
SenileOldGit said:
How much of a voltage drop will there be through a 70mm cable, that short distance? What will the difference between the two sets of cables be? I imagine it will be miniscule.
Click to expand...

Given that the cables appear twice as long, twice as much as the other.

SenileOldGit said:
Saying "It does matter" and then "How much, it depends" is ludicrous. It might matter a tiny, tiny amount, so little that it doesn't affect the inverters at all... And I even gave you a clear photo of the length difference of the cables, and the (massive, 70mm) size of them...

Do you think my inverters will be able to detect 0.00027245V difference on the rare occasions (probably never) that I'm taking 10kW from my three batteries? (I set my inverters to charge at a max of 80A, so even on a sunny day, the voltage drop will be even lower than what's shown in the screenshot below.)

View attachment 188939
Click to expand...

FIRST you argue with a career electrician...

THEN you demonstrate you have no clue how to use a tool and post a result nearly two orders of magnitude wrong.
 
SenileOldGit said:
Sorry - my mistake.

The voltage drop is 0.0118339V - I don't think the inverters are going to notice that either.
Click to expand...

timselectric said:
Voltage drop is affected by size of conductor, length of conductor, and amperage flowing through the conductor.
When the two inverters are connected together (acting as one system).
If in parallel, the unit seeing the higher battery voltage will carry more of the load.
If in series (split-phase), when the voltage difference is enough. The units will error and shutdown.
Click to expand...

This.

@SenileOldGit you need to get an electrician that knows what they're doing.
 
Appeal to authority fallacy.

I do have a clue 'how to use a tool', I just made a simple error because I always refer to the cable as '70mm' rather than '70mm squared', no big deal.

So are you saying there will be a problem with this setup? If so, why? Based on that hundredth of a volt difference? How much more of the load will the inverter seeing the higher voltage carry? We are dealing with a hundredth of a volt difference at voltages around 48V.
 
SenileOldGit said:
Sorry - my mistake.

View attachment 188947

The voltage drop is 0.0118339V - I don't think the inverters are going to notice that either.
Click to expand...
My uncalibrated eye measures about 4' more cable in the total round trip. That would make the voltage drop difference about .05V. Not enough to worry about and probably far less than voltage drops on the fuses, connectors, etc. Even if you double the calculated drop and call it .1V, it is not enough to worry about.
 
SenileOldGit said:
Appeal to authority fallacy.
Click to expand...

When the authority is known to understand this issue, is consistent with education and training materials, and has implemented their own major system, it's not a fallacy.

SenileOldGit said:
I do have a clue 'how to use a tool', I just made a simple error because I always refer to the cable as '70mm' rather than '70mm squared', no big deal.
Click to expand...

Simple or complex error doesn't matter. You aren't paying enough attention to know that 70mm is not a diameter but an area, and blind reliance on a tool because it gives you the answer you want is confirmation bias.

SenileOldGit said:
So are you saying there will be a problem with this setup? If so, why? Based on that hundredth of a volt difference?
Click to expand...

The consequences have been stated. It's an inherent system imbalance meaning the inverter connected via shorter wires will carry more of the load. Will it be a problem? Maybe. Maybe not. Is it consistent with best practices? Nope.

Funny how everybody understands how important it is at that each parallel battery have the same length of conductors, but for some reason the length of the cable to the devices sharing loads suddenly doesn't matter?

If you see your electrician using oversized and/or unequal length for the AC side, fire them on the spot. Same reason but notably more significant.
 
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