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Do the positive and negative cables from the battery to the inverter need to be the same length?


PowerUser
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On 2021/08/14 at 12:48 PM, Quwatush Shams (Suly) said:

Yes,same length (shortest as possible if makes sense) and size

Shortest possible and same size is given but that's not what I'm asking for.

Why the same length would "make sense"? I've been looking around but failed to find a proper argument supporting "makes sense". Could you perhaps have a proper technical explanation, you can point me to?

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7 hours ago, branderplank said:

This is the general accepted practice and is mentioned in most manuals etc.

Just wondering if there is any authority on this that can be specifically cited?

With modern day Lithium BMS systems, is this still the case? Or will the BMS manage any problems due to wire length?

"General accepted practice" by who? Which manuals? My battery manual does not mention this. Yes, when connecting batteries in parallel or in serial, the cable lengths between the batteries have to be equal because of Ohm's law but which manual specifies equal lengths for the positive and negative cables? Can you please point me to one?

I have been looking at Victron's recommendations and they show the following:

 

 

vic-bat.PNG.6d4d0e1ec2d660a7484de94a003529d0.PNG

 

Link to the full document: https://www.victronenergy.com/upload/documents/Wiring-Unlimited-EN.pdf. There is nowhere in the entire document anything mentioned for the +/- cable lengths.

Design 1, 3 and 4 show as correct, with different lengths for the positive and negative cables.

Besides, if you open your car hood, you will see your car battery has different lengths for the +/- as well. Did all the engineers fail in their design?

I'm just trying to understand the argument for equal +/- lengths. That's why I' asking all those questions and that's why I started this thread.

 

Edited by PowerUser
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19 minutes ago, 87 Dream said:

The goal is for both parallel paths forming the main terminals to have the same series resistance. If they don't, the load won't be distributed evenly between the batteries.

The Victron pic is showing this exact fact.

 

 

I’m not talking about the cables connecting the different batteries. Yes they have to equal. No argument about that. 
 

I’m talking about the main +/- cables to the inverter. And unless I’m blind, the Victron pic is showing different lengths.

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To my thinking no, the shorter overall length and less the resistance, the better. From a power delivery point of view, the battery to Inverter +- lengths do not matter, just the shortest length possible. The lower the resistance, the less the DC ripple will be. Remembering that every crimp, every connector and every connection add to resistance as does wire length. Ersatz, the less the better. Also the reason to use good quality cable.

Edited by Sarel
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Positive and negative cables from batteries to inverter do not have to be same length.

 

The whole same length theories is more whenever you parallel electrical circuits.

 

To be honest most of us use different length positive and negative cables from batteries to inverters.

 

Keep high current DC wires as short as possible yes, when parallel  circuits like between batteries yes keep them the same length.

 

The modern BMS will do the voltage, balancing and current monitoring for you and not so much up to the inverter.

 

Sorry see now... what Sarel above said :}

 

 

 

 

 

Edited by jlr
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@PowerUser https://www.victronenergy.com/upload/documents/Wiring-Unlimited-EN.pdf

Please read documentation above will give you all you need to know,as well as technical explaination for all our esteemed professionals🤔

Edited by Quwatush Shams (Suly)
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1 hour ago, Quwatush Shams (Suly) said:

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

Please read documentation above will give you all you need to know,as well as technical explaination for all our esteemed professionals🤔

 

wall o text.png

 

 

ALL 69 PAGES?

😆 Agreed that it's good doc, but that's why The Powerforum exists, ain't nobody got no time for that.😁

Edited by YellowTapemeasure
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On 2021/08/15 at 4:50 PM, PowerUser said:

Shortest possible and same size is given but that's not what I'm asking for.

Why the same length would "make sense"? I've been looking around but failed to find a proper argument supporting "makes sense". Could you perhaps have a proper technical explanation, you can point me to?

Hi,

I think this attachment will help. 

B9522A7C-A85E-4CD2-BEEA-519C87B79F6E.jpeg

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1 hour ago, Quwatush Shams (Suly) said:

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

Please read documentation above will give you all you need to know,as well as technical explaination for all our esteemed professionals🤔

If you bothered to read the thread, you can see, I have posted pictures and quoted the very same document, which supports that positive and negative cables DO NOT have to be the same length. Please, if you think, you can contribute to the discussion, post something worthwhile!

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2 minutes ago, PowerUser said:

This is for paralleling batteries. This thread is not for that.

Please read the thread title again and post something relevant to the topic!

& l quote - Do the positive and negative cables from the battery to the inverter need to be the same length?

Not parallel batteries but parallel inverters - you also didn’t specify hence l sent an answer regarding parallel inverters. 

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No.

The combined positive and negative cabling length of parallel batteries should be the same so that voltage drops over cabling is the same.

Which in turn means that parallel batteries are equally charged/discharged at the same rate.

The same logic would apply to other parallel devices that are likely to draw high current during operation.

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1 hour ago, 87 Dream said:

My last limited contribution to this thread before being told by the author I have nothing sensible to say 😁🥺

Lol, it’s all good @87 Dream 😁 I really appreciate your contribution, research and coming back to the thread  

I was just trying to find out if anyone can justify properly the “yes” answer. And in general, I hate it when wrong information is repeated again and again on the forums, until many take it for granted truth. 
 

But anyway, I know the answer now. 
 

No, the positive and negative from the battery to the inverter don’t have to be the same length. 

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@PowerUser In your opening post you ask battery (singular) to inverter, but then later posts pictures with batteries is parallel. Surely you can see have that could've led to people misunderstanding your question.

In the scenario of a single battery and inverter, the positive and negative cables can be of different lengths. The longer the length of the cable (Pos and Neg cables summed), the higher the resistance and the higher the voltage drop will fall over the cable (it doesn't matter whether this is on the Pos or Neg cable). As the Pos cable, battery and Neg cable form a closed, serial circuit the current will flow through all the components, equally.

Voltage (V) = Current (I) x Resistance (R)

 

image.png.c67ed048d022b7fb6e738725988c446a.png

 

If you are using 35sqr welding cable, the voltage drop over the cable will be ~0.8 milli-Volt per Amp per metre.

Example 1 - Equal cable:

Voltage drop over Pos will be = 0.8mV x 50 x 2 = 80mV = 0.08V

Voltage drop over Neg will be = 0.8mV x 50 x 2 = 80mV = 0.08V

Therefore, the Battery will "see" 50V - 0.08 - 0.08 = 49.84V

 

Example 2 - Unequal cable:

Voltage drop over Pos will be = 0.8mV x 50 x 1 = 40mV = 0.04V

Voltage drop over Neg will be = 0.8mV x 50 x 3 = 120mV = 0.12V

Therefore, the Battery will "see" 50V - 0.04 - 0.12 = 49.84V

 

So, both scenarios are the same (electrically) from battery and inverter point of view.

 

Side note:

However, the both cables should be kept as short as possible to reduce lost energy which is dissipated as heat by the cables. The power (W) lost can be calculated as P = V x I.

The higher the loss over the cable, the higher the temperature due to heat dissipation. The thinner the cable, the higher the voltage drop over the cable = more loss = higher heat, which in a worst case can lead to burning if underspec'd.

 

 

 

 

Edited by Mier
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Positive and negative cables from battery to bus bar simply serve to close the circuit and there is no need for them to be the same length at all.

The oft-mentioned statement above that they should be as short as possible is basically applicable to any cable anywhere in any system - if it is longer than necessary, then losses - however marginal - will be higher. 

Likewise, a positive and negative from the busbar to the inverter or charge controller also do not have a requirement to be the same length.

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Of course. I had also often heard the 'equal length' statement, and had never really though about it.

But if you look at a battery stack connected to an inverter, then the equal length proposition falls out straight away since the cables connecting the batteries in the stack are very short, and obviously, the cables connecting the stack to the inverter are a lot longer. If the equal length cable proposition held any water whatsoever, this would be a problem.

Thanks for the sanity.

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My current battery bank consists of 12 x 260 Ah SLA batteries wired series/parallel to 48v, so essentially 3 x 48v batteries.

Had a bloke in Melbourne make up the cables for me, the lugs are machine crimped and insulated, a veritable work of art.

All the connects and interconnects are the same length, so are the positive and negative leads to the controller, the way the controller is situated the positive lead could have been about 400mm shorter but it made sense to me for them to be equal in length.

Some things I just take as read like not disconnecting AC whilst under load, I'm assuming doing that could let the magic smoke out, not something I want to get familiar with.

So far this bank has served me well for 3 years now, when it fails I plan to replace it with a Li Ion battery (with equal length connectors)

 

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