Jump to content

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


PowerUser
 Share

Recommended Posts

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?

Link to comment
Share on other sites

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?

Link to comment
Share on other sites

Posted (edited)
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
Link to comment
Share on other sites

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.

The shortest possible wiring to your inverter for sure. Otherwise losses. The losses are your responsibility. How much of it you accept is your problem. Does this affect how the battery will discharge? No, they will discharge in a balanced way. 

You won't find an official document about this because it's a well taken for granted scientific fact. You set this up & play around with it & the results will be revealed. 

The same is true about so many things Solar. The proof is in the eating of the pudding. The idea is to ask as many bakers as possible & decide for yourself based on good judgement.

87

 

 

Edited by 87 Dream
Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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
spelling
Link to comment
Share on other sites

We are merely stating the fundamentals. I think you already have the answer with many guys posts. You accept the losses if you want the longer cable go ahead. The same length not a requirement. The shortest possible is best due to losses in terms of resistance & as @Sarel has said all these things add up. You will not find any official document stating this as it's taken as something learnt at School Physics or even at Uni level Physics 154.

87

 

Link to comment
Share on other sites

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
Link to comment
Share on other sites

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
Link to comment
Share on other sites

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

Link to comment
Share on other sites

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!

Link to comment
Share on other sites

3 minutes ago, LeeMun said:

Hi,

I think this attachment will help. 

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

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

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

Link to comment
Share on other sites

3 minutes ago, LeeMun said:

Full Page. This is from a Manual - 5kVA/5kW Axpert inverter. 

8C628A15-A4E0-449F-AB39-24903B69533A.jpeg

Again, this requirement is for paralleling batteries and inverters. Nothing to do with the thread topic. 

Link to comment
Share on other sites

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. 

Link to comment
Share on other sites

Posted (edited)
8 hours ago, Warlok said:

Yes

Why? Any technical justification of your answer?

I still haven't seen even one "keyboard expert" above to be able to technically justify the "yes" answer.

Edited by PowerUser
Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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

To answer the question: No, lengths don't have to be the same.

Deep in the same manual the below is stated about resistance. 

So horses for courses here gentleman & ladies. I would also add with the tiniest pitch of salt. Nobody here is getting paid to provide your answers to your question. It's on a public forum & it's not a subscription required to answer.

Let's all be free on this forum to post & contribute. By telling people: read the question & if they haven't nothing valuable to add is a tad harsh.

Because so many times the wrong information can lead you to the correct information, so I for one actually enjoy every person's contribution right or wrong.

I learn from either.

87

 

Screenshot_2021-08-17-09-31-25-29_f541918c7893c52dbd1ee5d319333948.jpg

Link to comment
Share on other sites

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. 

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...