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Parrallel connection of batteries - is there a correct and incorrect way?


flyfisherman

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Hi,

 

When batteries are in parallel, does it matter if the positive and negative from the same battery is connected to the inverter? Or is it better to have the positive from one battery and the negative from the other connected? I have read mixed reports.

 

Refer to images below. First image is from the Dyness manual, it clearly states the correct way according to them is to have the positive from one battery and the negative from the other connected to the inverter. (so 3 wires max in/out of a battery if bank of two batteries - either 2 in positive and one in negative or vice versa)

 

Now see the images below that - this is from the installation that was done at my house. The installer here clearly connected positive and negative from the same battery to the inverter. (First battery 4 wires in / out, 2 each on positive and negative and second battery one wire into positive and negative respectively)

 

Both are still technically connected in parallel, and dip switches seem correct as they system is functioning as it should, but I am wondering if the installation as I have it now could void warrantee or are there any other unforeseen balancing issues etc

 

How much does wire resistance play a role here in terms of both batteries depleting and charging equally? I have done some reading on this and I get mixed reports - some say no problem some say it is a problem. 

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1.JPG

Edited by flyfisherman
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32 minutes ago, Nexuss said:

You definitely want it like the manual is showing. Currently your one battery will put in much more work and it gets worse the more batteries you have and the larger currents you pull.  Eventually they get very unbalanced . Will Prowse did a video about that a while back . https://youtu.be/oXBT8lSC25M

 

As stated in the video on large banks with a lot of batteries it is a problem. When only using 2 it is still great to keep both the leads of the same length. 

When looking at his video one will see the 3 or more banks also have different lengths of cable. Further by having fuses in only the positive lead the fuse resistance can be as high as 8m cable. Thus his positive cables have a resistance much higher than the negative. We just don't see the extra resistance. 

A great video to indicate how one applies over current protection to each battery and not the common reason mostly given that you add fuses to protect only the cable to prevent a fire. 

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24 minutes ago, Scorp007 said:

As stated in the video on large banks with a lot of batteries it is a problem. When only using 2 it is still great to keep both the leads of the same length. 

When looking at his video one will see the 3 or more banks also have different lengths of cable. Further by having fuses in only the positive lead the fuse resistance can be as high as 8m cable. Thus his positive cables have a resistance much higher than the negative. We just don't see the extra resistance. 

A great video to indicate how one applies over current protection to each battery and not the common reason mostly given that you add fuses to protect only the cable to prevent a fire. 

Thanks for the info.

Out of curiosity, when you say "both leads the same length" what exactly do you mean? When looking at the Dyness install instructions, the lenght of the + lead and - lead is not the same for the top battery for example? The top battery in the image has a shortish - black wire running to the breaker directly and the + red cable distance of that top battery is much longer to the breaker as it goes via the short sections via the other batteries? So when people say cables should be equal, length, what are they referring too? To each battery the + and negative length cable should be the same or the total sum length of + and - cables of the batteries together should be the same? (sorry hope this makes sense what Im trying to ask?)

Edited by flyfisherman
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48 minutes ago, flyfisherman said:

Thanks for the info.

Out of curiosity, when you say "both leads the same length" what exactly do you mean? When looking at the Dyness install instructions, the lenght of the + lead and - lead is not the same for the top battery for example? The top battery in the image has a shortish - black wire running to the breaker directly and the + red cable distance of that top battery is much longer to the breaker as it goes via the short sections via the other batteries? So when people say cables should be equal, length, what are they referring too? To each battery the + and negative length cable should be the same or the total sum length of + and - cables of the batteries together should be the same? (sorry hope this makes sense what Im trying to ask?)

It is seen as the total length of cable. If you look at the picture in your post the left side as per manual is the better way by a very small margin. 

Your install the battery on the right has longer cables than the one on the left as power goes from inverter to the 1st battery and only then to the 2nd battery. Thus 2nd battery will do less work and take a bit longer to charge. Having only 2 batteries it is not a train smash. The 2nd one will catch up when charging and also discharge well. 

This is like so many things in solar we always want the BEST of everything. 😀😀

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9 minutes ago, Scorp007 said:

This is like so many things in solar we always want the BEST of everything. 😀😀

I think this is the case not only for solar but "modern" human nature for everything :) 

 

Thanks for the input and advice over the last days for my system setup, much appreciated.

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3 hours ago, flyfisherman said:

the lenght of the + lead and - lead is not the same for the top battery for example?

That has absolutely no importance to anything. Understanding electric circuits!

Another story is the the difference of lead lengths from different batteries to the inverter. I have written about it in other threads. The importance is overemphasized by most. People assume that the internal resistance of same type batteries are equal. My experience shows that this assumption is wrong. I have 4 packs of same type batteries acquired sequentially, wired in a row. The newest and last in row therefore has the longest leads. Yet it takes the highest charge and discharge currents. Apparently my newest pack benefits from improved manufacturing with the lowest internal resistance. That proves that differences of internal resistance between batteries might be higher than the resistance difference of unequal lead lengths.

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8 minutes ago, Beat said:

That has absolutely no importance to anything. Understanding electric circuits!

Another story is the the difference of lead lengths from different batteries to the inverter. I have written about it in other threads. The importance is overemphasized by most. People assume that the internal resistance of same type batteries are equal. My experience shows that this assumption is wrong. I have 4 packs of same type batteries acquired sequentially, wired in a row. The newest and last in row therefore has the longest leads. Yet it takes the highest charge and discharge currents. Apparently my newest pack benefits from improved manufacturing with the lowest internal resistance. That proves that differences of internal resistance between batteries might be higher than the resistance difference of unequal lead lengths.

OK so what you are saying? It does not really matter then in what parallel configuration they are wired...sequentially or "diagonally" as in the manual?

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1 hour ago, flyfisherman said:

It does not really matter then in what parallel configuration they are wired...sequentially or "diagonally" as in the manual?

That's what is my opinion. And it is also my experience that unequal recharging of packs due to such difference is automatically equalized when they reach full charge. Provided that the BMS do not intervene with false information to the inverter.

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3 hours ago, Beat said:

That's what is my opinion. And it is also my experience that unequal recharging of packs due to such difference is automatically equalized when they reach full charge. Provided that the BMS do not intervene with false information to the inverter.

100% my feeling as well but not what the clever people want us to belief. 

Just look at house wiring where the N is much shorter than the L going down to light switches and back and all circuits and equipment function correctly. AC has nothing to do with it functioning correctly. The load just gets a voltage to do the work needed. 

We do however try to give each battery as close as we can the same voltage to charge and to discharge. It is the run between battery and inverter we want to keep as short as practical possible but need not be measured to the mm/cm the same length.

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5 hours ago, Superfly said:

BTW I found this quite interesting as well ... I do think running at 1C  or say 200A charge/discharge is what they seem to focus on  - I don't have the need or cable thickness to exceed 80A .. so probably a moot point.. others tho' may find a need for it.

Well, one can exaggerate everything. Who wants to charge with 1C? I set my inverter to charge from utility with 0.1C to preserve battery life. PVs charge at rare peaks 0.2C for short times. That's 80A on 25mm² cable, capable of 160A continuously. My inverters have total 10kW max power. The cooks very rarely manage to pull 8kW for short moments. That's 160A or 0.4C on the batteries if nothing is coming from PVs. Most of the time the load runs between 350W and 3.5kW.

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Thans again to everyone for their valuable advice over the last couple of days on the two threads I posted. I know exponentially more about the system I have installed now from your feedback and other threads on here. Much appreciated. Realy.

Just makes me wonder why installers, who are highly recommended etc. and have all the certifications etc do things in certain ways and not according to manufacturer spec. This particular installer, when asked about why the batteries don't charge to 100%, said "ah these Dyness sometimes go to 90% and sometimes to 80%, its just the way they are made" .... something we have determined is clearly not the case, but rather was the settings on the inverter.

Anyways, thanks again for all the advice and input. I think just to be safe, I will get them to come re-wire as per Dyness manual/spec, just to hopefully cover my own arse should there be a warrantee claim one day because Dyness seem to require pictures of the wiring for warrantee registration.

Edited by flyfisherman
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