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Battery cables & wiring questions to Sol-Ark 15k?

RTL44

RTL44

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Hi,
Although I have learned a lot from research and this forum, I am still pretty new to this. I finally was able to purchase 8 - 48V LiFePO4 server rack batteries (5.12 kWh each) to use with my Sol-Ark 15k. The batteries come with 8 inch 2AWG cables per unit.

My questions are:

1) Is 2AWG adequate for connecting 8 of these together?

2) Would there be any advantage/disadvantage to putting all 8 in a server rack and then into one of the battery inputs of the Sol-Ark vs. connecting 4 into input 1, and the other 4 into input 2?

3) I assume it would be better to use bus bars in a cabinet vs. parallel/diagonal wiring. If so, what dimensions should my bus bar be to adequately handle the 8 batteries?

Thank you for any help you can provide. I live in Mexico and it is not easy to find equipment or experts in this field.

Attached are some pics/specs and a link to the manual of the batteries & the Sol-Ark manual
 

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RTL44 said:
1) Is 2AWG adequate for connecting 8 of these together?
Click to expand...
Well... the most concise answer is NO!!

EDIT: I was looking at the manual again and realized I used the wrong spec for calculating the battery draw.
1671004414229.png

The numbers are still crazy big, but not as bad as what I posted. I will update/correct the rest of this post with the correct numbers.

Sorry for any confusion.

Let's start with what draw the inverter might have. In the manual we find this:

1671004503098.png

If we assume a 95% efficiency, that means the batteries could be called on to provide a total of 12000W/.95 = 12632W.

The worst case scenario is when the batteries must supply this when they are low and the battery voltage has dropped to 48V. In this scenario,
the batteries must provide 12632W/48V = 263A.

Let's assume you wire like this:

1671001167910.png

Each of the batteries have a 100A breaker, so you could get away with 4AWG or even 6AWG between the battery and the busbar.

The wires between the busbar and the inverter are another story alltogether.

Since you never want to run a fuse at 100% of its trip value, the fuse should be 263A x 1.25 = 329A. You might find a 350A fuse, but it is more likely you will have to go to a 400A fuse (We are getting into crazy big numbers here).

The wires from the busbar to the inverter would need to be big enough for 400A!!!!

1671001463370.png

If you found and used 4/0 AWG 105C wire, it would work but that is not something I would ever build. 400A circuits is beyond what I am comfortable with.

RTL44 said:
2) Would there be any advantage/disadvantage to putting all 8 in a server rack and then into one of the battery inputs of the Sol-Ark vs. connecting 4 into input 1, and the other 4 into input 2?
Click to expand...
I think my previous answer gives a hint about the answer to this. By splitting the batteries we can cut the current on the wires in half.

1671002322768.png

Now the worst case on the wires from the bus bar to the inverter is 263A/2 = 132A and each fuse needs to be 132 x 1.25 = 164A. You might get might find a 175A fuse, but you will probably need to go to a 200A fuse That is still a lot of current, but it is at least out of the bat-shit-crazy zone. If we look at the chart we see that 1/0 90C wire would work.

Edit: Clarified that I was talking about 90C-rated wire in the above paragraph.

This is probably why solarc provides two inputs.

RTL44 said:
3) I assume it would be better to use bus bars in a cabinet vs. parallel/diagonal wiring. If so, what dimensions should my bus bar be to adequately handle the 8 batteries?
Click to expand...

I will assume the batteries are grouped in two sets. With that, each busbar needs to be able to carry up to 200A.

The following table gives the ampacity for rectangular copper busbar.

1671003619043.png

In order to keep the batteries balanced, it is best to oversize the busbars.
Since we will be drilling holes in the bars, I like wider busbars instead of thick busbars.
I find 1/4" to be the thickest I like to work with, but I prefer 3/16" or 1/8" thick.

I do not know what the mounting constraints are but in the absence of other constraints, I would recommend something like 3/6"x1"
EDIT: After thinking about it for a bit, I think I would go with 1/4" x 1". This would keep any imbalance between the batteries minimized and it would ensure the busbars remain reasonably cool.
 

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The Sol-Ark already has built in breakers on both of the battery inputs (which you need to use both of), so no need to fuse those wires. Fuses/circuit breakers for each individual battery are still recommended though.
 
niktak11 said:
The Sol-Ark already has built in breakers on both of the battery inputs (which you need to use both of), so no need to fuse those wires. Fuses/circuit breakers for each individual battery are still recommended though.
Click to expand...
The Fuses are needed to protect the wire BEFORE the inverter. If a short happened someplace between the bus bars and the inverter, there would be significantly more than 400A through the short before the battery breakers start popping. I maintain my recommendation of putting a fuse on the wire as close to the bus bar as possible.

There is debate about whether the breakers that are built into the batteries are adequate to protect the circuit from the battery to the busbars.
 
RTL44 said:
Thank you for any help you can provide. I live in Mexico and it is not easy to find equipment or experts in this field.
Click to expand...
Here's a video, adding fusing to a battery rack.
If you have busbar stock available and racks that are similar to his, this might be an option.
I haven't installed any yet, so far I've kept the opposite cables separate as much as possible. I'll be replacing that white sch40 flex pipe with 1½" Liquid-tight conduit in the future too.
 
I used 4/0 on everything
 

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RTL44 said:
OK..Thanks. Better safe than sorry. I'll go with the 4/0
Click to expand...
It’s like buying gold but I have very little cable loss and almost no wire heat. I measure about 5 degrees temp increase when charging at 210a over a couple of hours. All cable lengths are equal.
 
RTL44 said:
Thanks you also!! Great setup, too. The pictures are helpful. Are you using 4/0 wire from your buss bars to your 15k?
Click to expand...
Yes, I used double runs of 4/0 battery cable (very flexible). I'm still looking for a simple and clean way of adding fusing.
Just not liking the thought of more connections and parts, when I'm only going a short distance through a cement block wall.
So, while trying to protect against shorting opposite cables, I'd be creating more areas where a fault could occur. Bolting a fuse block straight to my bus bar might be an option :unsure:
 
Spunky said:
Yes, I used double runs of 4/0 battery cable (very flexible). I'm still looking for a simple and clean way of adding fusing.
Just not liking the thought of more connections and parts, when I'm only going a short distance through a cement block wall.
So, while trying to protect against shorting opposite cables, I'd be creating more areas where a fault could occur. Bolting a fuse block straight to my bus bar might be an option :unsure:
Click to expand...
I did it this way but keep in mind these are not class T fuses which I probably should have but I am fused at each battery, at the bus bars and at the inverter. All battery terminals hydraulic crimps, and torqued to specs. I also have temp monitoring on my wire from the inverter to the bus bars. One precaution I have taken is the installation of a new fire extinguisher and fire blankets close but separate.
 

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I went ahead and e-mailed Sol-Ark about this and here was their response:

Me:
I have ordered 8 - 48v LiFePO4 100A server rack batteries to use with my 15k.

2 questions:

1) Is it better to use 2 banks of 4 batteries (1 to each battery input terminal) or all 8 into 1 input?

2) Is 2/0 awg wire ok to run from bus bars from battery banks, or should I use 4/0? I assume the 15k will accommodate 4/0.

Sol-Ark:
Thanks for reaching out to Tech support.

If you have the battery rack the best approach is to send 2 positives (2/0) and 2 negative conductors (2/0) from the rack busbars into our unit, in that way you can use both 200A breakers and achieve max current flow.
 
FilterGuy said:
each fuse needs to be 132 x 1.25 = 164A. You might get might find a 175A fuse, but you will probably need to go to a 200A fuse
Click to expand...
RTL44 said:
Sol-Ark:
Thanks for reaching out to Tech support


If you have the battery rack the best approach is to send 2 positives (2/0) and 2 negative conductors (2/0) from the rack busbars into our unit, in that way you can use both 200A breakers and achieve max current flow.
Click to expand...
It is gratifying to see that my numbers are in line with theirs. I always worry I might get it wrong and give someone bad advice.
RTL44 said:
OK..Thanks. Better safe than sorry. I'll go with the 4/0
Click to expand...
Assuming it fits and if it's not too expensive, 4/0 is better, but 2/0 should work fine if the batteries are split or you have two lines from a single battery bank.

1671245744590.png

I like to use the very flexible 90c Marine wire and technically 1/0 would work with that. However, I would almost certainly step up to 2/0 to give it plenty of headroom. (The expected max continuous current is 132A and 2/0 90c Marine wire is caple of 285A.... over 2x headroom)
 
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FilterGuy said:
It is gratifying to see that my numbers are in line with theirs. I always worry I might get it wrong and give someone bad advice.

Assuming it fits and if it's not too expensive, 4/0 is better, but 2/0 should work fine if the batteries are split or you have two lines from a single battery bank.

View attachment 125051

I like to use the very flexible 90c Marine wire and technically 1/0 would work with that. However, I would almost certainly step up to 2/0 to give it plenty of headroom. (The expected max continuous current is 132A and 2/0 90c Marine wire is caple of 285A.... over 2x headroom)
Click to expand...


Yes and thanks again. I May still use 4/0 just to be sure and allow room to expand, but I am truly humbled by you and others who willingly take time to help and educate others. I’m a retired schoolteacher, so I’ve always tried to share what I know with others. To be the recipient of that is really great.
 
RTL44 said:
I May still use 4/0 just to be sure and allow room to expand,
Click to expand...
I hear ya. When dealing with that much current, being safe is way better than being sorry.
Just be sure you have a quality crimp tool and everything is properly tightened down.
 
FilterGuy said:
In regard to the attached diagram, (which is how I will be doing mine), is it still important to use the diagonal wiring of the 4/0 leads to the Inverter even though they are coming off of the bus bars and not the top/bottom battery terminals? I ask to be sure my 4/0 leads are long enough.
Click to expand...
 

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In regard to the above diagram, (which is how I will be doing mine), is it still important to use the diagonal wiring of the 4/0 leads to the Inverter even though they are coming off of the bus bars and not the top/bottom battery terminals? I ask to be sure my 4/0 leads are long enough.
Click to expand...

Is it better to do the diagonal? Yes. Without a doubt.

Is it important to do the diagonal? That is a more difficult question to answer.
  • The larger the bus bars, the less the voltage drop across them will be and the importance of the diagonal connection diminishes.
  • At some point other factors such as the internal resistance of the BMSs can be a larger factor than the advantage of diagonal wiring.
  • The objective of doing the diagonal is to ensure a more even wear on the batteries. However, if the batteries are only cycling once per day, the calendar life of the battery may get you before uneven wear does.
If you are buying all those batteries, the cost of the extra copper should not be that big of a deal. Consequently, unless there is some other physical constraint preventing the diagonal hook up.... I would recommend doing it as a 'better-safe-than-sorry' move.
 
FilterGuy said:
Is it better to do the diagonal? Yes. Without a doubt.

Is it important to do the diagonal? That is a more difficult question to answer.
  • The larger the bus bars, the less the voltage drop across them will be and the importance of the diagonal connection diminishes.
  • At some point other factors such as the internal resistance of the BMSs can be a larger factor than the advantage of diagonal wiring.
  • The objective of doing the diagonal is to ensure a more even wear on the batteries. However, if the batteries are only cycling once per day, the calendar life of the battery may get you before uneven wear does.
If you are buying all those batteries, the cost of the extra copper should not be that big of a deal. Consequently, unless there is some other physical constraint preventing the diagonal hook up.... I would recommend doing it as a 'better-safe-than-sorry' move.
Click to expand...
One more thing.

The more batteries, the more value a diagonal connection offers. If there are two batteries, hooking them diagonally fully balances the load. For each additional battery, the balance gets worse. With more than two batteries in parallel, a diagonal is still a lot better than a 'straight' connection, but not 'perfect'.

There are various wiring techniques to keep things perfectly balanced above 2 batteries, but at some point, it is just not worth it.

Also, there are two diagrams above. One with 8 batteries in parallel, the other with 2 parallel banks of 4 batteries. 2x4 arrangement is better balanced than the 1x8 arrangement.
 
Have you considered not having two separate bus bars instead one longer bus bar and landing your Sol Ark connections on each end with the batteries attached in the middle. This is how I have mine and my batteries all show fairly well balanced cells and voltages. I have had zero battery charging issues and they all stay fairly close when charging and discharging. I have 4 Trophy's as you know. 3 of the 4 came in at 230Ah and one came in as 237Ah so I see a little more variance with that one but the rest are pretty much in lock step
 
jfpetesn said:
Have you considered not having two separate bus bars instead one longer bus bar and landing your Sol Ark connections on each end with the batteries attached in the middle.
Click to expand...
I am not following. Could you do a drawing?
 
Yes when I get a min but in your drawing you show 2 positive and 2 negative bus bars with the two positives and two negatives each getting a feed out of the SolArk. I suggest you drop that down to 1 positive and 1 negative longer bus bar and bring the two positive and two negatives out of the Sol Ark and attach them to each end of your bus bar. Then attach your batteries in the middle. That way you are feeding power from each end and the batteries are drawing from the middle. In my mind the bus is energized from each end equally across the entire length. If I energize a bus from one end then the energy at the furthest point will be slightly less. If you energize both ends then potentially the middle of the bar would be the less energized but In actuality I believe you are providing the most energy possible to attached batteries by energizing both ends of the bus. I believe this is the theory behind X configuration. I am not an electrical engineer but if you think about a water pipe. The back bath at the end of the line has less pressure. If you brought another pipe from your water meter to that back bath then you would be feeding the pipe from both ends and you would have significantly better pressure on all faucets. And if you had any pressure differential it would be in the middle not at the end of the line. Just my thoughts.
 
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