Bus Bar to Bus Bar Question

[AlaskanNoob]

I've currently got my bus bar (two I guess, one for positive and one for negative) below my inverter/MPPT and above my batteries in a shed. Shown with the red and black boxes in this image.



The Alaska winters are crushing my batteries though despite my sad attempt at insulation. So I want to move the batteries straight down (under the floor they're sitting on now) into the root cellar below, so I can keep them above freezing year round. I'll be putting them in a custom shelf that will be super insulated and enclosed as well.

Down in the root cellar I'll hook up a second bus bar just below the floor, drill some holes, and then connect that bus to the original bus shown in the image using 4 x 4/0 cables (4 x positive bus bar cables connecting the two positive bus bars and 4 x negative bus bar cables connecting the negative bus bar). That way I can I keep the power flowing to and from the batteries evenly and can minimize loss.

The 4/0 cables connecting the two bus bars would be about six feet in length.

Any issues connecting bus bars in this way?

 

[Suijkerbuijk]

Yes you make a extra break punt .
So that cost your volts and it can heat up

The best is always use a direct connection
All lugs , bars , fuse all thake some volt and that means lost and heat

 

[AlaskanNoob]

Yes you make a extra break punt .
So that cost your volts and it can heat up

The best is always use a direct connection
All lugs , bars , fuse all thake some volt and that means lost and heat

Yes I can do it this way, but it will cost me volts and possible heat? Or am I misunderstanding?

 

[Suijkerbuijk]

Yes I can do it this way, but it will cost me volts and possible heat? Or am I misunderstanding?

Yes.
It can cost a volt drop and it can make more heat if the connection is bad .
Al parts between the wires always make some lost .
And it can always make problems.
That is the reason the best way is to set a cable direct .
For safety for the cable we set a fuse
If the same Cable have more break parts between like more busbars and connectors it make a risk..

 

[AlaskanNoob]

Yes.
It can cost a volt drop and it can make more heat if the connection is bad .
Al parts between the wires always make some lost .
And it can always make problems.
That is the reason the best way is to set a cable direct .
For safety for the cable we set a fuse
If the same Cable have more break parts between like more busbars and connectors it make a risk..

Thank you for the info, much appreciated.

 

[HRTKD]

To paraphrase, every connection has a consequence. Some are minor, some aren't. In the end, you need to do what's best for the batteries.

Minimize connections, minimize cable lengths.

 

[AlaskanNoob]

Having cables made right now. The 48V bus bars will be connected by 4 x 13 foot 250kcml cables. Lots of copper between them so voltage drop should be mitigated. Will get a thermal camera to inspect the bus bar connections and make sure no hot spots develop. The cables from the second bus bar to the batteries will remain the same ones that came from Pylontech although I shortened them a little bit. They're rated for 100A but I'll only be sending 70A max through them. They're each going through a t-fuse on the positive side (not shown below).

Not shown on Bus Bar A is that is where the MPPTs and the inverter connect. Bus Bar B will simply connect to the batteries and also to an Orion to feed a 12V bus bar.

Damn those 250 kmcl cables were expensive!

 

[AlaskanNoob]

4 of these 13 foot fat 250 cables per bus bar should hopefully make this no issue. Each cable will carry a max of 140 amps at 48V so should be negligible voltage drop and (hopefully) heat build up.

 

[HRTKD]

What's up with the squared off end of the lugs?

 

[AlaskanNoob]

What's up with the squared off end of the lugs?

No idea. The first lugs they had were "flex" lugs or something, they were also squared off. But they didn't have enough for our order so we went with these.

Do you think it being squared will be an issue?

 

[HRTKD]

Do you think it being squared will be an issue?

Seems like the amount of surface contact is less than what I have with my lugs. But yours look to be wider than mine.

 

[AlaskanNoob]

Seems like the amount of surface contact is less than what I have with my lugs. But yours look to be wider than mine.

I put these on the bus bar and the entire lug is flat on the bus bar so I think the contact should be good. They are wide for sure. Two of them on the bus bar are flush with each other and touch. But when I set it up there won't be two of them next to each other so that won't happen.

Edit: actually the bottom of the lugs hang off the bus bar just a bit and width wise they don't quite touch (the flex ones did though).

 

[AlaskanNoob]

Got it all set up and so far so good. Eight fat 13 foot cables connecting the above ground bus bar to the underground bus bar and batteries. Measured 51.2 volts on both above and underground bus bars and the batteries are happily chugging in a place where Alaska's freeze won't get them under the earth and in an insulated shelf. Eventually I'll insulate the shed too.

Fingers crossed there are no issues. Double checked all the torque settings but I'm always amazed when this stuff works.

The only potential issue I see at this point is that with this wiring, the batteries won't all be getting the exact same juice since it's not quite centered. I should have put them all together in the center although it still wouldn't be perfect. I'll just check the cell balances to see if it matters and probably re-arrange tomorrow, but hopefully the Pylontech BMS handles it tonight. Usually the difference between high and low cells is .001 or .002 and so far I've seen the difference as much as .004 volts which still seems like a small difference.

 

[altenergy21]

Instead of individual nuts, you should put a rectangular flat washer with holes drilled in it.
This way Alaskan earthquake won't shake them lose individually. If you lose one, the rest goes like dominoes.
Can't these noobs do anything right?

 

[AlaskanNoob]

Instead of individual nuts, you should put a rectangular flat washer with holes drilled in it.
This way Alaskan earthquake won't shake them lose individually. If you lose one, the rest goes like dominoes.
Can't these noobs do anything right?

Does anybody sell something like that? I really don't want to put my own DIY stuff on something so important, but it does make sense and I'll look into it. These things are torqued pretty tight, I would imagine any shaker that pulls one of those off of there is going to result in much bigger headaches for us. I'm going to secure the cables to the wall here soon too.

 

[AlaskanNoob]

Seems like the amount of surface contact is less than what I have with my lugs. But yours look to be wider than mine.

@HRTKD I noticed that a slight bit of the lug hangs off the bottom edge of the bus bar which isn't optimal. Not sure how big an issue it might present though. It's still much more contact area than what I had before with the much smaller cables and lugs, and the most amperage that will be flowing through each individual cable will be about 140 amps once I get the other arrays set up. Right now the most that will flow is about 50 amps through each cable max.

 

[altenergy21]

You need the ones with hole not the open ended ones which can falls off. If you have temperature variation it could do that.

 

[AlaskanNoob]

You need the ones with hole not the open ended ones which can falls off. If you have temperature variation it could do that.

Do you have a link to such a product so I can look at it?

 

[altenergy21]

Do you have a link to such a product so I can look at it?

Just go to hardware store, you'll see them, the end tabs with holes. For rectangular washers, I saw metal rolls with holes in them. They're soft but you could stack them up for added strength.

 

[AlaskanNoob]

Just go to hardware store, you'll see them, the end tabs with holes. For rectangular washers, I saw metal rolls with holes in them. They're soft but you could stack them up for added strength.

I'll skip these given I don't know how they'd handle the voltage and amps and such. if a solar supplier had such a product I'd give it a look. I think we'll be fine with the singular studs on the bus bar torqued appropriately.

 

[robbob2112]

S

Thread 'Calculation of parallel string battery currents'

Apr 5, 2022

Hi, everybody! I'm a retired EE. I didn't want my EE skills to fade away after retirment, so I exercise them when I can. Whenever I see a non-trivial circuit analysis problem on one of various forums I frequent, I challenge myself to solve it. Recently I watched Will's video about current sharing in a parallel battery string, and it occurred to me that the string is just a circuit. It could be solved using standard circuit analysis techniques giving theoretical values for the battery currents. I wondered if this has been done before and posted on the web. I found a link on this forum...

• Solarod
• Replies: 141
• Forum: General Battery Discussion

• 

 

[HRTKD]

Since he's running equal length cables from each battery to the battery busbar, I think the batteries will be well balanced. I don't think you can get much better than that.

The only way to get a better distribution of power would be to use a circular busbar. But that's like going from 99.9% to 100%. Not worth it.

 

[AlaskanNoob]

Since he's running equal length cables from each battery to the battery busbar, I think the batteries will be well balanced. I don't think you can get much better than that.

The only way to get a better distribution of power would be to use a circular busbar. But that's like going from 99.9% to 100%. Not worth it.

So far the max difference in voltage I'm seeing is .004v still. It fluctuates from .002v difference to .004v. It's about two times the difference I used to see back when I had it wired differently (to include the battery cable lugs doubled up on the bus bar).

 

[HRTKD]

I would be more interested in how many amps are going to/from each battery.

 

[JoeHam]

So far the max difference in voltage I'm seeing is .004v still. It fluctuates from .002v difference to .004v. It's about two times the difference I used to see back when I had it wired differently (to include the battery cable lugs doubled up on the bus bar).

I don’t see a problem with that, personally I would be on to the next project.