House battery bank interconnection

[J.R. Good]

Hi All,

I have 8 Trojan T-105 225ah wired series/parallel for a 12v 900ah house bank. I had heard some time ago about "cross connecting" to better balance everything, and then read about it again in Nigel Calder's book. I welcome any feedback from folks who know about such things. Diagram of the existing set up attached.

Thanks very much,

Randy Good

 

[Socal_Cecil]

I don't think I would do it.

If you have an open cell in a battery instead of going from 4 series connected 12v batteries you end up with 3 series connected 12v and an extra 6v in parallel with just the one side of 6 volters. The spare battery is then going to have to push through the 3 batteries. There's a reason we like to keep the numbers even and matched.

If you have a shorted cell all the other batteries are going to try and charge it either way unless you have a fuse between each series connected battery. It would also become harder to jumper out.

If everything is working normally it will more evenly load each battery but ultimately your charge still ends up at the max of the weakest cell. You might be able to get higher max discharge later in life though but hopefully you don't get near max discharge on a house bank.

If you do it you only need the centers connected on one side. The distance between the left center and right center is so small it is all the same point electrically. Did they say why not to do it down all 4 banks instead of 2 banks of 2?

 

[DavidM]

What you have now is a form of cross connecting. The positive lead goes to one battery and the negative to another as far as possible from the first. This balances out (more or less) the voltage drop in the interconnections, so each battery is seeing near exactly the same voltage.

But FWIW, I think cross connecting is unnecessary. The voltage differences are tiny.

David

 

[J.R. Good]

Thanks for the reply. Do you know where I might find a diagram of the very best way to configure this bank? Nigel Calder's diagram only shows 4 batteries, and I have 8. I know that one bad cell anywhere spoils the pot - that's a given no matter what, but I'd like to maximize what's healthy for the batteries if possible. The physical layout of the batteries themselves has to stay as drawn
Thanks again!

 

[SteveK]

Hi All,

I have 8 Trojan T-105 225ah wired series/parallel for a 12v 900ah house bank. I had heard some time ago about "cross connecting" to better balance everything, and then read about it again in Nigel Calder's book. I welcome any feedback from folks who know about such things. Diagram of the existing set up attached.

Thanks very much,

Randy Good

Randy, Interesting.
why are the left 4 & right 4 not connected in the middle in this plan.

 

[Socal_Cecil]

Batteries are actually a collection of cells and each cell is typically around 2 volts. So a 6v battery has 3 cells in series and a 12v battery has 6 cells. Instead of having your 6 cells in one case you have them in 2 cases. Once you connect 2 in series treat it like a single 12v battery with just the outer terminals now used. I'll include a picture of the wiring for a large bank of 6v's. The picture is from the internet, not mine. If it doesn't show up you can right click and open in a new window.

Use a wrench too short to reach both terminals or electrically safe too. A direct circuit with no load can cause lots of problems.

Battery health will be far more impacted by how you use it and how you maintain it. Check the recommendations for your brand on depth of discharge and get a good charger that is specific to your battery type. Follow the checks required on schedule for whatever it calls for if any.

 

[Insequent]

This article has been posted before. Its worth making the effort for Method 3 I think

SmartGauge Electronics - Interconnecting multiple batteries to form one larger bank

 

[J.R. Good]

Randy, Interesting.
why are the left 4 & right 4 not connected in the middle in this plan.

That’s actually one of my main questions. The left 4 make 12 volts with 450ah, as does the right 4. I figured if I parallel those two sets I get 12v 900ah. Will connecting the two sets in the middle change the voltage or amperage? I keep confusing myself about that?

 

[SteveK]

I think you are getting 900Ah already as shown. If any one battery becomes dead it is bypassed, but would it still drain the others.
Still trying to understand the wiring plan for how it is better.

Actually post 47 link method 4 looks to be the same idea.

 

[ranger58sb]

Randy, Interesting.
why are the left 4 & right 4 not connected in the middle in this plan.

That’s actually one of my main questions. The left 4 make 12 volts with 450ah, as does the right 4. I figured if I parallel those two sets I get 12v 900ah. Will connecting the two sets in the middle change the voltage or amperage? I keep confusing myself about that?

I'm struggling to understand why the left 4 (two pairs) are each also connected in the middle... and then why the right 4 (two pairs) are each also connected in the middle...

Haven't had my coffee quota yet, though...

-Chris

 

[Bacchus]

This article has been posted before. Its worth making the effort for Method 3 I think

SmartGauge Electronics - Interconnecting multiple batteries to form one larger bank

Thanks for the reply. Do you know where I might find a diagram of the very best way to configure this bank? Nigel Calder's diagram only shows 4 batteries, and I have 8. I know that one bad cell anywhere spoils the pot - that's a given no matter what, but I'd like to maximize what's healthy for the batteries if possible. The physical layout of the batteries themselves has to stay as drawn
Thanks again!

JR
Brian's link has a wiring diagram for 4 12V battys. Just substitute a pair of 6V battys w just +/- jumpers for each and you have a balanced bank.
That is exactly what I have in my all electric motorhome and that diagram is what the mfg recommends.
In cases where there are more cable connections than desired on "main" terminals you can simply move a set to a mirror image location.
Ex: Main house draws on as shown and an inverter / charger on the mirror image set.
It's fairly easy to test for balance... each circuit through a pair of 6V battys should have the same # of short and long jumpers to the inlet / outlet connection points. Whether they are on the + or negative side doesn't matter.

 

[Iggy]

I'm struggling to understand why the left 4 (two pairs) are each also connected in the middle... and then why the right 4 (two pairs) are each also connected in the middle...

Haven't had my coffee quota yet, though...

-Chris

Me too, note really understanding it.

If it was me, I would not install all the interconnecting wires. But install one jumper from the battery with 1st battery in line and than to the 3rd battery down. On both Pos and Neg side. This would give another path to better balance the whole bank. I think that would work too with out all the added jumpers.

 

[J.R. Good]

Calder's explanation:

"In a series/parallel setup, it is an excellent practice to cross-connect the positives and negatives on the individual 6-volt batteries, or 2-volt cells, as shown in Figure 1-21. This minimizes differences in the way the batteries work and perform."

As has been pointed out, the voltage differences are so small that the cross-connects are not necessary. But, maybe worth it? And should the 2 middle pair in my 8 battery diagram be cross-connected as well? I'm almost done overthinking this, promise. Thanks for all the input!

Figure 1-21 from his book attached.

 

[Socal_Cecil]

Because the wire if large is such a minimal gain compared to the resistance you add with an additional ring connector you would be better off just running one center vertical line. You can alternate which side of center it goes down through. The terminals provide most of your installation resistance. Anything on either side of a wire connection can be considered the exact same point electrically if only a couple feet. Get professionally crimped interconnects if you don't have the right tool.

Does he show a setup with proper fusing? I got my training in the navy and haven't read his book. We had a big battery though. Over a hundred cells that were 4 ft tall. We did not interconnect every cell, just banks of 12 cells And remove them equally from the banks when they started going bad ( only when bad though).

If you fuse each positive terminal and one blows it would normally remove both batteries from the circuit and keep it a balanced bank. If you have the center interconnects and blow a terminal fuse but it is because of the first battery, the first battery will still be connected and drawing current. That doesn't seem safe to me. If your not doing fused terminals for each set of 2 batteries you'll blow the overall battery fuse either way.
I typically look for how things are going to fail, not how they run when they are working correctly. I'm told I'm a pessimist.

 

[Frank60]

Recent video by Nigel Calder:

https://youtu.be/cN_fTL_0r_8

 

[IWAB]

What is the proper way to inter-connect 3 house batteries? They are 8d’s and it would be nice to be able to pull power from the center battery without a bus bar mounted an equal distance away. I know pulling power and charging from the center terminals would be better than from the end battery, but I would like to see a diagram of a correct set-up with my 3 8d house bank. Help anyone?

Thanks

 

[Bacchus]

Here's the diagram for 6 - 6V so just consider each series pair one 12V 8D.
By wiring main cables to the end battys and keeping all interconnect jumpers the same length all batty circuits contain the same # & L of cabling... 2 jumpers. Doesn't make a difference if they are on Neg, Pos side or split.