Cross connecting, in serial-parallel systems?

[jdege]

I'm reading through the chapters on electrical systems in Nigel Calder's "Boat Owner's Mechanical and Electrical Manual", and he says:

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.

I've never seen anything quite like this, and none of the examples of series/parallel I've seen online do this.

It's this normal practice? It's it a good idea?

 

[MisterSandals]

It's this normal practice? It's it a good idea?

Yes and Yes, if it yields the battery voltage you seek.
If those batteries pictured were 12V 100Ah batteries (connected 2P2S) it would yield a 24V 200Ah battery bank.
If you doubled this to 2P4S, it would yield a 48V 200Ah battery.

 

[jdege]

It's the connection of the inner positive-to-positive and negative-to-negative that looks unusual, to me.

Pictures of similar setups I've found online aren't connecting parallel at every pair of terminals, but only at the ends:



That is, what Calder is doing in his book is wiring two pairs of batteries in parallel, then connecting the two pairs in series.
What the above website is doing is connecting two pairs of batteries in series, then connecting the two pairs in parallel.

I've never seen any discussion about which is preferred, of even any mention that there are two options, except for this reference in Calder's book.

Does it make a difference?

 

[MisterSandals]

It's the connection of the inner positive-to-positive and negative-to-negative that looks unusual, to me.

Connecting in parallel, keeps the voltage the same but adds the amp hours. Perfectly normal.

What the above website is doing is connecting two pairs of batteries in series, then connecting the two pairs in parallel.

It works out the same regardless of the order. 2P2S is the same as 2S2P.

It makes a difference for lithium cells because of the limitations of how BMSs can be wired.

 

[Bud Martin]

The main object of wiring the multiple batteries together is so that each battery see the same wiring resistance, and with minimum resistance as possible.

 

[MisterSandals]

What the above website is doing is connecting two pairs of batteries in series, then connecting the two pairs in parallel.

That description implies that it depends on the order in which the connections are made:

If you put wires 1 and 3 on first, then you are connecting in parallel first, then series connecting.
If you put wires 2 and 4 on first, then you are connecting in series first, then connecting the 2 pairs i parallel.

I don't know how they would describe putting wires 1 and 4 on first. Or wires 2 and 3 on first. But still you end up with the same thing...

 

[jdege]

In Calder's diagram there are six wires connecting the four batteries, in the photo there are only four.

They are not the same.

 

[Chesapeake]

Connecting in parallel, keeps the voltage the same but adds the amp hours. Perfectly normal.

It works out the same regardless of the order. 2P2S is the same as 2S2P.

It makes a difference for lithium cells because of the limitations of how BMSs can be wired.

Yes and Yes, if it yields the battery voltage you seek.
If those batteries pictured were 12V 100Ah batteries (connected 2P2S) it would yield a 24V 200Ah battery bank.
If you doubled this to 2P4S, it would yield a 48V 200Ah battery.

I have utilized Calder's cross connection scheme from his book, but wonder how this would look in an 8 battery (6 v) 12 volt bank. Would there be a cross connection between two sets of 4 as well as what his schematic shows for one bank of 4?
Thanks!

 

[MisterSandals]

Would there be a cross connection between two sets of 4 as well as what his schematic shows for one bank of 4?

It actually gets more complicated. Fortunately @Solarod did the math and tested the configurations with real cells.

between two sets of 4

Are you referring to 4S2P?

Here is the thread, i'll let you find the configuration you are referring to (or reply with the config and i'll give it a look):

Calculation of parallel string battery currents

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...

diysolarforum.com

Its a long thread but fascinating reading. Once you read thru a few configs you can follow along by looking at the pictures...