Wire sizing between parallel strings

[Computerizer]

I'm putting together a 13S4P lithium pack, as shown:

I'm experienced enough and comfortable enough to figure out the wire sizing for all the solid-colored wires in my diagram, which are meant to carry the load. However, the BMS needs to be able to measure the voltage of parallel groups of cells and to discharge them for balancing. The dashed wires on my diagram are for that.

The BMS will only draw 42 mA when discharging, so for that purpose the wires can be quite small. However, I'm concerned that current could flow between the parallel strings via these wires. For example, at initial setup the third cell of the first string may have a lower voltage than the third cell of the second string, which seems like it would result in current flowing from the third cell of the second string into the third cell of the first string via these dashed wires.

Obviously I would try to ensure all the cells are relatively balanced BEFORE connecting them, but it's not going to be perfect. And perhaps there are other circumstances where an imbalance between parallel cells would occur that I'm not thinking of.

So ultimately the question is, do these wires need to be sized according to just what the BMS might draw, or do they need to be sized the same as the solid-colored main wires, or something in between? And why? (And, if something in between, how do I figure that?)

Thanks!

 

[sunshine_eggo]

What you're proposing won't work. The BMS wires need to be very thin to essentially be self-fusing, but you're using them as parallel interconnects. Not good.

A 13S4P pack typically is four separate batteries, each with their own BMS, in parallel.

You want a 4P13S battery, where cells are grouped in 4 via parallel and then the 13 4P groups are placed in series.

Several examples here:

Cell Configurations for 12V 24V and 48V LiFePo4 Batteries

This short deck shows 1P and 2P cell configurations for 12V, 24V & 48V LiFePO4 batteries. To get the deck, click on the orange button at the top of this page. Note: Reviews, Comments and corrections are welcome Document Revision History...

diysolarforum.com

4P is difficult to ensure you get the same current through each cell.

2X 2P13S batteries each with their own BMS is likely the better solution.

 

[Computerizer]

Ah, okay. That's very helpful, thanks. I can see how the BMS would work if it's parallel-first. I'll weigh my costs etc for having 2 or 4 BMSs (with different current limits) and decide where to go from there.

 

[Computerizer]

So it seems like in my case having 4 BMSs, each with a 13S battery, would work great. I attempted to purchase 4 BMSs from Daly, and got a message from them saying that they cannot be used in parallel on the same circuit, so if I intend to do that I should cancel my order. I sent them the attached diagram and they confirmed that it won't work.

I was specifically looking at this one: https://smile.amazon.com/DALY-Battery-Management-Protection-Balance/dp/B0876NGKLZ?sa-no-redirect=1

I asked if they have other models that work in parallel (perhaps their more expensive ones), and they said they do not.

What is it that makes them not suitable for this? That one will shut off and then the others will suddenly take the amps?

How should this be handled? Is there a different brand of BMS that can handle this? Or are Daly and I misunderstanding each other?

Thanks!

 

[sunshine_eggo]

Some try to use multiple BMS on the same battery for increased current, i.e., multiple BMS attached to the same cells. I believe this is what DALY is referring to.

You would have 4 independent batteries, each with its own BMS, wired in parallel at their main terminals. This should not be an issue, and there have been several examples of DALY BMS operating that way on this site.