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2 12volt battery bank help
#1
Hey guys. I’ve just purchased 40 headway 38120 cells wanting to make 2, 20 cell batteries 12v batteries, question is do I do wire batteries up with 4 banks of parallel cells then wire for the 4s after? Or wire the batteries into series first then branch off for the 4 banks of parallel groups. ? . I want to get the cell placement.Down property so I don’t mess up.
Any help would be amazing sorry for being a newbie but I’ve looked everywhere for info on cell placement making packs with no luck
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#2
One consideration that jumps out at me would be the BMS aspect. Seems like if you do 4 banks (4 seperate 'batteries') then you need 2 BMS(s) per larger battery. If you do 1 bank per battery then you'd only need 1 BMS. I presume its a hi-amp BMS situation... A recent @jehugarcia youtub titled "4000A BMS?" shows a potential solution on headway 38120s.
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#3
(08-04-2019, 02:53 PM)jiggyjonboat Wrote: question is do I do wire batteries up with 4 banks of parallel cells then wire for the 4s after? Or wire the batteries into series first then branch off for the 4 banks of parallel groups. ?

If you wire 4p then 4s, which is a 4s4p configuration, then you'd only need 1 bms, as OffGridInTheCity mentions. If you wire it up as 4s then 4p, which is a 4p4s configuration (I know, can be confusing on the naming conventions), then you'd need 4 bms units, 1 for each string of series.

I personally would recommend 1 bms, or 4s4p. Reason being, is if you have multiple bms sharing a common output, it's possible 1 of them could shut down prematurely, which would cause a shift in current flow from the other strings and over load those as well. Depending on the current draw, this "could" burn out the FETs on the bms, requiring replacement.

This is, of course, if you are using a cheapo bms from chinesium manufacturing. If you are using something quite a bit more robust and smarter, then that makes it more of a funds available question than anything else.

In both situations, 4s4p or 4p4s, they are electrically identical on the absolute outputs. Charging/Discharging would be more or less the same. The only issue that "could" come up with 4p4s, is that one of the cells in one of the strings could become a different voltage than one of the other cells in another string. This would cause a little imbalance in the system. This would be the "primary" reason to have a smart bms that talks to the others, or go with 4s4p and use 1 bms from the get go.
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#4
My 220ah lifepo4 battery bank is 2x110ah 4s80p batteries, I just connect both batteries to a y connector that is connected to a bms and then you connect the balance connectors on both batteries together and to bms. I had it like that for the past 2 years running perfectly.

I'm building a smaller 30ah lifepo4 battery, it will be 3x 10ah 4s6p connected together with a connector that can handle all 3. Same principle as above. 

Thats the best way to do it, it makes for a more compact footprint, also you can disconnect one of the batteries or add more batteries to increase capacity, and you only need one bms.

Just make sure all batteries are same voltage when you join them together.

This is a picture of a parallel board, but I make my own with xt60 connectors. Same principle. Once batteries are joined together, they charge/discharge as one big battery.

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#5
(08-04-2019, 07:16 PM)jonyjoe505 Wrote: .... I just connect both batteries to a y connector that is connected to a bms and then you connect the balance connectors on both batteries together and to bms. 
This sounds as if you are (by definition) connecting them in parallel...  and I would think the small wire size for the balance leads could easily burn up under load.   
Perhaps I missing something here Smile
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#6
The main connector is connected using a Y connector. The balance leads are connected using the smaller white connectors in the image (I'm guessing as such)
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#7
I would love to see a simple drawing of how this would look, I know it will be 4s4p but I want to see how you would wire cells and secondly wiring up the bus balancing leads
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#8
So if my set up is 4s4p. What is this considered ? I get the 4s part but what is the parallel portion of this and how would you label this set up ?
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#9
(08-05-2019, 03:04 AM)jiggyjonboat Wrote: So if my set up is 4s4p. What is this considered ? I get the 4s part but what is the parallel portion of this and how would you label this set up ?

This is 4s (4 in series) and 8p (8 in parallel).   


If you cut the battery in this picture in 1/2 from left to right - you would have 2 batteries of 4s4p.

This would 1 of the 2 batteries of 4s4p
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#10
(08-05-2019, 03:04 AM)jiggyjonboat Wrote: So if my set up is 4s4p. What is this considered ? I get the 4s part but what is the parallel portion of this and how would you label this set up ?

I think you also need to read the FAQ on the main page as this question is answered in detail with several example images.
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