Paralleling Battery Packs


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gpn

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I have a couple 15s17p li-ion packs. Each one was once rated for just over 2kwh of storage. I was going to put a 15s smart bms in each pack and then parallel the output from the BMS to support a larger load. Then I came across these boards: https://www.amazon.com/Amass-Parallel-Charger-Board-Balance/dp/B075CBGX5K
Obviously that isn't for a 15s pack but would something like that work for connecting more than one 15s pack together? If I understand correctly the + and - are paralleled like normal. The difference is that all the balance leads are also paralleled. I don't quite get that how that would work, if it would in fact work.
 

daromer

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Potentially yes. Above is not high enough though
 

Korishan

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Yes, it could work. All this would do is make it so that all the outputs of the packs are paralleled together, which you were doing anyways, but it would also connect each of the smaller paralleled cells in parallel as well so you only need 1 bms.
The biggest issue with this is that you need to make sure that "all" groups of cells are at the same voltage "before" connecting them to the balance board. Otherwise, you could melt the small wires used for the balance leads.
So not only do both packs need to be the same voltage, but every parallel section of cells need to be the same as well, at least with it's twin in the other pack.
 

gpn

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daromer said:
Potentially yes. Above is not high enough though
I do realize the s count isn't high enough. That board just got me thinking about it.


Korishan said:
Yes, it could work. All this would do is make it so that all the outputs of the packs are paralleled together, which you were doing anyways, but it would also connect each of the smaller paralleled cells in parallel as well so you only need 1 bms.
The biggest issue with this is that you need to make sure that "all" groups of cells are at the same voltage "before" connecting them to the balance board. Otherwise, you could melt the small wires used for the balance leads.
So not only do both packs need to be the same voltage, but every parallel section of cells need to be the same as well, at least with it's twin in the other pack.

Thinking about strategy here... I could fully charge a pack and make sure it was balanced and then fully charge the next pack and make sure it was balanced and then connect the two in parallel on both the power and the balance leads.

Thanks for the help. This didn't quite make sense to me but if the experts think it is okay then I'll go with it!
 

OffGridInTheCity

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If parallel2batteries that delivery 10a@12v each I can get 20a@12v out it.
IF I join the balance leads like this:

image_azdyno.jpg

And if the balance leads are 'thin' (e.g. can only handle 10a). Don't I run the risk of 20a going thru them... and melting them thru?

My question is - how does current flow thru the main wires vs balance wires when you parallelize a couple of batteries as shown in this drawing.
 

daromer

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They flow to Keep balance.

Here have i done it

 

Korishan

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OffGridInTheCity said:
My question is - how does current flow thru the main wires vs balance wires when you parallelize a couple of batteries as shown in this drawing.

Current on the string level will primarily run through all the packs to the end, it won't go from one string to the other through the balance leads. And if it does, it's negligible. Or there's a bigger problem; like one of the packs is not as strong as the other packs and is pulling the whole system down.
 

gpn

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I find that often when people want to build packs everyone tells them to build them as parallel groups and then put those groups in series and note them such as 7s10p.

Along the same lines as my original question is this: if I built ten 7s1p pack and then paralleled all the packs I would have the same amount of power as I mentioned on the line above. The difference is that I would need 10 7s BMS units to control each pack. Why wouldn't I just make a jumper board that allows 10 of the 7s balance plugs to parallel into the board with one output to the bms. Then I would have a more modular pack (I could take a 7s1p pack out of service and keep my main voltage just with 10% less pack capacity) and still only require one BMS.
 

Korishan

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gpn said:
I find that often when people want to build packs everyone tells them to build them as parallel groups and then put those groups in series and note them such as 7s10p.

Along the same lines as my original question is this: if I built ten 7s1p pack and then paralleled all the packs I would have the same amount of power as I mentioned on the line above. The difference is that I would need 10 7s BMS units to control each pack. Why wouldn't I just make a jumper board that allows 10 of the 7s balance plugs to parallel into the board with one output to the bms. Then I would have a more modular pack (I could take a 7s1p pack out of service and keep my main voltage just with 10% less pack capacity) and still only require one BMS.

There is nothing wrong with this, and that is exactly what OffGridInTheCity put in his diagram. We aren't saying this is wrong.

And there are several builders on SLS that have done just this, just with different packs in series, and with different count of cells in parallel in each group.
 

OffGridInTheCity

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@daromer - yes I have indeed watched that youtube on parallizing balance leads with great interest and it makes sense.
@Korishan - thank you for your comments about current flow - it makes sense.
Both of you - I haven't tried the balance lead parallelization as I was unsure about current flow but I do understand what you both are saying in this thread plus I have confidence in your expertise and can put it to rest in my mind. I may very well use this technique as I continue building battery packs.
 

gpn

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I am definitely going to try it myself! Now just need to order a bms!

On a different note... what is the best way to parallel 15s packs? Is there a simple wiring adapter y-type plug out there already?
 

daromer

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For up to 14s (That is more a standard...) you can get Y connectors in terms of balancing wires. You can see them in the video I attached.
Im not sure what you can find for 15s. But with that said yoiu can have smaller contacts but more
 

Crimp Daddy

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You can easily rig up your own Y-cable or bus bar blocks to connect each pack. For the balance leads, I typically use some sort of multi-pin quick disconnect like those seen in automotive, or even a old ATX power supply harness can be used to make a "y-cable"
 
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