Cell Arrangement - 2p8s single or 8s x 2 double

RvNaut

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Apr 4, 2022
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I am building a 560ah battery using 16 280ah 3.2 cells.

My initial path was to build two 280ah 24v batteries using 2 8s BMSs, and then putting them in parallel.

In talks with someone, I was advised that this in not the way to build it. They said first build up the 560ah cells by putting pairs of the 3.2 in parallel, then series them to gain the 24 volt and using a single BMS. I questioned attaching a single BMS to the doubled up cells. The answer was that if I am using grade A cells that are properly balanced, it is a non-issue.

So, is it?

My draw is going to be heavy.. running computers, mini split, and was aiming at a 200 Amp DALY 8s BMS.... now I am not so sure... the advice was that .. parallel cells to gain AmpHours and then put in Series to gain voltage, versus putting in series to gain voltage and then parallel to gain amphours.

What say you?
 

Mahendra Gomanie

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Mar 8, 2021
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I am building a 560ah battery using 16 280ah 3.2 cells.

My initial path was to build two 280ah 24v batteries using 2 8s BMSs, and then putting them in parallel.

In talks with someone, I was advised that this in not the way to build it. They said first build up the 560ah cells by putting pairs of the 3.2 in parallel, then series them to gain the 24 volt and using a single BMS. I questioned attaching a single BMS to the doubled up cells. The answer was that if I am using grade A cells that are properly balanced, it is a non-issue.

So, is it?

My draw is going to be heavy.. running computers, mini split, and was aiming at a 200 Amp DALY 8s BMS.... now I am not so sure... the advice was that .. parallel cells to gain AmpHours and then put in Series to gain voltage, versus putting in series to gain voltage and then parallel to gain amphours.

What say you?
Two bms's is better .
You will have more amp out put plus monitoring of individual cells
 

TorC

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Jan 13, 2022
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I'd go with two BMSs, and preferably big enough to normally run most any likely load on one. If nothing else that gives you redundancy. Should one battery fail all you need to do is repair it while the other runs your system, get SoC to match, and plug the repaired string back in. I've bought 3 BMSs for my soon to be built 16S2P with the same cells to have a spare on hand in case that should be the part to fail.
 

RvNaut

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...What about the battery build "must do" of running parallel first, then series....

This was the major point being made... that S then P is the wrong way to assemble "a battery" using lithium cells....

I also have a question about having two batteries being charged by one charger? and how that may effect the battery(s) or the charger itself???
 

MisterSandals

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...What about the battery build "must do" of running parallel first, then series....
I have a 2P4S battery that i made with 8 cells in my RV. Its been running flawlessly for a couple years. I REALLY wanted to have a single BMS and I did not have an issue with max loads exceeding the 120A of the single BMS.

One thing that I recommend if doing a 2P battery is to match the cell pairs to be as equal as possible. I did this my ranking the capacity/strength of each cell. Then i paired strongest with weakest. Then second strongest with second weakest. ...

This makes the pairs as equal to one another as i could.
I also have a question about having two batteries being charged by one charger? and how that may effect the battery(s) or the charger itself???
Charging 2 batteries in parallel is easy: attach charger + to + on battery 1, and charger - to - on battery 2.

Both ways work. As mentioned above, if you need more load amps than a single BMS can handle, paralleling 2 batteries can deliver the sum of the BMSs. Also, if you want to monitor each individual cell (as opposed to cell pairs) then building 2 batteries each with their own BMS allows that.
 

RvNaut

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Thanks for that answer....

... but it still does not address the main point this person was making... in that assembling the battery in series first and then parallel is the wrong way to do it.....

I guess what I amlooking for is "why it is better to parallel the cells to gain the AH prior to putting them in series to gain voltage?" Is this something any of you have heard of before? If this is *the way* to build a battery, why is it not more well known or supported?
 

MisterSandals

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"why it is better to parallel the cells to gain the AH prior to putting them in series to gain voltage?"
Any time you parallel cells or batteries or solar panels, the amps are added. Any time you put cells or batteries or solar panels in series, the volts are added. What is the mystery?
If you have 3.2V cells and want a 12V battery (12.8V nominal) you MUST put 4 cells in series to achieve this voltage. This is a battery.
If you do this to make a separate battery, even if you intend to put them in parallel later, its still a battery.

So, 2P8S is a single battery and requires a single BMS
Conversely, 8S2P is 2 batteries and requires 2 BMSs.
 

toms

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Thanks for that answer....

... but it still does not address the main point this person was making... in that assembling the battery in series first and then parallel is the wrong way to do it.....

I guess what I amlooking for is "why it is better to parallel the cells to gain the AH prior to putting them in series to gain voltage?" Is this something any of you have heard of before? If this is *the way* to build a battery, why is it not more well known or supported?

You are looking in the wrong place. I’ve been building large LiFePO4 packs for a long time, and the only time i’ve seen people use series first is when they have cheap FET based BMS that can’t handle the large currents.

If you want a 400ah pack, the simplest way is to use 400ah cells.

Parallel connection of 4 100ah cells gives you a 400ah cell.

If you are scraping the alibaba barrel and getting wildly mismatched cells then putting a bunch of them in parallel is a bad idea.

Ultimately you need to closely define your needs, both ways can work well - or fail spectacularly!
 
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