2p8s or 8s2p

[Pneuma]

Hello. I hope y’all are having a good day/night.
I was wondering in configuring a 24v system with 16 cells 3.2v 100ah which is better configuration, 2p8s or 8s2p?

I think I know 2p8s is better but I do not know why?

Please let me know what you know

And also if you still wanted to upgrade that 16 cells to a I dont know, say a 32 cells How would you go about putting those cells together?

Thank you ?

 

[MisterSandals]

I think I know 2p8s is better but I do not know why?

2P8S will only let you/your BMS monitor pairs of cells but will only require 1 BMS.

8S2P will allow individual cell monitoring but each 8S pack will require its own BMS.

I run a 2P4S battery without issues. I spent a lot of time matching weak and strong cells into pairs which i believe made a big difference.

 

[Pneuma]

2P8S will only let you/your BMS monitor pairs of cells but will only require 1 BMS.

8S2P will allow individual cell monitoring but each 8S pack will require its own BMS.

I run a 2P4S battery without issues. I spent a lot of time matching weak and strong cells into pairs which i believe made a big difference.

Thank you! ?

 

[Pneuma]

Is that the only difference?

I was watching a video of Wills and in it he talks about how it’s bad to put cells in series First then parallel. But he doesn’t say why!
And so I though there was something seriously wrong with putting my batteries in 8s2p rather than put couple of them in parallel then put those in series as one cell

 

[MisterSandals]

Is that the only difference?

I think that is the important difference(s).

The folks here with the serious systems, especially 24v and 48v really prefer series first. There is an added level of expense and setup but you are able to monitor at the cell level.

 

[Steve_S]

Simply put, if you place say 4 cells in parallel, those 4 cells act like one (as far as a BMS is concerned) and therefore it can only read the "Average" of the 4 cells. Unless you have exactly perfect Matched & Batched Cells problems will arise, especially with higher capacity cells.

If the cells in the parallel set are at 2.75V, 3.00, 2.90, 3.00 the BMS will see 2.915V
0% SOC = 2.5V, 100% SOC = 3.65V

The real bugger is the Internal Resistance of the cells. Fully Matched & Batched cells have identical IR through the voltage ranges but regular Commodity Cells which are only Voltage & Base IR Matched the IR's will vary across the voltage ranges. The IR affects how fast a particular cell can Charge/Discharge. When cells are paralleled without being fully matched, they can diverge further between themselves if their IR varies too much, and so as some cells reach 100% SOC, the lower cells will still be demanding more, in parallel, they "should" pull from the other sets in parallel but this doesn't quite work out for the BMS, because it can only see the "average".

Matching & Batching os cells is a long tedious process which get's longer with the bigger cells. Cells are mounted to a "rig", charged fully and then discharged at various rates with the IR being read at the set points, the cells are taken down to 0-SOC and then recharged, while also testing the IR at the set points. This cycle is usually repeated 3 Times with all the data being recorded & logged. Then cells which have matching IR through the voltage ranges get sorted & batched together. This process can add $50 a cell or more even and that adds up fast.

Case in Point: Battleborn, Relion, Fullriver, Trojan, Rolls Surette, Victron and ALL the High Quality Drop-In or ESS Battery systems are Matched & Batched Cells and THAT is one reason they cost so much more.... EV (Electric Vehicle) batteries are ALL Matched & Batched - THEY HAVE TO BE as Pack Consistency is critical, especially with High Demand battery systems. Yet another reason why EV Battery Packs are co expensive BUT it's als a warning about used EV Packs as often they may have one or more weak / bad cells within and were warranty pulled (EV makers do not repair EV packs, they just Re & Re them).

 

[toms]

Simply put, if you place say 4 cells in parallel, those 4 cells act like one (as far as a BMS is concerned) and therefore it can only read the "Average" of the 4 cells. Unless you have exactly perfect Matched & Batched Cells problems will arise, especially with higher capacity cells.

Incorrect, it doesn’t matter how many times you say this it will still be incorrect.

If you are using large format prismatic cells, each cell has many individual segments in parallel. There are tiny differences in IR in each segment of the cell, and in time they even out (the stronger segments get dragged down to the level of the weaker segments).

The voltage of each segment is always identical, as it is when you parallel two LFP cells.

When you parallel two cells, it is the same as what is already happening within each cell.

 

[Steve_S]

Toms that is your opinion.
1 LFP 280AH cell weighs 5 Kilograms +/- 200 grams (1/2 Lb = 227 grams)
They are NOT 100% consistent and even, do you not think that such variations are reflected within the cells ?
Segments in the jelly roll ? okay....

 

[Supervstech]

Incorrect, it doesn’t matter how many times you say this it will still be incorrect.

If you are using large format prismatic cells, each cell has many individual segments in parallel. There are tiny differences in IR in each segment of the cell, and in time they even out (the stronger segments get dragged down to the level of the weaker segments).

The voltage of each segment is always identical, as it is when you parallel two LFP cells.

When you parallel two cells, it is the same as what is already happening within each cell.

Well, since you are calling out someone as wrong, I gotta chime in.
An assembled prismatic cell staying internally balanced, is quite different from external possibly mismatched parallel banks of cells connected with bussbars or jumper wires, and connected externally.
Batteries can and do drain, and charge differently depending on connections, balance, I.R. and other factors.
So, yeah... paralleling can create issues in time.

 

[Pneuma]

This is Very interesting! Thank you all for Sharing your knowledge.

So In a 16 peiece battery bank there is really no way of avoiding Parallel connection. Bassed on what I am picking up Im better off doing the 8s2p with 2 BMSes so I can monitor EACH cells Performance. At first I though well If I paraller 2 and then put them in series those two should balance each other.

 

[Steve_S]

Below is a Typical 2 Pack Battery Bank with each pack in Parallel to make up the one battery bank.
For detailed wiring information and the best ways to make sure all is configured & balanced properly refer to this Victron Document, starting from Page 17 especially.
VICTRON Wiring-Unlimited-EN.pdf

 

[MisterSandals]

So In a 16 peiece battery bank

Just out of curiosity, why are you set on 16 small cells instead of 8 large ones?
Most of the issues that are concerning you would be eliminated with an 8S battery.

 

[John Frum]

So In a 16 peiece battery bank there is really no way of avoiding Parallel connection.

A 48 volt nominal lifepo4 battery is 16 cells in series.

 

[Pneuma]

Just out of curiosity, why are you set on 16 small cells instead of 8 large ones?
Most of the issues that are concerning you would be eliminated with an 8S battery.

Well I was thinking about going for 280 Ah cells and start with 8 but later on down the road I want to upgrade and the All in one unit is only designed for 24 v so Im thinking I am going to keep things at 24v even later on when Iwant to upgrade my battery bank

 

[Pneuma]

Below is a Typical 2 Pack Battery Bank with each pack in Parallel to make up the one battery bank.
For detailed wiring information and the best ways to make sure all is configured & balanced properly refer to this Victron Document, starting from Page 17 especially.
VICTRON Wiring-Unlimited-EN.pdf

View attachment 25437

If Im not mistake I see that you are using the bussbars to parallel connect the two battery pack instead of paralleing them fist then runnning the wires to the busbars. I was wondering about why? Thank you

 

[MisterSandals]

so Im thinking I am going to keep things at 24v even later on when Iwant to upgrade my battery bank

Okay, so 8 cells still makes a 24v battery bank. Its really confusing to me why you are so focussed on 16x 100ah cells to make 8S2P or 2P8S when 8S with larger cells is so simple.

 

[Pneuma]

Okay, so 8 cells still makes a 24v battery bank. Its really confusing to me why you are so focussed on 16x 100ah cells to make 8S2P or 2P8S when 8S with larger cells is so simple.

Its becasue I'm thinking of future :D I am no longer considering the 100ah cells. because well my usage seems to need bigger battery bank.

Right now I have the 280ah Cells in mind which easily will make a 24V. The talk about the 16 cell is for the future when I want to still increase my battery bank by buying 8 more 280ah cells.

 

[MisterSandals]

The talk about the 16 cell is for the future

Adding 8 cells to an 8 cell battery bank is STILL easier than adding 16 cells to a 16 cell bank. There is NOTHING future proof with your scenario, as stated, that makes 16 cells more flexible.

Edit: same goes for adding 8 cells in the future to a 16 cell bank. 16 cells up front has zero advantages based on what you've said so far.

 

[Supervstech]

Adding 8 cells to an 8 cell battery bank is STILL easier than adding 16 cells to a 16 cell bank. There is NOTHING future proof with your scenario, as stated, that makes 16 cells more flexible.

Edit: same goes for adding 8 cells in the future to a 16 cell bank. 16 cells up front has zero advantages based on what you've said so far.

Except he has the option to switch to 16S to power the inverter immediately...

 

[MisterSandals]

Except he has the option to switch to 16S to power the inverter immediately...

What 48v inverter?
I am not seeing that requirement.