8s for 12v pack ?

[f1991]

Hello.
I have 8 280ah cells lying around, I was wondering if it was possible to make a 12v pack.

If I make two 4s packs and I connect these two packs in parallel is what I can do so 12v 560ah ?
In theory I can, with a single bms 8s, do this project?
Thank you for your advice and experiences.

 

[MisterSandals]

If I make two 4s packs and I connect these two packs in parallel is what I can do so 12v 560ah ?

Yes but you need a BMS for each.

Alternatively you can combine cells in pairs (2P) and then connect the pairs in series (2P4S). There is a trick to make the pairs as equal to other pairs as possible: pair srrongest with weakest, second strongest with second weakest…
This make for a better battery.

Advantages to both 4S2P and 2P4S designs.

 

[Steve_S]

If you are going to Parallel the cells then all you need is a 4S BMS for 12V.
This will still give you the theoretical 12V/560AH battery pack IF the cells are matched and behave properly.
The parameters will still be a Max Output of 280A @ 1C Rate Discharge and a max of 140A Charge @ 0.5C Rate

If you built TWO 12V/280AH Packs then each could handle up to 280A 1C Rate Discharge & 140A Charge Rate.
Set in Parallel, these would divide any Charge or Discharge in half.

NB: If you intend t parallel cells within a pack, it is Very Very Strongly recommended to use only properly matched & batched Grade-A Cells and not B or Bulk Cells, nor cells which have only been Voltage & IR Tested at static storage voltage (what most vendors do).

 

[f1991]

If you are going to Parallel the cells then all you need is a 4S BMS for 12V.
This will still give you the theoretical 12V/560AH battery pack IF the cells are matched and behave properly.
The parameters will still be a Max Output of 280A @ 1C Rate Discharge and a max of 140A Charge @ 0.5C Rate

If you built TWO 12V/280AH Packs then each could handle up to 280A 1C Rate Discharge & 140A Charge Rate.
Set in Parallel, these would divide any Charge or Discharge in half.

NB: If you intend t parallel cells within a pack, it is Very Very Strongly recommended to use only properly matched & batched Grade-A Cells and not B or Bulk Cells, nor cells which have only been Voltage & IR Tested at static storage voltage (what most vendors do).

View attachment 139391

Hi, ahhh ok I see more clearly.
What I don't understand is to use a bms 4s?
Because if two cells are in pairs, they will necessarily have the same voltage?
I have 8 EVE cells bought directly from EVE factory, never used, just top balanced

 

[Steve_S]

2 cells in parallel appear as one cell to the BMS. BMS Sense Lead is attached to the [+] bus bar on the paired cells.
If the paired cells have identical IR throughout the working range they should be ok. If they don't then weird issues can & will arise. Hence why many who build battery packs with paired cells in Parallel end up changing their builds later (and generally won't talk about it) to simple Series Only battery packs.

 

[f1991]

---

2 cells in parallel appear as one cell to the BMS. BMS Sense Lead is attached to the [+] bus bar on the paired cells.
If the paired cells have identical IR throughout the working range they should be ok. If they don't then weird issues can & will arise. Hence why many who build battery packs with paired cells in Parallel end up changing their builds later (and generally won't talk about it) to simple Series Only battery packs.

Thanks you all.
I will test the 2p4s.
It seems to me to be the easiest way to make a large capacity battery.
Its likely to be hard to find a box to accommodate 8 cells ?

Thank you community for your valuable information.

 

[740GLE]

I am running a 2p4s with 230 cells, 200a jbd BMS. I’m not trying to squeeze every AH out of it (300w max draw) so it’s been perfectly fine for my needs.

 

[Marinepower]

The parameters will still be a Max Output of 280A @ 1C Rate Discharge and a max of 140A Charge @ 0.5C R

Set in Parallel, these would divide any Charge or Discharge in half.

Steve, I assume you don't mean that a 2P- 4S would be limited to max 280ah output current (continuous) due to the limitations of the single BMS in this configuration?

Why would the current output not double with the battery capacity? Doesn't that go against conventional wisdom? For eg. battle born batteries are made up a number of cells in parallel and they are not limited to the output current of one cell? Also, the prismatic cells themselves are made up of multiple foil cells in parallel.

I'm sure you have a point here but could you explain further?

I am interested as I have a 4P-4S Frey 100ah ("Fortune") battery planed (matched and batched from the factory - apparently) and will need more than 100ah continuous a times.

MP

 

[Supervstech]

The bms will limit max output amps.
The Ah will go up to 560, but your bms will not allow higher ampdraw.
Its why two bms units and two parallel packs are a better use of the cells.

 

[sunshine_eggo]

The bms will limit max output amps.

I know what you're saying, but the wording might be misleading to some.

To clarify, the BMS will not "limit" or regulate the max output amps. It will cut it the heck off. It's an on off switch, not a regulator. Just didn't want there to be any confusion. Exceed the limit, and the BMS will cut the discharge to 0A. It will not limit it to the BMS max.

In other words, if you have a 200A BMS, and you try to pull 300A, it will not limit you to 200A. It will cut you to 0A when you exceed 200A.

The Ah will go up to 560, but your bms will not allow higher ampdraw.
Its why two bms units and two parallel packs are a better use of the cells.

+100

 

[Marinepower]

The bms will limit max output amps.
The Ah will go up to 560, but your bms will not allow higher ampdraw.
Its why two bms units and two parallel packs are a better use of the cells.

Yes, fully agreed that a FET based BMS will likely be the weakest link for max current output in an eight 280 ah cell, 2P - 4S configuration.

But I don't think that is the point that Steve is trying to make. I think he is saying that 2P-4S is inherently unreliable - so you must de-rate the max current output to the amount of a single 280ah for a safety factor.

But I will let him chime in on this....

In my case, I have an Electrodacus BMS that uses external mechanical contractors (Blue Sea ML - 500 amps continuous) , so my BMS is not my limiting current factor...

MP