is there such thing as a a 12v 12s BMS?

[LarryDorneles]

new to this so thanks in advance. Im building a battery from 12 cells. I think I have to build 3 4s 12v batteries and put a bms on each but was wondering if I could instead configure them as 4 3s 3.6v and run 1 12v 12s BMS?

 

[LakeHouse]

You’re a little confused on some terminology, I think. I’m guessing you have 12 LFP cells and you’re looking to make one or more 12V batteries using all 12 cells, right?
You can do this as 3 separate battery with four cells in series each, that would be three 4s batteries. You could do it that way and use a separate BMS for each battery, then connect them all in parallel. This is probably what I would do, but there are other options.
You could parallel sets of three cells and then put three such sets in series, that would make a single 3p4s battery, which I think is what you’re asking in the last sentence. You only need a single BMS for that arrangement, but it’s still a 4s BMS.

 

[Steve_S]

If you are using LFP (LiFePo4 / Lithium Iron Phosphate) cells:
To build a 12V Battery Pack, it requires 4 cells in Series. Nominal Voltage for every cell is 3.2V X 4 = 12.8V
You can place cells in Parallel within a pack to increase the AH. Cells in Parallel act & appear as "one cell" but there are GOTHA'S!
It is extremely important to have matched cells in parallel otherwise they can imbalance and create issues.
Each Parallel "Set" of cells get's ONE BMS Sense lead, therefore you need a 4S BMS.

Battery Packs in Parallel (meaning identical 12V Packs) divide & share both Load & Charge.
If one fails within a parallel bank the others keep going unless they are all discharged.

Below are some graphics & guides I have written up to help.
Below is a Graphic showing standard LFP Battery Pack layout for 12V.
A Guide on how to assemble your own battery pack(s) with lots of handy ref info: Luyuan Tech Basic Lifepo4 Guide V1.0A
And a few more reference graphics below that you may want to download.

Note that each 12V Battery Pack will require it's own 4S BMS (Battery Management System) and fused appropriately.
It is best to "Right Size" BMS to the pack. IE: 100AH Cells use a BMS that supports 100A Discharge, if using 280AH Cells use a BMS capable of 200-250A Discharge.



---> LFP CHEMISTRY Voltage Range Chart.

---> VOLTAGE CURVE TO SOC (basic)

---> AN A-TYPICAL COMPONENT BASE SYSTEM

 

[LarryDorneles]

You’re a little confused on some terminology, I think. I’m guessing you have 12 LFP cells and you’re looking to make one or more 12V batteries using all 12 cells, right?
You can do this as 3 separate battery with four cells in series each, that would be three 4s batteries. You could do it that way and use a separate BMS for each battery, then connect them all in parallel. This is probably what I would do, but there are other options.
You could parallel sets of three cells and then put three such sets in series, that would make a single 3p4s battery, which I think is what you’re asking in the last sentence. You only need a single BMS for that arrangement, but it’s still a 4s BMS.

thank you for explaining. would the BMS be able to manage the 3 cell packs? I imagine it treats all 3 batteries in that pack as a single unit so any imbalance in the individual cells will go unnoticed....

 

[LakeHouse]

thank you for explaining. would the BMS be able to manage the 3 cell packs? I imagine it treats all 3 batteries in that pack as a single unit so any imbalance in the individual cells will go unnoticed....

Yes, one BMS would be fine. With them connected in parallel, the cells will always have the same voltage. The parallel cells won’t exactly be ‘balanced’ necessarily, except at the top and bottom of the charge curve where cell voltage and SoC correlate well, but you wouldn’t notice and it doesn’t really matter. Above 3.45 all parallel cells will be fully charged, and below 3.0 they all be close to fully discharged. In between, they may diverge and that’s ok.
What really needs balancing is between sets of cells, and that’s what the BMS is for.