Mixing Battery Capacities

[gmcconnell]

I have 30x 3.2Ah and 18x 3.8Ah 26650 LiFePo4 cells. I plan on making a 4S4P with the 3.8Ah for a 12.8V 15.2Ah battery. That will leave me with 30x 3.2Ah and 2x 3.8Ah cells for another project.

I'm planning on making a 4S7P out of the 3.2Ah cells for a 12.8V 22.4Ah battery. That will leave 2x of the 3.2Ah and 2x of the 3.8Ah.

So, my question is... Can I throw those final 4x cells into my 7P battery to make it an 8P? They are all LiFePo4, and I'm thinking it would de-rate the 3.8Ah capacity to 3.2Ah to match the rest of the battery which I'm fine with since I don't have another project in mind for those cells. It could bump my battery up to 25.6Ah.

I'm not thinking this is a good idea as I am worried about the voltages between the cells being too different at varying state of charges but wanted to check before I left some potential capacity on the table.

Thanks!

 

[sunshine_eggo]

I have 30x 3.2Ah and 18x 3.8Ah 26650 LiFePo4 cells. I plan on making a 4S4P with the 3.8Ah for a 12.8V 15.2Ah battery. That will leave me with 30x 3.2Ah and 2x 3.8Ah cells for another project.

4S4P implies you're series-ing the cells first, then paralleling them - 4 separate batteries. If the intent is to parallel 4 cells, then string those 4 groups in series, it's typically expressed as 4P4S.

I'm planning on making a 4S7P out of the 3.2Ah cells for a 12.8V 22.4Ah battery. That will leave 2x of the 3.2Ah and 2x of the 3.8Ah.

7P4S

So, my question is... Can I throw those final 4x cells into my 7P battery to make it an 8P? They are all LiFePo4, and I'm thinking it would de-rate the 3.8Ah capacity to 3.2Ah to match the rest of the battery which I'm fine with since I don't have another project in mind for those cells. It could bump my battery up to 25.6Ah.

No. You will be causing the individual series elements to be different capacity. You want to avoid that. IMHO, I would simply keep the cells as spares in the event that any of them fail.

I'm not thinking this is a good idea as I am worried about the voltages between the cells being too different at varying state of charges

You're correct.

but wanted to check before I left some potential capacity on the table.

You're not leaving potential capacity on the table. You're leaving problems on the table and keeping them out of the battery pack.

 

[gmcconnell]

4S4P implies you're series-ing the cells first, then paralleling them - 4 separate batteries. If the intent is to parallel 4 cells, then string those 4 groups in series, it's typically expressed as 4P4S.



7P4S



No. You will be causing the individual series elements to be different capacity. You want to avoid that. IMHO, I would simply keep the cells as spares in the event that any of them fail.



You're correct.



You're not leaving potential capacity on the table. You're leaving problems on the table and keeping them out of the battery pack.

Thanks for the detailed response!

 

[BroomJM]

I have a follow-on question to this thread.

I am looking to build a 4P4S battery from older 26650 LiFePO4 cells. They came in a 7P5S configuration, which I have broken down into 5 packs of 7P that are no longer in series. I then removed 3 of the cells from the 7P pack, to fit the space available in my application. I will be able to fit a 4P4S arrangement into this space. My question is about the BMS. Do I buy a simple 4S BMS and treat each of the 4P segments of the battery as a single cell? Is this simply a 4S battery, despite the 4 cells having a 4P arrangement?