I'm looking into building my own li-ion battery pack. I read about linking the cells in parallel and in series, and how to add a BMS. I would like to build a 12V pack with a very large capacity (mAh). So I need to build a 3S pack and then link the 3-series connected cells in parallel.
What I don't understand is: does the BMS need to know about the number of parallel connected cells, or can I simply keep adding them up?
For example, one BMS I was looking at had this illustration:
It shows 2 parallel connected packs, but can I also make it 3P, or 10P, or 150P?
Best Answer
There are two reasons you need to be concerned about the number of parallel cells in your battery pack
a) The current cutoff threshhold when doing the final CV charge is dependent on the battery capacity, which is dependent on the number of parallel cells. You simply need to know the numbers, and program your charger correctly.
b) In the event of a failure of one cell, the other undamaged cells in parallel with it may discharge into the damaged one, prolonging and exacerbating any 'vent with flame' event, turning a mess into a catastrophe, turning a damaged battery into a damaged vehicle or building. I talk from experience, my brother lost an eBike and the van in which it was being charged, and would have damaged any adjacent vehicles as well but for being parked in a remote corner of the carpark.
You can mitigate this latter problem by connecting a fuse in series with (ideally) each cell, or at a minimum with each small group of parallel cells. The smaller the group of cells, the closer it can be fused to protect it.