Let's assume I am going to build a Li-ion battery pack with 12 18650s, where I connect four cells together in parallel and then the three sets of four in series.
My understanding is that a BMS (Battery Management System) keeps an eye on the voltage and keeps it from going too high or too low. Thus, would I then use a BMS module that connects three batteries in a series, or would I need to have a BMS with 12 connections, including the cells that are connected in parallel.
TBH, I am not at that level yet, but just trying to understand the nuances of li-ion batteries before I get my hands dirty.
Best Answer
For the most part, putting cells in parallel just makes them behave like a bigger single cell. So, if you take four cells and hook all of them together in parallel, it appears to a circuit to just be a single cell with four times the capacity.
BMS's are built to manage cells in series. Along with current and voltage protections, it monitors each "cell" in the pack to make sure its voltage is within limits, and if any one cell dies prematurely it will cut off the whole pack to prevent the other cells from reverse-charging the dead cell.
Your configuration is "3s4p" - three groups of four parallel cells wired in series. Thus, you need a BMS that can manage three cells in series - a "3S" BMS. Generally speaking, it's irrelevant how many cells you put in parallel in each cell group, as long as all the groups have the same number of cells at similar capacities (i.e. you do not want to put one parallel group of 3 cells in series with a parallel group of 4 cells), since the BMS will see your parallel groups as single larger cells and will manage accordingly.