Most BMS are made cheap. Thus the electronic is very basic, hence why it does feed up only of one row of cell rather than feeding equally on all cells. Plus if it is a balancing BMS, and if you use your eBike regularly, the BMS will balance the cells and you wont even notice the unequall draw of current on certain row of cells. But beware, many so-called "BMS" do not balance. These are more acuratlyu refered to as protectiv PCB (that have for function to protect cell from overdischarge, over-charge and overcurrent... But they do not balance).
Some high quality BMS might pull equall draw on all cells, but you would have to research their specs. And most battery builders do not care about very high quality and opt for cheap BMSs.
Being the cells are all interconnected why shouldn't the BMS parasitic drain be similar among all the P-groups?
Because it's cheaper for BMS manufacturers to do it that way.
It's not that the cheap BMS is faulty. It's just that it's design was not thought out to be made for people who plan on storing their batteries for years. Anyways, if you let a Balancing BMS do it's magic balancing every 10 or 20 charge cycles or so, your battery should stay balanced without a problem. But if it a so called "BMS" that does not balance, you are out of luck and need to manually balance the cell yourself.
What's the best KIS way to determine whether it's a cell problem or BMS problem? Or are we safe to assume it's the BMS. Could that disfunctional BMS have been caused by some abuse to the battery pack?
You could use an ammeter to check for the BMS parasitic current value (aka how much uA it pulls), and to check if the BMS pulls that parasitic load on all cells or just some specific rows of cells.
Otherwise if you need to test the cells themselves, it's more tedious. You need to test them for residual capacity (mAh), DCIR (millioms), voltage stability over time (if the cell has an internal short), check if they are old (with the manufacture code on the cell), etc. You can load test the cells to estimate their internal resistance (measure voltage drop of specific cell groups by pulling a know load (in amps, measured with ammeter wire in series) to calculate Internal resstance (using ohms law: voltage drop (delta V) = R x I ).
Matador
