The BMS is the Battery Management System.
It performs several functions. The two fat wires (red and black) from the charger will “bulk charge” the pack until it gets very close to being full, and then the charger will switch over to using a very low charge rate as it gets closer to being full. A 3A or 5A (continuous) charge rate is very common for the bulk charge.
This charging profile is called CC/CV, for Constant Current / Constant Voltage. It’s a simple and inexpensive way to accomplish a subtle goal.
We all want an affordable battery pack, so…we buy mass-produced cells. This means that there will always be very minor differences in the internal resistances of each cell. To use the example of a theoretical 7S/4P pack…each 4P cell-group is “seen” by the charger and controller as one large cell. The parallel connecting metal strip ensures that they all constantly equalize to each other, so we must discuss them as if they were in fact, one large cell.
There are seven of these “in series” to get 24V. Now, we then set our dumb bulk DC power-supply as a charger to 3A (with NO BMS), and we use (7S X 4.1V =) 28.7V as our fully-charged goal. It works like a dream. However, only five of the P-Groups are actually at 4.1V. One P-group is at 3.9V due to high internal resistance, and another P-Group is at 4.3V due to low internal resistance. Since our dumb charger only reads the 28.7V of the assembled pack when it shuts off, it has no idea of the trouble that is brewing…
High-resistance cells run hotter than an “average-resistance” cell, but for this discussion, let’s just assume that it never gets “too hot” to cause trouble (over 140F / 60C). Also…cells located at the heart of a pack run hotter than cells at the edge, since the edge-located cells shed “some” heat to the outside shell of the pack.
That leaves the low-resistance cell for us to consider. It will dump amps faster than the other cells (when accelerating), and it will also gulp the charge faster, too. It will actually run cooler than the other “average resistance” cells, but…a bulk charger will overcharge it. If letting a pack sit overnight at 4.2V per cell will cut the life of the charge in half (compared to 4.1V per cell).
What will letting one cell sit overnight at 4.3V do? It will lose its capacity rapidly.
And that means that, the one bad cell will cause the entire P-group to experience voltage-sag near the end of a ride, and then…that one low P-group will cause the entire pack to experience voltage-sag. And this means that…for a split second on acceleration, the LVC will “think” that the entire pack is too low, and it will cut off ALL power in order to “save” the pack (one of its most important jobs).
The pack will still “work”, but…your accelerating days are over. You may have planned on buying a new pack in three years (or more), but because of one “slightly” bad cell, the entire pack may now be useless to you after only a few months. This is why the bulk-charge phase (CC/Constant Current) only takes the pack to about 4.0V per cell. For the rest of the topping-charge, the Constant Voltage / CV phase is accomplished at low amps, with some sensitive electronics thrown in…
Side note: If your battery pack dies suddenly for no obvious reason, it is usually from some component in the BMS failing, leading to a complete drain of the pack down to zero-volts, or…a gross overcharge above 4.2V. This is why they are sometimes called a Battery-Murder-Suspect.
