If we take a look at configuring two individual 18650-format cells, it will help new ebikers to understand why batteries are they way that they are. Each cell has two electrodes, a positive cathode end, and the other end is a negative anode. If we attach the positive of one cell to the negative of the other, that is a “series” configuration, and it doubles the voltage, but keeps the Amp-hours (Ah) of range and current-output capability the same.
If instead, you took those same two cells and attached them side-by-side, and then connected both cathodes together with a wire, and both anodes together with another wire…that would be a “parallel” configuration. Doing that results in a 2-cell sub-pack that has the same voltage as a single cell, but…the Ah of range and the current-output capability has been doubled.
The majority of lithium-based cells that you will find in ebike battery packs have a “nominal” or average voltage of 3.7V per cell (and per paralleled sub-packs of cells). They can be fully charged to a voltage of 4.2V per cell. They are commonly considered to be “empty” when they get down to 3.0V per cell.
[as a side note, there is a significant increase in the cycle life of a pack if it is large enough for you to be able to only use it between 3.3V as the empty setting, and 4.1V per cell as the full setting, between charges]
It is possible to group a bunch of your cells into series sub-packs first, and then connect the series sub-packs into parallel groups. However, almost all battery pack builders group them into parallel groups first, and then they series those parallel sub-packs. The reason is that…if you do it that way, it is electronically easier to treat each parallel sub-group as a single large 4.2V cell.
Just as an example, a common battery pack might be a 48V / 15-Ah pack.
If you use a 3000-mAh cell, then 5 of them per parallel group (5P), would result in 15-Ah. Then, you would need 13 of those 5P sub-packs to make a pack that is 13 sub-packs “in serial” (13S). The resulting pack would be 13S / 5P
Since each parallel sub-pack electronically “performs like” a single large 4.2V cell, a BMS with 13 channels can monitor and adjust each of the 13 “5P” groups to keep them healthy.
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