PCM is wired to cells in series.
PCM applies LVC (low voltage cutoff): when one cell is getting too low, PCM shuts down the whole battery. No more discharge... neither the battery asa whole, nor any cells that constitute the battery will be allowed further discharge. Even is they are imbalanced.
PCM applies HVC (high voltage cutoff): When any cells is getting too high voltage (typically around 4.25 to 4.28V), PCM shuts down whole battery so that it does not accept any more overcharging of the battery as a whole, nore for any of the cell that constitute the battery.
PCM also applies a maximum dicharge current cutoof (see PCM specsheet). If you exceed it, battery shuts down power via the PCM.
Now a BMS is a PCM that also has a balancing circuit attached to it.
When the highest voltage cell group in the serie reach a certain voltage (called balancing voltage: typically around 4.20 to 4.22V) during charge process, the balancing circuited discharges only that series group (until it reaches a voltage release value. e.g. 4.10V). Then battery starts bulk charging all cells agains. Eventually, the highest voltages cells are discharged enough that they reach the same voltage as the lowest cell... then bulk charging all the cells back to 4.20V and the battery is completely charged....
A PCM will not balance anything... And as soon as one cell reaches the HVC (eg 4.25V), the whole battery pack stops charging, even if most of the other cells are at 3.83V... or so...
With a PCM, you are garanteed that the pack will eventually go out of whack.... Even more than without any PCM, because the PCM will have parasitic current drain, likely on the first one to first three cells of the group (uneven parasitic drain). But at least it will prevent overcharging and battery fires..... So as long as you manually balance charge one in a while... you're good.
BMS is way better, as long as you charge up to balancing threshold voltage (at least once in a while).
So basically, a BMS is a PCM with a balance circuit.
On certain website (Like the BesTech Power website), you can actually buy the complete BMS of a given model, just the PCM or just the balance circuit... That can be usefull is you mistakenly burn one component during the wiring process...
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