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Is PCM good enough to replace BMS?

Posted: Nov 25 2013 4:03am
by tmho
I am asking a friend (he has a factory in China producing cells and battery packs) to build a 48V 15AH LiFePO4 battery pack for my ebike. It will be 16S5P using 26650 cells. He is suggesting to use PCM (protection circuit module, a small circuit board for each cell to protect overcharge and over discharge) instead of BMS for simplicity. BMS has the balancing function which PCM does not have. But if each cell has the PCM, every cell will finally reach the same voltage (set by the PCM) and this is a form of balancing. Will this mean if PCM is installed in each cells, the cells will also be balanced during (or may be at the end) charging? If this is correct, is PCM good enough to replace BMS?

Re: Is PCM good enough to replace BMS?

Posted: Nov 25 2013 2:10pm
by wesnewell
A single cell pcm won't balance anything, however at both top and bottom of charge that would be the same effect assuming they are all the same. I'm pretty sure that's the same way most BMS's balance too. My RC charger will balance at any voltage, but I'm not sure if all rc chargers will do that either. As long as the pcm will also limit amp draw to the cell max, then it should be just as good imo.

Re: Is PCM good enough to replace BMS?

Posted: Nov 25 2013 2:56pm
by liveforphysics
PCMs are typically more added liability than added safety in large packs.

Re: Is PCM good enough to replace BMS?

Posted: Nov 25 2013 10:14pm
by tmho
But there are 16 groups of cells in series. When 1 group has reached the maximum voltage, it will block the current. Then will it also stop the other groups of cells from being charged?

Re: Is PCM good enough to replace BMS?

Posted: Nov 26 2013 2:12am
by wesnewell
I don't like or use BMS's, so someone can correct this if I'm wrong. When charging with a BMS, it monitors each cell group (one of more parallel cells). When that group reaches the max voltage limit, the charger stops charging. Once that group has bled down to a predefined voltage, the charger will start charging again. It will continue to cycle like this until all cell groups are fully charged and thus balanced. On discharge, when any of the cell groups go down to LVC level, the BMS then disables output. Using PCM's, I would assume charging current would continue, thus still charging other cells until the pack reaches the chargers max voltage.

Re: Is PCM good enough to replace BMS?

Posted: Nov 26 2013 1:05pm
by Toorbough ULL-Zeveigh
if you could control the Protection Circuit Module via Pulse Code Modulation with the cells imbedded in Phase Change Material.

Re: Is PCM good enough to replace BMS?

Posted: Nov 26 2013 2:15pm
by nicobie
Toorbough ULL-Zeveigh wrote:if you could control the Protection Circuit Module via Pulse Code Modulation with the cells imbedded in Phase Change Material.

That's what I do. :mrgreen:

Re: Is PCM good enough to replace BMS?

Posted: Nov 26 2013 9:40pm
by tmho
nicobie wrote:
Toorbough ULL-Zeveigh wrote:if you could control the Protection Circuit Module via Pulse Code Modulation with the cells imbedded in Phase Change Material.

That's what I do. :mrgreen:
Hi, could you please explain a little more on what that is?

Re: Is PCM good enough to replace BMS?

Posted: Nov 26 2013 11:30pm
by Gregory
It's a tongue in cheek statement about the overuse of TLA's.

PS i wouldn't want all those pcm in my battery.

Re: Is PCM good enough to replace BMS?

Posted: Nov 29 2013 1:22am
by tmho
I am sorry that I might have some misunderstandings on PCM. From the internet, I found that some PCM can have balancing function in addition to over charge and over discharge protection. Then what is the difference between PCM AND BMS? Could someone please tell me?

Re: Is PCM good enough to replace BMS?

Posted: May 14 2019 10:44am
by agniusm
tmho wrote:
Nov 29 2013 1:22am
I am sorry that I might have some misunderstandings on PCM. From the internet, I found that some PCM can have balancing function in addition to over charge and over discharge protection. Then what is the difference between PCM AND BMS? Could someone please tell me?
This my own explanation so dont take it word by word.
PCM is circuit protection module and it does just that. It has hardware preset limits e.g. balancing, overcurrent, under/overvoltage and generally is simple
BMS is battery management system. It is more sophisticated and will have some sort of mcu on it, communication protocols and advanced set of futures. Most of the limits and values will be cunfigurable by end user and it probably wont be tied to single chemistry battery. You could use BMS on LTO battery, and then transfer it to lifepo4, or cobalt or other and program it to that specific cell.
Liveforphysics stated that it is more liability but from my point of view its safer for diy people as it is less confusing and a lot simpler to use still giving some degree of protection.

Re: Is PCM good enough to replace BMS?

Posted: May 16 2019 12:37am
by Gregb
It should be ok. The big difference is a BMS will look at the total voltage and give you cutoff of the battery. I have no idea what will happen if one PCM module drops out. Will the rest of the battery continue to function at reduced voltage? I would include a total voltage alarm on the battery as well as on the cell rows. That's what i do as i dont have a BMS. I have only seen them up to 8 cells so you would need a straight voltmeter with alarm settings. Or rig up a couple...

Re: Is PCM good enough to replace BMS?

Posted: May 21 2019 11:24pm
by Matador
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...

Matador

Re: Is PCM good enough to replace BMS?

Posted: May 21 2019 11:43pm
by Matador
Here is an example from the "BesTech Power" website...

Example of Model HCX-D131A...

You can buy the whole BMS (PCM+Balancing board):
201211422736929.jpg
201211422736929.jpg (106.37 KiB) Viewed 581 times
Link: http://www.bestechpower.com/518v14spcmb ... -D131.html

Or you can buy just the "Balancing board" part of the BMS:
balance board.jpg
balance board.jpg (226.22 KiB) Viewed 581 times
Link: http://www.bestechpower.com/balanceboard/JH-D131A.html

Matador