Hi.
Usual settings: multiplus+cerbo+some US3000C pylontech modules.
Does anyone know how would the system react if one of the modules reaches 0% soc before the others? Does it shut off and the load is shared only among the remaining ones. Or does the whole stack of modules stops working as a whole?
I’ve never come close to anything like that but I’ve always been curious.
That’s because as far as I know pylontech modules of different sizes can be in the same stack. The only constraint is probably that the largest one is the master. If this is true then the stack should continue to work when one of the smallest modules reaches 0% soc. Obviously the maximum discharge current should reduce accordingly but that change should be communicated to the cerbo by the master module right?
Does anyone know how the system would behave?
Thanks
The state of charge to voltage curve of Lifepo4’s are very steep near fully charged and fully discharged. Therefore a steep voltage drop of the discharged module will make it receive current from the other modules. This fenomena will prevent this module from discharging even further. In other words you worry too much. The system will self balance itself.
OK, that’s interesting. But that’s from a physics point of view and that’s fine.
But I’m more uncertain about the software/firmware side of this problem. There’s probably a voltage threshold below which the firmware of the fully discharged module will take some action.
I was wondering if that possible action would block the whole stack or however it might affect it.
Hence I was asking if someone has actually witnessed a similar case in a working system.
Pylon’s aren’t rated to discharge 100% that wouldn’t be a great idea.
The BMS will intervene and shut the pack down, lowering system limits.
If it is the master, then comms will be lost and the inverter will timeout and power off with a BMS lost error.
It’s more likely that the voltages will drop below thresholds and the inverters will power off.
The way it affects the system is that in case of one “near empty” pack, the remaining maximum BMS current is reduced to the sum of the maximum currents of the non empty packs. So high loads could form a problem. Besides that the BMS’s should deliver the needed current into the empty pack to keep it from that low threshold where it the output current is cut-off by the BMS. In other words again you worry too much.
The problem with that reasoning is that then pylontech shouldn’t have as a selling point of that series of products that you can mix packs of different sizes. Because in that kind of configurations having the smallest pack go to 0% is the way it is supposed to work (otherwise the biggest packs would be useless). At least they should set the maximum level of discharge of a pack to a point where it does not damage itself.
As I wrote, mine was just a simple question born out of curiosity.
Where do you see me worrying in what I wrote?
Or is ending any answer you give to any question with “In other words you worry too much” your way of being funny? (mission failed, if that’s the case…)
If you mean electrically, they are in parallel as each one of them connects to busbars. But even if they were electrically connected in the usual way, i.e. with the small cables going from one to another, the small cables basically form two busbars, so they should be in parallel even in that way.
If you mean the coms cables, they are connected one after the other in the only way possibile, but again, I think electrically they form a bus so on pack dying should not affect coms with the others (except the master).
Thats a different scenario.
The bms will manage discharge and will never run one flat.
Depending on the speed of discharge the remaining batteries that are allowed to discharge will keep the other powered.
That’s true, but most BMS also use the lowest cell voltage to do a low voltage disconnect, so the first pack the reach the cell Vmin will probably disconnect even though the total voltage is the same as all the others.
Pylon like others, will offline a battery that has exceeded cell limits. The others will remain online.
Likewise if there is a big disparity between whole battery packs, it will block that battery. Throughout charge/discharge limits will change.
What models can be connected together, and what versions are required , what needs to be master is covered in their docs.
They have a mature bms, and it is tried and tested. Also, remember victron applies a custom charge algo to these batteries which always contains overvoltage by varying CVL.