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bkvanduk avatar image
bkvanduk asked

Min SOC is not reached - Pylontech US2000C - sustain

Some of our installations are going to sustain mode before they are reaching the min SoC set to 10%.

The installation: Pylontech US2000C battery modules, MultiPlus II 48V Inverter, ESS assistant is loaded.

I know I should change the dynamic cut off voltage and the sustain voltage settings: https://www.victronenergy.com/media/pg/Energy_Storage_System/en/controlling-depth-of-discharge.html

At the moment I use the recommended settings:

ESS Parameter Settings
Sustain voltage. 48V
Dynamic cut-off values set all values to 46V.


I tried to decrease the sustain voltage on a system to 47V and tried to increase the dynamic cut off on another system to 46,5V. --> But the same thing happens.

Any recommendation?


Multiplus-II
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seb71 avatar image
seb71 answered ·

First, decide where you ask a question. Don't ask the same question in multiple threads.


You say that you increased the Dynamic cut-off voltages. That will make the system to enter the sustain mode sooner (at higher voltage), not later.


If you set the Minimum SOC at 10%, that means the battery is very close to empty. BMS managed batteries (such as Pylontech) are balanced at the top (the aim of the BMS is to get all cells at the same voltage at the top - when fully charged). This means that if the cells are not well matched, when the battery is close to empty, some cells could reach lower voltages than others.


Also usually rated capacity of the battery given by the manufacturer is between some wide ranges of cell voltages. For instance minimum voltage they consider might be well under 3V (cell voltage). In real usage, I would not recommend to drop under 3V (cell voltage).


Keep in mind that Pylontech 48V batteries have only 15 cells, not the typical 16 cells.


You did not said what is the typical current you have during discharging (especially when close to empty).


SOC is an approximate quantity. Voltage is king.


In my opinion, if you have to set Minimum SOC to 10% and you frequently reach that level, the battery capacity is undersized for the usage.

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bkvanduk avatar image bkvanduk commented ·
I increased the cut of voltage because sometimes the system discharged below minSOC. I set the minSOC to 10% and it discharged to 8%. Now it's exiting the sustain mode on a higher voltage, then before.


On another system I changed the sustain voltage from 48V to 47V. Now the discharge is stopping at 11%, this point the voltage is ~47.3V, sustain mode is entered, I can see low battery alarm too. And it's charging itself back to 12%, here the voltage is ~48V. Sustain mode is exited ~48.2V.

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seb71 avatar image seb71 bkvanduk commented ·

because sometimes the system discharged below minSOC. I set the minSOC to 10% and it discharged to 8%.

That is normal if it happens when there is no solar power to charge the batteries (such as over night). The inverter, GX device, etc. are still powerered from the battery.

Now the discharge is stopping at 11%

Most likelly it is stopping because a low voltage condition (battery voltage or cell voltage) is triggered. So not based on SOC.

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bkvanduk avatar image bkvanduk seb71 commented ·
So as I understand, it's not possible to go down to 10% SoC because the voltage drops too low and the battery is flagged as empty. Even if I change the ESS assistant settings, the same thing will happen.


Sometimes the batteries are going to sleep at low SoC. Maybe this is happening due to protection. My other problem is that they are not waking up if solar is available. Is the only solution here to change minSOC to higher than 10%? So the voltage won't drop too low to send the batteries to sleep?

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seb71 avatar image seb71 bkvanduk commented ·
Can you see cell voltages?


Try to find out which is the actual factor which stops discharging: battery voltage, cell voltage, Minimum SOC.


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nickdb avatar image nickdb ♦♦ seb71 commented ·
Depending on the firmware, you may not see this via the GX and would need the pylon tools to get more detail.

The GX should however be reporting battery voltage and discharge current.

Like you, I don't see the reason to repeatedly discharge a battery that low, it is just undersized if that is a requirement.

Pylons also have terrible discharge rates so if there are too few batteries and too large a load, only bad things will happen.

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