Can you add the BMS CCL to that widget please.
The system is actively dropping it’s setpoint so it is clearly instructing the DC chargers to stop production.
Here is the graph with CCL added.
The BMS does reduce the Charge Current Limit during the day, however the CCL remains at approximately 120 A for most of the period when the MPPT output becomes unstable.
120 A at approximately 52 V still represents more than 6 kW of allowed charge power, so I am trying to understand why the MPPT repeatedly drops to 0 W while the battery is not blocking charging and Fronius continues exporting power.
Allowing 120A doesn’t mean the battery wants it, you can see the battery voltage sharply ramps up at the top of charge as lithium batteries do. When the voltage reaches the charge voltage setpoint charging stops and the mppts stop producing. When the setpoint raises above the battery voltage the mppt’s will produce again.
With pylons DVCC automatically lowers the charge voltage based on cell differentials to aid balancing. So it would be good to see a min/max cell chart covering the same time period.
At the same time your PV inverter is covering all loads and exporting.
the MPPT is stopping only because the battery reaches the charge voltage setpoint, can you explain why a Cerbo GX restart immediately restores normal MPPT production while the battery remains full and all battery limits remain unchanged?
After a Cerbo restart, the MPPT starts producing normally again and exports together with the Fronius inverter, even though the battery is still at 100% SOC. If the battery voltage alone was the reason, I would expect exactly the same behavior before and after the restart.
How long does that last for? Remember that DVCC is running on the GX which is coordinating and calculating limits. Without a GX being online (while restarting) the DC mppt’s would shut off regardless and the inverters would go into passthrough.
Do you have any override values set in DVCC?
.After a Cerbo GX restart, the MPPT starts producing normally immediately and continues working normally for the rest of the day.
I am not using any DVCC current override. The only manual DVCC setting I use is the maximum charge voltage limit set to 52.5 V.
Apart from that, DVCC follows the limits provided by the Pylontech BMS.
That is why I am trying to understand why the MPPT repeatedly drops to 0 W before the restart, but then works normally for the remainder of the day after a Cerbo GX reboot, under the same battery conditions
You definitely have some cell differences going on.
There isn’t any value settling a voltage limit in dvcc when it already clamps the voltage lower.
differences, but they are only around 50 mV. What I still do not understand is why a Cerbo GX restart restores normal MPPT operation for the remainder of the day if the cell voltages, SOC, CCL and battery conditions remain essentially unchanged.
Can you explain the mechanism that would cause that behaviour?
Can’t answer that.
How are comma to the fronius?
Can’t be a dvcc reset as it would just calculate the same values unless something changes during the restart.
It’s too difficult to try guess without proper access.
The Fronius Symo is connected to the Cerbo GX via Ethernet and communication appears stable.
After a Cerbo GX restart, the MPPT starts producing normally immediately and continues working normally for the rest of the day.
I am not using any DVCC current overrides. The only manual DVCC setting is a 52.5 V maximum charge voltage, which was recommended by Victron and my installer as an additional safety limit.
I also know several users with very similar systems (ESS + Fronius AC-coupled PV + Victron MPPT + Pylontech batteries) who observe the same behaviour, where the MPPT intermittently drops to 0 W while the Fronius continues exporting power.
That is why I am not convinced this is purely related to cell balancing. The restart behaviour and the fact that it has been observed on multiple similar installations suggest there may be another factor involved.
Do you know of any known ESS, DVCC, MPPT or Fronius interaction that could explain this?
My current suspicion is a possible ESS/DVCC control logic issue, because the battery conditions do not significantly change after a Cerbo GX reboot, yet the MPPT behaviour immediately returns to normal.
If it were only battery balancing, I would expect the same behaviour before and after the reboot.
error-active is misleading, that is the normal state.
When disconnected it is passive.
There are no bad packets with the exception of a tiny amount on the mppt.
would like to add that this does not happen all the time. Today I observed the MPPT briefly drop to 0 W while the PV voltage was around 180 V and the battery voltage around 52.3 V. The status remained “External control” and after a few seconds the MPPT resumed normal production by itself.
However, on some days the MPPT can repeatedly drop to 0 W for longer periods until a Cerbo GX restart is performed.
This is why I still suspect there may be an ESS/DVCC interaction involved rather than a communication issue, battery cell imbalance or a hardware fault.
Have you seen similar behaviour on other systems with ESS, Fronius AC-coupled PV, Victron MPPT and Pylontech batteries?
Thinking on this issue is that moving AC PV to AC OUT typically resolves the behaviour.
Hi Nick,
Thank you for your suggestion.
Unfortunately, I cannot move the Fronius inverter from AC IN to AC OUT. My system is registered under a government-approved feed-in arrangement and receives a Green Bonus subsidy, so changing the AC-coupled topology is not possible.
Do you think the behaviour could be related to the ESS interaction between the AC-coupled Fronius and the DC-coupled Victron MPPT?
Thank you for your help.
It is more likely related to inverter behaviour under fringe conditions, that include AC IN PV, AC charging and DC feed-in enabled.
Not easy to identify as often there are other causes that can trigger similar symptoms.
There was another case, iirc, that was resolved by moving PV.
Thank you, Nick.
That makes sense. My system uses:
Fronius AC-coupled PV on AC-IN
Victron SmartSolar MPPT 250/100
6× Pylontech US5000
ESS with DC feed-in enabled
The MPPT occasionally drops to 0 W for only a few seconds and then immediately recovers by itself. It does not happen every day.
Battery cell balance looks good (typically below 30 mV difference) and there are no CAN communication errors.
Do you know if this behaviour is already known to Victron engineering, or if there is any ESS debug information I could collect when it happens again?
Thanks again for your help.