I understand that there are delays between different manufacturers and control systems, and I agree that some oscillation is unavoidable with AC coupled systems.
But in my case the issue seems different.
My Fronius on AC input is exporting only around 2 kW while my export limit is set to 6.6 kW. House consumption is already covered, batteries are full, and there is still about 4.5 kW export headroom available.
Despite that, the Victron MPPT sometimes drops to 0 W or stays stuck at a few hundred watts instead of using the remaining export capacity.
After a Cerbo reboot everything immediately works normally again.
That behavior makes it look more like an ESS / power limiting logic issue than normal control loop delay between devices.
The system:
3x MultiPlus 5000
Fronius Symo on AC input
Victron MPPT on DC
Pylontech US5000
EM540 meter
ESS with export limit
I think the issue appears specifically in mixed AC PV + DC MPPT + ESS systems when batteries are near/full and export headroom still exists.
I would like to provide some additional information and graphs regarding the ESS / MPPT behaviour I am observing on my system.
System configuration:
3x MultiPlus 5000
Fronius Symo on AC input
Victron SmartSolar MPPT on DC
6x Pylontech US5000
EM540 energy meter
ESS with export limit set to 6.6 kW
What I am observing is the following:
When the batteries are full, the Fronius inverter continues operating normally and exports around 2 kW, while there is still significant export headroom remaining before reaching the configured 6.6 kW export limit.
At the same time, the Victron MPPT sometimes suddenly drops to extremely low power levels or even close to 0 W, despite:
house consumption already being covered,
export headroom still being available,
and no apparent reason for aggressive throttling.
The attached graphs show:
Fronius production remaining relatively stable,
while the DC MPPT output repeatedly collapses.
Most importantly:
after rebooting the Cerbo GX, the MPPT immediately returns to normal operation and starts producing correctly again.
This behaviour makes me believe the issue may not only be caused by normal regulation delays between different manufacturers or control loops.
Instead, it appears possible that ESS / dynamic power limiting logic may occasionally become stuck in an incorrect throttled state in mixed AC PV + DC MPPT systems when:
batteries are full,
AC PV is active,
DC MPPT is active,
and export limit headroom still exists.
I wanted to provide this feedback together with real production graphs, because the behaviour appears reproducible on my installation.
Thank you for your time and for any feedback or suggestions.
This is indeed very different from what I described, from my situation, in regards to power control delayed reaction.
I’m not a specialist with regards to those setups, can’t give good advice on it, sorry.
Have you looked into other ‘external power limiters’ like AC voltage and frequency? Although those would not be changed by a restart of the controller.
How are the fronius and cerbo communicating over Ethernet or WiFi? Maybe the communication drops and isn’t able to restart its connection or protocol. I can see such a thing being solved by a restart. Might be worth a look, if only to discard as a possibility.
Sounds like a config or install issue. We see occasional reports like that, but usually there is a cause (grid code was one, CCL being set to 0 was another).
Instead of trying to redesign ESS, which works just fine, you would be better off opening a specific topic to diagnose your issue of DC throttling in a PV inverter environment.
At the moment this topic is winding between both those disparate subjects.
Can you check the MPPT states in VRM during that time as well as the mppts reported chargecurrent limit? For the state, does it say “voltage or current limited?”
Do you have DVCC enabled?
Check your ACCurrent Limit AND Peakshaving limits are not set to low by accident. If either limit is reached, DC PV is the first thing to be throttled.
I am using a Victron SmartSolar MPPT VE.Can 250/100.
DVCC is enabled and the system communicates directly with 6x Pylontech US5000 batteries.
The issue is not related to AC input current limits or Peak Shaving settings.
When the batteries are near 100% SOC and the system reaches absorption or float, the MPPT occasionally drops to 0 W, stays there for a few seconds, then starts searching for a new MPP and recovers again.
During this time, the Fronius inverter continues producing normally.
A Cerbo GX restart immediately restores normal operation, which makes me suspect the issue is related to GX/ESS control logic rather than PV input limitations.
The problem has been reported by multiple users with similar Victron ESS + Fronius systems.
Next time it happens, I will check the VRM logs and report the MPPT state (Voltage limited / Current limited) together with the reported charge current limit.
CCL is not 0. The batteries still allow charging and the issue disappears immediately after a Cerbo reboot. If CCL was the reason, a reboot should not instantly restore MPPT production.
I also know another installation with a very similar configuration that shows the same behaviour:
Fronius AC PV
Victron DC MPPT
ESS
Pylontech batteries
The issue mainly appears near/full SOC while export headroom is still available.
That is why I suspect this may be related to ESS / dynamic power limiting behaviour in certain mixed AC PV + DC MPPT scenarios rather than a hardware issue.
already use DVCC with a manually limited charge voltage of 52.5 V, so my system never charges to 53.2 V. Therefore I do not think the issue is caused only by the higher Pylontech charge voltage
have also attached my ESS export settings.
Both AC-coupled PV and DC-coupled PV export to grid are enabled.
The export limit is set to 6600 W and active.
Therefore, when Fronius is exporting only around 2 kW, there should still be significant export headroom available for the MPPT.
At the moment the system is in Bulk because the battery is only at 87% SOC.
The issue typically appears when the batteries are near 100% SOC and the system is entering Absorption or Float.
I will monitor it and collect screenshots the next time it happens, before rebooting the Cerbo GX.
Also, my water heater relay is configured to switch on at 97% SOC. This means there is already a significant load available before the batteries reach 100%.
Therefore, the system should have enough time to react to excess PV power and gradually reduce charging power if required.
From a practical point of view, it does not seem like a sudden surplus event.
One thing I find interesting is that the issue does not occur every day. Under seemingly similar conditions, the system can operate normally for an entire day with the batteries at 100% SOC. On other days, the MPPT drops to 0 W and starts a new MPP search cycle. That makes me think it may not be a simple configuration issue.
The battery is at 99% SOC and export to grid is enabled. Both AC-coupled and DC-coupled PV export are allowed, and the export limit is set to 6600 W.
At the time of the screenshots, the Fronius inverter was producing approximately 4.3 kW, while the MPPT was producing only around 100 W.
Since the export limit was far from being reached, there was still several kilowatts of export headroom available. Therefore, I would expect the MPPT to continue producing and exporting power instead of being limited to almost zero output.
This is the behavior I am trying to understand.
Another interesting observation is that the issue does not occur every day. Some days the system behaves normally, while on other days the MPPT drops to very low power or
I have also recently moved the Fronius communication from Wi-Fi to Ethernet, so I will continue monitoring the system for the next few days to see if this makes any difference.
Thank you for your time and assistance. I am simply trying
Nick,
Thanks. I created the requested widget and attached the graph.
Red = VE.Bus charge voltage setpoint. Blue = MPPT voltage.
The battery is at 99–100% SOC, all 6 Pylontech US5000 modules are online, no charge/discharge blocks, cell voltage difference is only about 26 mV and temperatures are normal.
Despite this, the MPPT voltage becomes unstable and the MPPT repeatedly reduces output or drops to 0 W, while the Fronius inverter continues exporting power to the grid.
The issue is reproducible and usually disappears after a Cerbo GX reboot or when a significant load is switched on.
I have also attached the MPPT power and Fronius production graphs from the same period for reference.
