thomasw-1 avatar image

Throttled Fronius wastes up to 90% of capacity - improvement required on Zero-feed-in

Hi All,

With this documentation I like to demonstrate that Fronius Zero-feed-in algorithm ignores up to 90% of available energy in a zero-feed-configuration, although all produced energy could be fed into battery at that moment.

@Victron&Fronius-Developers: Please read this article and translate improvements into next software releases.

Generic problem description

My PV panel power balance between MPPT-fed and Fronius-fed energy ratio is about 45% vs. 55% (my complete topology is described below). This screenshot demonstrates that this ratio can be also effectively generated and loaded into AC-IN and battery as long as the grid is disconnected from MultiPlus-II.


Only 30 seconds before when grid was still connected, I took that screenshot. You see that MPPT has nearly the same output, but the Fronius delivered only 28% of its actually currently possible capacity. Since the previous screenshot was taken before, it demonstrates clearly that at that moment there was no restriction of the battery taking additional energy (charging limit of 242A current allowed by BYD at that moment of time).

Frist conclusion
Fronius Zero-feed-in feature in Victron ESS wastes plenty energy (>75% in average – up to 90% in peak)

Closer analysis
Following analysis demonstrates that Fronius Zero feed-in mechanism only works as long as you have a steadily deviating power consumption. Otherwise it wastes plenty energy.

1. AC load on AC-OUT consumption is currently low at 249W and Fronius feeds in with 934W only (compared to screenshot in step 3 you see that was is only 50% of the available capacity at that moment).

2. Switch on a mentionable load on AC-OUT1 activates the Fronius within a few seconds to leverage full available capacity.

3. After having switched off the additional load, Fronius continues to produce similar but already slight lower volume. The produced capacity is fed mainly into battery.

4. As long as there is no peak any more on AC-OUT1, Fronius lowers its production steadily down. In that example its returned to 23% of currently available capacity.
Battery SOC was 52%, the currently applied current 52,9A with readiness to take 242A at that moment.


Please improve zero-feed-in feature so that also battery will be effectively loaded with available capacity on Fronius attached PVs.

My environment

  • MultiPlus-II 48/5000/70-50 in 3-phase (firmware 477) with a ESS zero feed-in configuration
  • ESS zero feed-in configuration
  • Multiphase regulation on single phase
  • Grid setpoint defined at 80W > currently returned to 60W again
  • Fronius Symo 6.0-3-M (firmware 3.16.7-1) with 6.030 kWp attached (55% of total PV) – connected to AC-OUT1 (critical loads) – no additional smart meter - sunspec activated
  • Victron MPPT VE.can 250/100 rev2 (firmware 3.02) connected via VE-Bus with 5.025kWp attached (45% of total PV)
  • PV modules all with absolutely identical direction and no shadowing
  • BYD LV 15.4 (allowed charging and discharging current during the test procedure 243A)
    BMS connected via CAN-bus (firmware 1.16)
  • VenusGX (firmware 2.60) > currently tested with 2.62-beta5
  • Any component with latest firmware as of today


ESSBYDFroniusfronius and victronzero feedin
00-wasting-90.jpg (342.2 KiB)
01-wo-grid.jpg (307.6 KiB)
01-with-grid.jpg (339.6 KiB)
02-start.jpg (354.9 KiB)
topology.jpg (109.1 KiB)
2 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.


Is it possible you have missed something in the settings. I have seen a working zero feed in system, 3 phase with fronius eco on ac out and charge the batteries from excess pv no problems?

Maybe battery life setting? The system does not seem to be interested in charging the battery for some reason.

Hi Alexandra,
No, the setup was checked tripple trimes, also via remote access by the Victron distributor.
Tests were performed with and without battery life options.

In the diagram below you can see clearly, that in red marked sections the battery did allow a charging current between 73A and 243A. But during both periods the Fronius inverter got limited up to 100%. The deviations on left example were impacted by direct energy consumption. As soon as direct consumption was ended, the limit was reestablished. Right example is mucher clearer and evident!

So for sure there is a high interest of the batteries getting charged, but MultiPlus-II unfortuntaly don't.

You mentioned that in your environment it would work. Just assuring that we have a comparable environment?

  1. Do you also have a mixed setup with MPPT and Fronius inverter or the a Fronius inverter only?
  2. Do you also have a zero-feed-in configuration activated?



1607768890116.png (36.7 KiB)

Another view on Battery current, Charging Limit and Throttled Fronius Power. During red phase more power could have been loaded into battery, however Fronius got throttled. That view is restricted on second day only (right part of previous diagram).


1607772700715.png (51.9 KiB)
5 Answers
thomasw-1 avatar image
thomasw-1 answered ·

Hi Folks,

With VenusOS 2.65 this issue has been fixed.
Fronius doesn't get throttled anymore and charges battery with full available capcity.

Although my PV-modules are suboptimally installed with a 17° angle, this week the peak production reached 102% of nominal peak module capacity. Batteries were charged above 160A. Fronius runs at its maximum limit ;-)

That is, what I expected.
Thanks to Victron team, especially to Izak for delving into that topic and eliminating the route cause.

2 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

bulraydin avatar image
bulraydin answered ·

We have almost the same setup and same suspicion about how the communication works between Victron and Fronius. Could it be the case that Fronius produces equal energy on all the phases , however the zero-feedin misses to regulate properly when the loads are unbalanced ? The question here is, why Fronius is lower its production although the battery is craving for energy ??

somehow the multiplus are reluctant to charge the batteries higher when the loads are quite low.

We are eagerly waiting for an answer to this question.

1 comment
2 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

I am also wondering that I haven't received any feedback by Victron experts on my Fronius submissions so far. Does silence potentially mean desperately knowing about the issue, but not having any solution on that issue?

ossi avatar image
ossi answered ·


what is your ESS Setpoint?

I presume 50W from screenshots, plz try to set it to 150W, then there is a broader gap between the setpoint for the Feedin Setpoint of PV AC Power and ESS Charging Setpoint.


Also the Fronius Modbus TCP is quite slow- room for improvement ((esp. with slave invters via SolarNet), so the values for the production are very conservative incase AC Loads drops suddenly.

1605962426058.png (20.2 KiB)
2 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

thomasw-1 avatar image
thomasw-1 answered ·

During year change I performed an additional test, which brought more clarity into that issue. It could help Victron to isolate the route cause and develop a fix for that issue.

1. Island mode (green area in below diagram)
I started the environment disconnected from grid and consumption network.
As expected Fronius inverter and MPPT charged full available power into battery.

2. Grid connected and disconnected Consumption (blue area in below diagram)
In next step I connected grid fuses.
After a short time Fronius power limit got activated, but it was more than surprising that still full capacity was loaded into batteries. The Fronius power limit was corresponding obviously to the total power capacity that Fronius produced at that moment of time.
You can compare in following diagram the green line that represents the parallel MPPT power production to exclude potential sunshine deviations.
This example shows from my perspective that Power throttling does not impact the battery charging process in every condition.

3. Standard Operation ( grid and consumption network connected- red and yellow areas in below diagram)
Shortly after the consumption network was connected and first peak loads appeared, the Fronius inverter got throttled, so that it produced less than 50% of it's current available potential, although battery charging would have been still possible (red part of the diagram).
Unfortunately I have not chosen the optimal time to exemplify that phase for a longer period. In the yellow phase the battery got throttled since it reached nearly full charging level already. But also during that phase (yellow part of the diagram) you can easily reproduce that actual battery charging current was for about 20 minutes still below actual allowed limit!

As soon as consumption gets in, the AC-Out related power production is not properly redirected anymore via MultiPluses with the consequence

  1. not charging the battery with actual available power capacity and
  2. not to compensating required power on other phases. Instead power is partially fed into grid (see documented zero-feed-in issue).

For those who want to have a look on the native raw data, I also attached the Excel.eval.jpg


eval.jpg (207.7 KiB) (110.4 KiB)
1 comment
2 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

I've been observing similar behavior on our system when limiting or disabling feed in power.

It may be related to battery life state, but I could not nail that down.

Fronius feed in limit is constantly updating depending on ac load, battery charging is ramping up when ac loads are constant for a couple of seconds. As soon as a high ac load appears, power is drawn from grid, battery charging drops to 0 and Fronius feed in limit goes up, all in about 2-3 seconds. As the high ac load disappears, there is an overshoot to the grid, then Fronius feed in limit drops, keeping grid at 0W. After about 3-5 seconds, battery charging goes up again and also feed in limit goes up slowly.

So in summary, at zero or limited feed in, ac load spikes constantly altering the pv power output and hindering battery charging because of a high latency.

Since we can feed in up to 70%, I've decided to set the feed in limit to this value, resulting in a more consistent pv yield.

However, battery charging adjustment is still pretty sluggish.

thomasw-1 avatar image
thomasw-1 answered ·

Hi Ossi,

Thanks a lot for your reply.

Grid setpoint is crrently at 80W.
I tried different seetings between 30W and 200W, but without any effective change in behaviour.

From my point of view the delayed reactivity could be related with my second post on the Feed-In issue, but I don't think, that this is realated to that problem. There is a timely difference up to 45 minutes. In that period there should be a proper communication on Modbus possible.

1 comment
2 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.


Try 800W for the ESS Setpoint

See if grid is regulated to 0W (Zerofeedin setpoint) or 800W Consumption (ESS Setpoint)