micmak avatar image
micmak asked

Fronius throttled by Quattro despite being needed (AC/DC coupled system)

Hello all,

I have an AC/DC coupled system as seen below:


Half way through the day, the batteries got to 100% and I had a low load of around 1000W, which the DC coupled system handled while the AC coupled is throttled down to 0. Load eventually got higher than the capability of the DC coupled system, and yet the Fronius remains throttled.

BYD BMU raised the CCL a while ago and yet the Quattro didn't change its behavior:


And here's the Quattro throttling the Fronius:


I have two questions:

1. Obviously something is wrong here, Fronius should kick back in when CCL is above 0 to recharge the batteries

2. Why isn't the AC coupled prioritized when the batteries are full over the DC, given that would be more efficient for the AC load?

EDIT: toggling the dc switch of the Fronius didn’t help. Turned on my generator for few minutes then the Fronius started pulling power from its array. Turned off the generator and it is still going strong. A barbaric workaround though…

MultiPlus Quattro Inverter ChargerFroniusAC PV Coupling
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8 Answers
ponzoa avatar image
ponzoa answered ·

Perhaps it has to do with your batteries being pretty much topped up. Does this behaviour happen when the batteries are say at 80%? If it does, something is up. If not, I can only imagine it's throttled to look after the batteries.

When you switch on a generator, the systems try to charge the batteries as quick as it can so perhaps you're just tricking the system.

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Phil Gavin avatar image
Phil Gavin answered ·

Your frequency is 52.6Hz, which will throttle any PV inverter.

Fronius has dedicated comms, but I have no experience with that PV inverter, but I'd check to see if it is working, because you have locked out on high frequency. ( Throttled by frequency-shifting).

There are various conditions that reset this. It is documented on the Victron site .

From memory:

1. A 50 Hz supply on the input on the Victron.

2. Cycling the power switch on the Victron.

3. A 0.7V ( I think) drop in the battery voltage. ( It is higher for other battery chemistries, so check yours is set to Lithium).

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micmak avatar image micmak commented ·
fair point and when I turn on my generator number 1 recovers the fronius. Also, in the morning, the Fronius is working fine, so number 3 is the culprit here, as @ponzoa suspects as well. But I didn't find the configuration related to the voltage drop threshold. Any ideas?

Before adding the DC coupled part to my system, Fronius was handling the AC loads and keeping the battery charged after it reaches 100%. Now the Quattro is relying on the DC part of the system for that (which is only 6 panels facing south east, whereas Fronius is handling 15 panels facing south west'ish).

Is there a way to prioritize the Fronius inverter? First, it has way more panels and second, it makes more sense to have the AC coupled system handling the AC loads...

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Phil Gavin avatar image Phil Gavin micmak commented ·


I agree with about the DC vs AC preference in terms of efficiency, but I don't have a solution.

I have used the Lithium setting for LA batteries and it allows AC coupled PV inverters to reset faster.

It is a trick based on the following info from the Victron website:

"Q1 - The system is locked to 53 Hz and does not resume

Its locked to the upper limit because of a battery over voltage.

The frequency will reset itself automatically, in case one of the following conditions is met:

  1. the battery voltage drops below the re-bulk voltage threshold: the voltage that makes the system restart the charge cycle. For lithium batteries this threshold is defined as the float voltage minus 0.8V. Note that when float is configured to 54.8 volts or higher, then the battery voltage needs to drop below 54V. For lead batteries, the re-bulk voltage is defined as 5.2V below the configured float voltage. Unless that is configured to 56.8V or higher: then the battery voltage needs to drop below 51.6V.

  2. the system is connected to mains or generator power. As soon as it sees a voltage on AC-in, it will sync the AC frequency to 50Hz and then connect.

Alternatively, power cycle the system with the rocker switch on the front panel on the inverter/charger.

To prevent this from happening again, check your configured absorption voltage: setting it lower will prevent the overvoltage condition from happening again."

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micmak avatar image micmak Phil Gavin commented ·

I already have the Lithium option checked, and my floating voltage is set to 55, which means that the reset should happen when the battery voltage hits 54.2. This is not happening:


It even goes below 53V and still it doesn't reset. The reset in the graph is from when I turned on the generator. and here's my config:


Which matches section 4.2 from

I did notice in that article something about absorption voltage though:


Shall I try setting it to 56.5 instead of 55.2? what's the logic here?

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ponzoa avatar image
ponzoa answered ·

You're off-grid so yes, you should try the 56.5V. Definitely.

You should also activate modbus in both the Fronius and the GX and have them on the same network - wifi or wired or mix but the same network.

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micmak avatar image micmak commented ·
modbus is active and they are all on the same wired network. The configuration on the Fronius is correct when it comes to that as well. I am also connecting to all of the units through modbus from my homeassistant.

I am mostly offgrid, our electrical grid is an hour or two a day if it happens, which hasn't for over 2 weeks now. I guess 56.5V won't interfere with the grid if it ever happens to show up. I will modify the configuration and upload that. Will report back. Thank you both for taking the time, I appreciate it

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Alexandra avatar image Alexandra ♦ micmak commented ·

Also set your BMU to offgrid use.

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micmak avatar image micmak Alexandra ♦ commented ·
I double checked, this is the case, thank you

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micmak avatar image
micmak answered ·

I still couldn't reproduce the issue. In the past 2 days, once batteries are full, AC PV continue to handle the load instead of DC PV. While on the days I had the issue, the DC PV were handling the loads and AC PV went down to 0 and didn't reset. I changed the float charge for now to match the offgrid value recommended.

My concern is still about prioritizing AC PV over DC to avoid the problematic scenario. Is this possible in the configuration somewhere?

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Dave Rae avatar image Dave Rae commented ·
what ver firmware are your MPPT's on?
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micmak avatar image micmak Dave Rae commented ·
I don't know what was the mppt release when I had the issue, but I update them regularly and currently running 1.61. I also increased my float voltage to 56.5
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Dave Rae avatar image Dave Rae micmak commented ·
OK, my MPPT's started being biased towards always covering the loads during the day after i updated them to 1.61. Prior to updating them the Fronius was covering the loads. So, I suspect that the 1.61 firmware update on the MPPT's is the cause of the issue. The MPPT's will not accept downgrading to the previous version, i have tried to install a legacy version without any success.
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Dave Rae avatar image
Dave Rae answered ·

Yep, have the same issue with my system, Seems to be prioritizing the DC coupling over the Primo AC coupling, which as you stated if far more efficient at running the AC loads during the day.

Basically, what you have described is exactly what's happening to my system, should be a way to prioritize the AC coupled side.

When I turn off my two 150/70 MPPT's DC couple side only then does the Multiplus drop the frequency and bring the Primo back online.

Victron should just develop their own AC coupled string inverter as that would be controlled by the VE direct connection and would have instantaneous control on the output as opposed to the extremely slow frequency de-rating control method. This would also allow people to install longer higher voltage PV strings and reduce the overall cost and simplify the installation for the larger off-grid systems that are now becoming more mainstream.

But in the meantime would be great if the DEV team could sort out this AC coupled system bias towards the DC coupled side on the AC/DC coupled off-grid systems.

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iwan avatar image
iwan answered ·


I have a very similar issue and tried the tips (setting absorption and float voltages for off-grid) in this threat but this did not fix my issues.

I have a Fronius inverter AC coupled with my Victron Quattro units (3 in single phase setup), off-grid.

Most days this works perfectly ok, but once in a while the Quattros do not ramp down the frequency so the Fronius does not start producing any energy.

This is what my system currently looks like;


The DC Power is charge current coming from two Schneider XW MPPT80 units.

The batteries are 5p16s 304ah LiFePO4 cells managed by a REC BMS. (1520ah or 73KWh total)

The voltage limits are set to 2.8-3.5 V per cell. (44.8-55.0 V for the bank)

I’ve read that the Victron should reset the frequency to 50Hz when the battery is 0.8 V below the float voltage for Lithium batteries or 54 V, whichever is lower. The float voltage is set to 55.0 V so I would expect the Fronius to start working again at 54.0 V, however, the batteries are at 53.28 V and the frequency is still ramped up to 52.8Hz.

The Quattros and the assistants are all on the latest versions (500) and loaded into each of the Quattros.

These are the battery settings in the Quattros;


I have a couple of questions;

  1. Why are the Quattros not resetting the Frequency?
  2. Is there any other way (without shutting the system down) to reset the frequency?
  3. Ideally, I would like the Fronius to work any time the batteries are not at 100% and/or if the AC load is larger than the Fronius 5KW rating (which is happening daily when I get home and plug in my EV) as that would give me maximum power, especially because the solar panels connected to the Fronius are ideally placed for late afternoon sun. Is there any way to set that up?

Again, most days it works fine and the Fronius re-starts again in the morning.

These are the assistant settings;




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Keith Arnold avatar image
Keith Arnold answered ·

Hi. i have a similar setup but on grid. For some reason the multi will lock onto 53 Hz output (not sure exactly what logic/conditions are used to lock in 53, or close to 53, as it does not always do so.) and keep Fronius off until grid returns, then all is well again.

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Alexandra avatar image Alexandra ♦ commented ·
@Keith Arnold

It is throttled based on battery voltage as described above by @Iwan . It is possible that because rec bms is not fully supported there is some unknown interaction there, i cant say for sure. More here in a FAQ.

Grid coming back forces a synchronisation to 50hz so will wake up the Fronius. One work around i have seen is to set the absorption voltage a little lower If the bms is telling the inverter no more charge then that is also a cause of the frequency lock.

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iwan avatar image iwan Alexandra ♦ commented ·
@Alexandra I had absorption set to 55.5 before I changed it to 56.5 for testing purposes. In both cases the frequency would get stuck to around 52.8Hz sometimes.

It seems a bit strange there is no option to configure the Quattro to use all available power sources (including AC coupled Fronius) before starting to drain the battery. The easiest option would be to never tell the Fronius to ramp down unless both the batteries are charged at 100% AND the AC power draw is less than the Fronius capacity. That would solve all my problems!

If anyone knows how to contact Victron with a feature request? :)

Kind regards,


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Alexandra avatar image Alexandra ♦ iwan commented ·


That is what ESS does. But since you are using PV assistant. Then you are trying to be off grid?

With the assistant on the quattro configured and the GX to sort out the other options like depth of discharge and control of the Fronius with frequency shifting through the data connection.

Is the bms communicating with the system at all? Maybe it is not sending the right signals?

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iwan avatar image iwan Alexandra ♦ commented ·

@Alexandra Yes, this is an off-grid system. The BMS is talking to the system, have a look at my vrm; I have no issues with any part of my system except the Fronius being ramped down for no good reason.

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Alexandra avatar image Alexandra ♦ iwan commented ·
Had a look. Will have another look later. With your batteries full and the other mppts covering load, there is no need for the fronius at this point.

If you switch if your dc pv does it change anything?

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iwan avatar image
iwan answered ·

Hi @Keith Arnold @Alexandra @micmak

I have concluded it must be a bug in the Victron logic. Keeping the Frequency way above 50hz while the battery is discharging, and the voltage dropped way below the 54V, does not make sense. I had days where the battery dropped to 50% SOC and the Fronius was still throttled.

I have now removed the PV Assistant and switch the Fronius via the Generator Assistant and a contactor. I could do this as I have a dedicated AC copper pair between the shed where the Fronius is and my Victron equipment in my Systems Room. I hope Victron will solve the issues with the PV assistant at some point as I would much rather use that again! @mvader (Victron Energy)

Kind regards,


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