AC coupling Fronius + Victron MultiPlus-II: how to add a battery and a backup system to an existing Fronius inverter

You have a Fronius Primo or Symo has been running for years and you want to add a battery and a backup power supply in case of power cuts. Replacing the inverter with a GEN24 Plus is one option; the other – a very common one, which we at FV Componentes frequently install and sell – is the AC coupling: A Victron MultiPlus-II with 48 V batteries is installed between the grid and the property, and the Fronius is connected to the Victron’s output. In this way, the Fronius continues to generate power as usual, whilst the Victron manages the battery, self-consumption and backup.

How it works

• The Fronius connects to AC-out 1 from the MultiPlus-II, not directly to the mains.
• When the grid is connected, the MultiPlus-II allows power from the grid and from the Fronius unit to pass through; any surplus power either charges the battery (the Victron converts it to DC) or is fed into the grid.
• When not connected to the mains, the MultiPlus-II creates its own network via the AC-out and the Fronius continues to produce on it. If the battery is fully charged and there is no consumption, the Victron increases the frequency (50.2 → 53 Hz) and the Fronius reduces the power output to zero: this is the frequency shifting.
• The battery discharges overnight to meet demand, as with any ESS.

Rule 1:1

The power of the photovoltaic inverter connected to the AC-out terminal must not exceed the rated power of the MultiPlus/Quattro: 1:1 ratio. A Fronius Primo 5.0 requires at least one MultiPlus-II 48/5000 (or 2 × 3000 in parallel). With a three-phase Symo, a three-phase Victron system (3 × MultiPlus-II) of at least the same power is required.

Furthermore, the battery must be able to absorb the maximum output: the Fronius can deliver 5 kW when the house is consuming 300 W, and those 4.7 kW go to the battery. With the Pylontech/Pytes/ZYC models, this requires at least two 5 kWh modules (charging current ≥ 100 A).

Components

Team	Example
Inverter-charger	MultiPlus-II 48/5000/70-50 (one per phase in a three-phase system)
48 V battery	2-4 × Pylontech US5000, Pytes V5 or ZYC SIMPO 5000
GX Device	Cerbo GX + optional GX Touch screen
Network meter	ET112 (single) / ET340 or EM24 (triple) if there are loads before Victron
Type A CAN cable	GX → battery
Protections	DC fuse (Mega/NH) and isolator switch between the battery and the inverter; circuit breakers on the AC-in and AC-out circuits

If you have a Fronius Smart Meter, it is no longer required for management purposes: the Victron uses its own meter. You can leave it in place so that Solar.web continues to display consumption.

Fronius settings

1. 1. On the Fronius unit, network code / country settings: this must be set to a profile that supports frequency-dependent power reduction (P(f)) from 50.2 Hz to shutdown at 53 Hz. On the Primo/Symo, this is done in the EVU Editor → Frequency-based power reduction (requires an installer code); in Solar.web, it can be adjusted using the “MG50” (micro-grid) profile in recent versions.
2. 2. Disable any 0 W feed-in restriction that may be in place with the Smart Meter: the Victron will take care of it.
3. 3. Check that the Fronius switches off and on again correctly when the Victron is connected to the network (test with the network disconnected).

Victron Configuration

1. 1. VEConfigure: Spain network code, wizard ESS, ‘Charger’ tab with lithium values (52.0 / 51.0 V). See ESS step by step.
2. 2. In the ESS wizard, indicate that there is “PV inverter on AC output” and its power; active frequency shifting.
3. 3. Cerbo GX: DVCC enabled via CAN using the battery (DVCC guide).
4. 4. Settings → ESS: Optimised mode, minimum SOC (20–30 % if you want a reserve for power cuts), charging according to your contract.
5. 5. Add the PV inverter to the GX: Settings → PV Inverters. The Cerbo can read the Fronius output via the local network (SunSpec/Solar API) to display it in VRM. Enable “Solar API” on the Fronius (Data Manager → Settings → Solar API enabled) or Modbus TCP SunSpec.

Backup in the event of power cuts

With this setup, when the mains supply fails, the MultiPlus-II switches over in less than 20 ms and the property (everything connected to AC-out 1) continues to be powered by the battery and the Fronius. It provides full backup, without the need for additional switch boxes, and also works with three-phase power. See MultiPlus as a backup in the event of power cuts.

Please note: during a power cut, the available power is that of the Victron (5 kW at 48/5000), not the sum of the Victron and the Fronius; the Fronius only contributes if there is a load or the battery needs charging.

Common mistakes

• The Fronius switches to off-grid mode (code 108 / 107): The Fronius network profile does not support high frequencies; adjust P(f) or use the microgrid profile.
• The battery becomes overcharged or the BMS cuts the power: Frequency shifting configured incorrectly, or the battery capacity is too low for the Fronius’s power output.
• Overload on the Victron with the mains connected: Rule 1:1 has not been complied with.
• Fronius output is not displayed in VRM: Solar API disabled or inverter on a different subnet.

Alternatives

If the Fronius is a GEN24 Plus, the logical thing to do is to fit a high-voltage battery and use PV Point/Full Backup (guide). If you just want to make use of surplus energy without a battery, our management system EXCEDIA (Customised Cerbo GX) diverts surplus power to Shelly loads.

We are authorised Victron Energy installers and provide free technical support for equipment purchased from FV Componentes. If the problem persists, please open a ticket at Technical support including the model, serial number and the link to your VRM installation. You can find diagrams and parameters on our Victron Resource Centre.