Cerbo GX Generator Automation Using Auxiliary Relay on Digital Input for Grid Sensing

Been running a similar hybrid setup on my static caravan for the past couple of years, so this topic hits close to home.

My current arrangement uses a Cerbo GX with a MultiPlus-II 24/3000, and I've been experimenting with using the digital inputs for grid-sensing to trigger generator automation. The idea being that when shore power drops out or dips below a usable threshold, the Cerbo detects this via the auxiliary relay circuit and kicks the genny on without any manual intervention.

The bit I've found genuinely tricky is debounce timing. The Cerbo's digital inputs are sensitive, and any momentary fluctuation on the AC input — even brief brownouts — can cause false triggers. I've had the Honda EU22i fire up at 2am more than once because a voltage sag got misinterpreted as a full grid loss. Ended up adding a small delay via Node-RED running on the Cerbo's Venus OS, which helped considerably.

Worth also considering:

• AC input current limit integration — if your generator can't handle full inverter load, make sure the MultiPlus input current limit is configured correctly for generator mode separately from shore power mode
• Battery SOC thresholds — I cross-reference the digital input state with SOC logic so the generator only runs when the Fogstar 200Ah bank genuinely needs it, not just because grid is absent

Has anyone else used the relay assistant within VEConfigure rather than Node-RED for this kind of logic? I've read the relay assistant can handle some of this natively but I've never fully trusted it over a more visual Node-RED flow.

Curious what others have landed on for commercial-scale installs specifically — residential tweaks don't always translate.

@VoltAlison great timing on this thread — I went down almost exactly this rabbit hole last spring on the boat.

The bit that caught me out was the Cerbo's digital inputs needing the AC input ignored flag set correctly in VEConfigure, otherwise the MultiPlus would see the generator come up and immediately try to sync before it had stabilised. Caused some spectacular relay chatter that had me convinced something was dying.

Worth checking your generator warm-up time setting in the ESS assistant too — I had mine set to a frankly optimistic 5 seconds. Bumped it to 20 and everything settled down beautifully.

Fogstar cells in the bank meant the BMS comms were doing their own thing on top, which added another layer of "fun" to the diagnostics. 😅

What's your generator model? That changes the conversation quite a bit.

Really useful thread this one. @VoltAlison the key thing worth adding is that the Cerbo's digital inputs have a slight debounce delay that can cause the relay logic to feel sluggish if your grid signal is borderline. Worth going into Settings > I/O > Digital Inputs and checking which input type you've assigned — "Generator" vs "Grid meter" behaves differently in terms of how the GX responds downstream.

On my narrowboat I use DI-1 purely as a grid-present sense input wired through a small 24V relay coil off the AC-in rail, which keeps the logic clean and avoids any floating input issues. The MultiPlus-II then handles the actual transfer switching — you don't want the Cerbo trying to do that directly.

Also worth enabling the two-signal BMS wiring if you haven't already, as it prevents the generator relay from fighting your BMS charge inhibit signal.

Worth adding to what @ExSquaddie49 touched on — the auxiliary relay logic becomes more reliable if you configure the digital input as "generator running" rather than trying to use it purely for grid sensing. The Cerbo treats these differently internally, and the generator run-detection path has tighter hysteresis built in.

In my setup (garden office, MultiPlus-II 48/3000, Cerbo GX) I use a cheap voltage-sensing relay wired across the grid input — it feeds a clean dry contact to DI-1. Far more stable than relying on AC voltage thresholds alone.

Key settings I found made the difference:

• Minimum run time: 10 minutes minimum, avoids short-cycling
• Stop on high SOC: 90% for daily use
• Generator start battery voltage: conservative, around 46.5V on 48V bank

The Victron documentation on generator start/stop conditions is dense but worth reading carefully — the logic tree isn't obvious from the UI alone.

What @CarlBaker hasn't mentioned — and it caught me out on the narrowboat — is that the digital input's pull-up voltage sits at 3.3V internally. If you're sensing mains presence through a resistor divider or an optocoupler circuit, that matters enormously for your threshold calculations. I spent an embarrassing afternoon wondering why my grid-detect signal was reading ambiguous states before I actually checked the Cerbo GX manual properly rather than assuming it behaved like a standard 5V logic input.

Also worth noting: Victron's own documentation on this has been quietly revised at least twice. The version you'll find cached on various forums may not reflect current firmware behaviour. Always pull the PDF directly from Victron's site, filtered to your specific firmware version.

@SophieFisher pull-up voltage nearly had me too — my Cerbo on the cabin sat there completely ignoring my generator like a teenager ignoring chores until I sorted the voltage divider on the input signal. Worth noting for anyone using a diesel genny with a 12V aux output feeding a 24V system: a simple resistor divider is your best mate before you even think about relay logic, otherwise you'll be scratching your head at 2am wondering why VRM shows the grid as perpetually absent.