Incorrect AC Loads calculation on MultiPlus-II when no current-sense bridge or sensor is connected

Been scratching my head over something with my MultiPlus-II setup at the static caravan and wondered if anyone else has hit this.

Running Venus OS v3.80 on a Cerbo GX and noticed the AC Loads figure on the dashboard looks completely wrong when the system is running purely on inverter power. No grid, no external current sensor fitted — just the MultiPlus-II doing its thing.

From what I can tell, the system should derive AC Loads purely from what's being measured on AC OUT. But without a current-sense bridge or sensor connected, it seems to be pulling in phantom figures — the loads readout jumps around in ways that don't match anything I've actually got switched on. My battery shunt (Victron SmartShunt) confirms the DC draw is perfectly sensible, so it's definitely a display/calculation issue rather than anything actually wrong with consumption.

My questions for the thread:

• Has anyone else seen this on v3.80 specifically, or is it present on earlier versions too?
• Is fitting a proper current sensor on AC OUT the only clean fix, or is there a firmware/config workaround?
• Would a Victron EM24 energy meter on AC OUT sort it properly, or is that overkill for a simple off-grid backup setup?

The caravan only runs as emergency backup power during outages, so I'm not desperate to overcomplicate the installation. But inaccurate load readings make it tricky to know how long the battery bank will realistically last, which rather defeats the point.

Anyone who's dug into this in VRM or via the Venus OS settings — would love to know what you found.

@BoxerAdventure yes, classic issue this one. Without a current sense bridge fitted, the MultiPlus-II can only estimate AC loads by subtracting grid input from inverter output — and if there's no proper measurement on the AC-in side, the maths goes wonky.

Check in VEConfigure whether AC Input current limit is configured correctly, and also look at the Assistant settings if you're running any load management.

On my setup I added a Victron ET340 energy meter on AC-in which sorted the calculation immediately — Venus OS then uses actual measured values rather than derived ones.

Worth also checking:

• AC input L1 readings in the Remote Console
• Whether Has DC system is ticked (can throw figures off)

What firmware is your MultiPlus-II itself running? Sometimes the Cerbo and inverter firmware being mismatched causes reporting oddities independent of the sensor issue.

Interesting one this — does it affect the battery SOC calculations downstream as well, or is it purely a display issue on the VRM dashboard?

On my boat setup I had something similar where the AC Loads figure looked wildly off, and I eventually realised the MultiPlus-II was just doing maths based on what it thought was passing through rather than measuring it properly.

Would fitting a SmartShunt on the DC side at least give you more accurate overall consumption figures as a workaround while you sort the AC sensing side? That's what I ended up leaning on.

Also curious — are you running any AC-coupled solar through the system? I wonder if that complicates the estimation further when there's generation happening simultaneously that the unit can't properly account for.

@CornishBoater it absolutely drags the SOC figure off a cliff with it — my Victron setup was convincing itself the batteries were half empty whilst my Fogstar cells were laughing at it.

@KatePrice that's exactly what happened to mine last summer — Cerbo was adamant I'd somehow used 400Wh overnight while I was sat watching telly drawing maybe 80W. Felt like being gaslit by a £300 bit of kit.

Fitting the current sense bridge sorted it completely on my end. Took about 20 minutes if you're handy with a screwdriver, which presumably @BoxerAdventure is given the username suggests some mechanical background.

Worth checking your VEConfigure settings too while you're in there — make sure AC Input current limit is set correctly or it compounds the estimation errors further.

@CornishBoater — yes it cascades. Duff AC Loads figure → duff power balance calculation → SOC drifts. Your BMV or SmartShunt will eventually drag it back via coulomb counting but it takes ages.

@BoxerAdventure yes, hit exactly this on my narrowboat setup a while back. Worth checking in the MultiPlus-II settings whether AC Input current monitoring is configured correctly in VEConfigure — there's a known quirk where without a proper current sense path, the Venus OS will estimate AC loads by subtracting inverter output from a assumed input figure, which can go badly wrong.

Quick thing to try: in the Remote Console, navigate to Settings > System Setup and check what's selected under AC input. If it's showing "Not available" or similar, that's likely your culprit.

@KatePrice and @FormerMechanic — the SOC drift you're describing sounds like the MPPT or BMV is then trying to compensate for the phantom load in its calculations, which compounds things horribly overnight.

What firmware is your MultiPlus-II itself running, @BoxerAdventure? Sometimes there's a mismatch issue between the GX device and the inverter firmware.