Spare Multiplus with different

Running a 24/3000 as a temporary stand-in for a 24/5000 on my narrowboat taught me that the Fronius will absolutely throw a wobble if the frequency shifting headroom doesn't match up properly — anyone else had the AC-coupled Fronius effectively go on strike because the smaller Multiplus couldn't bluff its way through the power limits convincingly enough?

Ended up with the Fronius curtailing itself down to almost nothing on a sunny afternoon, which on a narrowboat in a rare burst of British sunshine felt like a genuine crime against nature.

The bit that catches people out is that ESS assistant settings and the VE.Configure frequency shift parameters need revisiting when you swap units — the 3000 has different power thresholds and if you've just slapped the old config straight across, you're setting yourself up for grief.

Also worth checking: does your 24/3000 have the older firmware that plays awkward with newer Fronius units? Had to flash mine before it'd even have a civil conversation with the inverter.

Curious whether anyone's running a mismatched Victron/Fronius combo as a longer-term solution rather than just a stopgap — the cost of a second-hand 3000 versus leaving yourself exposed with no backup at all seems like a reasonable gamble, but only if you actually configure it properly rather than just plugging and hoping.

What size Fronius are you running against it? That'll make a big difference to whether this is a minor inconvenience or a complete headache. 🔧

@FormerMechanic43 interesting — did you end up having to manually adjust the frequency shift range in the Multiplus settings to compensate, or was it more a case of the Fronius just refusing to derate properly?

Asking because I've been wondering about keeping a spare inverter-charger on hand myself and this is exactly the kind of gotcha I hadn't considered. I'd just assumed swapping like-for-like voltage was the main concern with a standby unit.

Was the Fronius still on default AC frequency settings when it threw the wobble, or had you previously tuned it to the 5000's headroom range? Presumably once you've optimised around one unit the headroom mismatch becomes more pronounced.

@FormerMechanic43 this is exactly the sort of thing that bites people at the worst possible moment — usually a Sunday afternoon in February when you're already cold and cross.

The frequency shift headroom mismatch with Fronius kit is a proper gotcha. What I found with my shepherd's hut setup (running a 24/3000 as emergency backup behind a 24/5000) is that the PowerControl parameters in VEConfigure need revisiting almost entirely from scratch when you swap units — the smaller inverter simply doesn't have the same frequency ceiling to play with before it starts throttling the AC-coupled PV.

@Mike1980 to your point — yes, manual adjustment in VEConfigure is the answer, but the frequency shift range setting alone won't save you if your AC input current limit is also sized for the bigger unit. Both need recalculating together, otherwise you're just moving the wobble rather than eliminating it.

@FormerMechanic43 ah yes, the classic "it'll do for now" swap that turns into a three-week debugging nightmare. Been there with my own setup — swapped in a spare Multiplus thinking it'd be a five-minute job, ended up knee-deep in VEConfigure at midnight with a cold cup of tea and the dog giving me the

@FormerMechanic43 worth checking your AC frequency shift upper limit in VEConfig — the 3000 has a narrower power output ceiling so the Fronius hits its curtailment threshold earlier and can't ramp down gracefully before it trips. I ran into something adjacent when temporarily swapping inverter sizes in my motorhome setup; the fix was tightening the PowerControl overload factor and dropping the Fronius' feed-in limit in the inverter's own web interface until the headroom matched what the smaller Multiplus could actually handle. Once those two figures are properly reconciled the wobbling stops. Also double-check your AC input current limit isn't fighting the frequency shift algorithm simultaneously — on a narrowboat with a generator input that interaction can get messy fast.