Hybrid off grid frequency dhifting

Been running a similar hybrid setup on my narrowboat for the past couple of years and this topic has been nagging at me lately.

I've got a Multiplus-II 5000 paired with a Victron MPPT 150/70 and recently added a grid-tie inverter on the AC input side to handle some extra PV. Since making that change I've been getting occasional high voltage warnings and what looks like DC ripple showing up in the VRM logs — nothing catastrophic, but enough to make me uneasy.

My understanding is that the Multiplus uses frequency shifting to throttle back the grid-tie inverter when the batteries are full — essentially nudging the frequency above 50Hz so the Fronius (or similar) backs off. Clever in theory, but I'm wondering whether this dance between the two devices is what's causing the ripple spikes.

A few questions for anyone who's been down this road:

• Did you find that cable sizing and routing between the inverter and battery made a meaningful difference to the ripple?
• Is there a recommended AC coupling limit relative to the Multiplus capacity — I've seen the 1:1 rule mentioned but not sure how strictly people follow it in practice
• Anyone using a Fogstar or similar LiFePO4 bank — does the BMS respond badly to these ripple events or does it generally shrug them off?

Also wondering whether the ESS assistant settings in VEConfigure play a role here, or whether this is more of a physical/wiring issue than a configuration one.

Would be good to hear from others running AC-coupled systems on 50Hz — seems like there's a lot of nuance that doesn't get covered in the Victron docs.

Hey @VE_Boats, interesting thread topic! One thing worth checking on your Multiplus-II 5000 is whether you've got ESS Assistant loaded vs just running it in standalone inverter/charger mode — the frequency shifting behaviour is quite different between the two configurations.

With ESS active, the Multiplus uses frequency shift to communicate with grid-tied inverters, but on a narrowboat setup you're obviously not grid-tied, so it's worth confirming your MPPT 150/70 is talking to the Multiplus via VE.Direct rather than relying on frequency signals.

What's your battery chemistry? LiFePO4 with a decent BMS can sometimes cause confusion with the absorption/float transitions which gets misread as frequency instability. Would love to know more about the symptoms you're actually seeing! 🚤

@VE_Boats this reminds me of when I first commissioned my cabin setup — spent three evenings scratching my head over frequency shifting behaviour before I realised the MPPT and Multiplus weren't quite "talking" properly through the VE.Bus.

One thing worth checking that nobody's mentioned yet: have a look at your DVCC settings in VEConfig. On my system, having DVCC enabled but with inconsistent SVS/SCS settings caused the frequency response to behave erratically under certain load conditions — almost like the inverter couldn't decide who was in charge of voltage sensing.

Are you monitoring this through a Cerbo GX? The live graphs on VRM can be really telling — you can often see the frequency creep in real time and correlate it with exactly what your MPPT is doing at that moment.

Really worth digging into your DVCC settings if you haven't already — on my shepherd's hut setup this was the thing that finally sorted erratic frequency behaviour between the Multiplus and the MPPT. They need to be talking properly or the MPPTs just do their own thing.

Also check whether your grid code is set correctly in VEConfigure. UK grid code vs "other" makes a surprising difference to how the frequency shifting headroom behaves in practice.

@ForestJenny the commissioning rabbit hole is real — I spent a weekend on mine convinced something was faulty when it was just a VEConfigure setting sat at default.

One other thing @VE_Boats — what firmware are you running on the Cerbo/CCGX if you've got one? Some older versions had genuinely odd ESS coordination bugs that looked exactly like frequency shifting problems.

@VE_Boats frequency shifting only works properly if your MPPT is set to Network mode via VE.Direct or VE.Can — if it's running standalone it'll completely ignore the Multiplus-II's throttle signal and just crack on like an enthusiastic labrador regardless of SOC.

Also worth checking: if you've got any Fogstar or similar third-party cells with a BMS that doesn't speak DVCC properly, the whole chain breaks down and you end up with the inverter frequency dancing around like it's had too much Guinness while the solar does whatever it likes.

Pull up victron.energy/live/ess:design-installation-manual — the frequency shift response curve section will tell you whether your setup is actually communicating or just pretending to.