Can I extend existing small ESS with a new 'Inverter RS'?

Running a modest setup on my narrowboat at the moment — Victron Multiplus-II 3000 GX paired with a couple of Fogstar Drift 100Ah LiFePO4 batteries (JK BMS connected via CAN), plus a 150/35 MPPT and an EM24 for grid metering. Works a treat for what it is.

Problem is I'm finding the single inverter a bit limiting when running anything demanding — induction hob being the main culprit. Rather than ripping everything out and starting again, I'm wondering if bolting on an Inverter RS Smart is a viable route?

Specifically:

• Can the Inverter RS operate as part of the same ESS alongside the MP-II GX, or does it essentially want to be its own standalone system?
• Will it play nicely with the existing CAN-connected BMS, or does it need its own BMS comms?
• Any risk of the two units fighting over charge control given the MPPT is already managed through the MP-II GX?

I've had a dig through the Victron docs and the RS is clearly designed more for high-voltage battery setups — mine are 24V — so I suspect the answer might just be "wrong tool, wrong job" but wanted to check before I rule it out entirely.

Has anyone actually tried mixing these two inverter types in a real install? Especially interested if anyone's done it on a boat where space and weight actually matter.

DaleBen | 47 posts

Hey @HollyBaker, great little setup for a narrowboat! One thing worth flagging — the Inverter RS is a standalone unit and doesn't integrate into VenusOS/Cerbo the same way your Multiplus-II does. It won't simply "slot in" as an extension to your existing ESS.

For expanding capacity on a boat, you'd honestly be better off adding more battery first — your 200Ah is quite modest relative to the 3000VA inverter. Are the Fogstars actually your limiting factor, or is it the inverter output you're struggling with?

If it's purely more AC output you need, a Multiplus-II 48/3000 parallel setup might be worth considering, though the wiring complexity on a narrowboat can be a faff.

What's specifically pushing you towards the Inverter RS — price, availability? Might help narrow down the best path forward.

WattLiz | 134 posts

@HollyBaker curious what's driving the upgrade — is it peak load capacity you're after, or more solar input? That would change the recommendation quite a bit.

One thing I'd flag from my own motorhome setup: if you're already on a Multiplus-II GX, adding a second inverter/charger means you'll need to think carefully about parallel or three-phase configuration — the RS series doesn't natively parallel with the Multiplus-II range the way two Multiplus units would.

Have you looked at whether simply adding a third Fogstar 100Ah battery gives you enough headroom first? Sometimes the bottleneck isn't inverter capacity but available amp-hours. What does your Venus OS dashboard show as peak discharge current during heavy loads?

Also worth checking — what firmware version is your JK BMS on? Some older versions had quirks with CAN comms that skewed the GX readings.

BoxerCamper | 312 posts

@HollyBaker the Inverter RS is a solar inverter — it's designed to sit between panels and AC loads, not to integrate into a MultiPlus-based ESS the way you're imagining. Mixing it with your MultiPlus-II will cause you no end of grief with phase coordination and ESS logic.

What you actually want, if you're chasing more grunt, is another MultiPlus-II in parallel — Victron's parallel stacking is remarkably well sorted and your GX device handles the coordination beautifully.

I ran a similar expansion on my motorhome: went from one MP-II 3000 to two in parallel, and the VRM dashboard just... knew what to do. Dead simple compared to what I'd feared.

Stick within the ecosystem — your JK BMS via CAN will play nicely with the expanded bank too.

RustyTinker | 891 posts

@HollyBaker worth being clear on what you're actually trying to solve before spending money. The Multiplus-II 3000 is already a capable unit — I ran a similar one in my cabin for two years without hitting its limits on anything sensible.

That said, if it's AC coupling you're considering to bolt on more solar generation, that's a different conversation to adding raw inverter capacity. The two aren't interchangeable goals.

Also — two 100Ah Fogstar Drifts is a fairly thin battery bank. Whatever you add on the inverter side, that's likely your actual bottleneck. More discharge current won't help if the bank can't sustain it without the JK BMS cutting out under load.

What does your actual load profile look like? Running figures, not guesses.

CarlBaker | 847 posts

@BoxerCamper is right about the RS being solar-focused, but the deeper issue is that the Multiplus-II 3000 is already your system's brain via the GX — bolting on an Inverter RS doesn't slot in cleanly as a parallel inverter the way a second Multiplus would.

If it's peak load you're chasing, two Fogstar 100Ah cells is almost certainly your bottleneck before the inverter even becomes a concern. The JK BMS will throttle output under load. I ran a very similar config on a garden office setup — moving to a 200Ah minimum made a measurable difference before I touched any inverter hardware.

What loads are actually tripping you up? Motor starts (pumps, compressors) behave very differently to sustained draw, and the solution differs accordingly.