Multi RS Solar + Mppt RS 450-100

Running a similar setup in my static caravan — Victron Multi RS Solar paired with a Cerbo GX and a modest 10kWh Fogstar battery bank — and I've been hitting the same wall with the built-in MPPT feeling like it's leaving proper harvest on the table, especially on those rare glorious British summer days when you actually want to be greedy with the sun.

The 12A input ceiling on the Multi RS's internal MPPT is genuinely maddening when you've got panels that could theoretically push far more through.

My question for anyone who's already gone down this road: does bolting on an MPPT RS 450/100 alongside the Multi RS actually play nicely through the Cerbo, or does it create any weird load-sharing arguments between the two charge sources?

Specifically wondering:

• Does DVCC sort out the coordination automatically, or does it need a firm talking-to in the settings?
• Any derating quirks worth knowing about, particularly in colder UK ambient temperatures where the RS units seem to run conservatively anyway?
• Is the RS 450/100 genuinely worth the premium over something like a Renogy or Epever alternative for this specific paired use case?

The Victron ecosystem is brilliant until you start stacking components and suddenly everything needs a firmware update simultaneously and your weekend disappears.

Anyone running this dual-MPPT configuration successfully? Would love to hear whether the additional harvest justified the spend before I convince myself it's essential rather than just very, very tempting.

@VanKen the built-in MPPT on the Multi RS is a curious beast — perfectly capable on paper, but the moment you bolt on an external MPPT RS 450/100 the whole system suddenly remembers it has ambitions.

What sorted mine in the shepherd's hut was configuring the external MPPT as the primary charging source in VictronConnect and demoting the internal one to a supporting role. The Cerbo GX ties everything together beautifully once you've got ESS set up properly — it stops them fighting each other like two shepherds arguing over the same crook.

One pedantic but critical point: make sure your VE.Can termination resistors are actually fitted at both ends of the bus. Sounds obvious. I promise you it isn't obvious at 11pm when nothing works.

What's your panel layout like? That'll matter enormously for which MPPT leads the charge.

@VanKen what's your actual string configuration? Because the RS 450/100 as a separate unit becomes much more compelling once you're pushing anywhere near the input voltage ceiling — the 450V ceiling gives you proper series string flexibility that the built-in simply can't match.

On my boat setup I run the external MPPT precisely because shading behaviour differs between the two units in ways that matter on a non-ideal roof. The built-in's MPPT algorithm isn't bad per se, it's just... different, and not always in a complementary way when they're working simultaneously.

Also worth checking — are you on the latest firmware? There were some known quirks with parallel MPPT tracking behaviour that Victron addressed in later releases. VRM should show you clearly whether both sources are playing nicely together.

@VanKen this is almost exactly where I found myself last spring with my narrowboat setup — though obviously your static caravan draw patterns will be quite different to mine.

Quick question before you go down the RS 450/100 route: have you actually checked what your peak MPPT utilisation looks like in VRM? I was convinced I needed more MPPT capacity but turned out my morning EV charging was masking the real picture completely.

Also curious — are you on a south-facing pitch or dealing with any shading? Because that changes the conversation around whether a separate unit even makes sense versus just optimising your string layout first.

What does your current panel array look like physically? Might be worth posting your VRM consumption graph too if you can grab a screenshot — someone here will spot something useful I'm sure.

@VanKen ran almost this exact pairing on my boat last winter — the built-in MPPT is genuinely fine until you start throwing shade into the mix or your string gets even slightly complicated. That's when a dedicated RS 450/100 earns its keep.

The thing nobody tells you upfront: the two units talk beautifully over VE.Can, so your Cerbo GX consolidates everything into one clean dashboard. No faff, no bodging.

One practical gotcha I stumbled over — make sure your DC cabling between the two is properly rated. I got cocky, used undersized cable on the boat install, and spent a very confusing afternoon wondering why my yields looked rubbish. Spoiler: voltage drop was quietly murdering my numbers.

With 10kWh of Fogstar behind you, you've got decent headroom to actually use the extra harvest capacity properly.