Charge limit when adding an external MPPT to Multi RS Solar (DESS)

Been tinkering with my cabin setup recently and this has been rattling around in my head for a while, so figured I'd throw it out there.

Running a Multi RS Solar with Dynamic ESS, and I'm looking at bolting on an external MPPT — probably a Victron SmartSolar — to pull in more harvest from a second array I'm planning on the south-facing roof slope. The cabin setup is built around a Fogstar Drift 48V bank, so battery care matters quite a bit to me.

My concern is around how DESS handles the charge ceiling once you introduce that second MPPT into the mix. From what I can work out, DESS sets its charge limit through the Multi RS itself, but does that instruction actually propagate properly to an external MPPT on the same VE.Can network? Or does the external unit just crack on regardless, potentially pushing the battery beyond what DESS has decided is sensible for that particular day's schedule?

I've had a poke around the Victron Community docs and it's a bit murky. There seems to be an assumption baked in that the Multi RS is your only charging source, which obviously falls apart the moment you add external generation.

Has anyone here actually run this combination — Multi RS Solar with DESS and a separate MPPT controller? Curious whether:

• The charge limit cascades correctly via DVCC
• You've had to do any manual capping on the external MPPT
• It's caused any odd behaviour with the DESS scheduling

Would love to know how others have approached this before I commit to the extra panels and wiring.

@JX_Boats curious timing on this — I've been down a similar rabbit hole with my tiny house setup.

One thing worth checking: when you add an external MPPT via VE.Can, does the Multi RS's DESS algorithm actually account for that extra charge source when calculating its own limits? My understanding is the system total charge current gets managed through Venus OS, but I've seen reports where the external MPPT effectively "bypasses" the DESS logic if it's not properly configured as a shared voltage/current source.

Are you using a Victron SmartSolar for the external unit, or something third-party? That'll make a big difference — VE.Can native devices should communicate properly, but anything relying on a generic protocol might confuse the charge limit calculations entirely.

What's your battery chemistry? LiFePO4 with a BMS asserting its own limits could add another layer of complexity here.

Ran into something similar when I was setting up the shepherd's hut — the Multi RS Solar's internal MPPT and an external one don't always play nicely under DESS without some deliberate config work.

Key thing I found: you need to set the charge current limit on the external MPPT via VE.Direct or VE.Can so that DESS can actually see and control it. If it's sitting outside the Victron ecosystem talking to nothing, DESS just ignores it and you get overcharging scenarios.

Worth checking:

• Is your external MPPT visible in VRM?
• Have you set the DVCC shared current limit?

Without DVCC properly enabled the whole thing falls apart tbh. Took me an embarrassing amount of time to realise that was my issue. @MultiPlusGeek did you have DVCC sorted before you started seeing sensible behaviour?

The real headache with DESS and external MPPTs is the charge current limit doesn't propagate properly unless you've got the external controller on the VE.Can bus and DVCC fully enabled. Without that, your Victron SmartSolar just does whatever it likes regardless of what DESS is asking for.

Had this exact mess with my cabin — external 150/35 was completely ignoring the system charge limit. Wasted a weekend on it.

Check DVCC > Limit Charge Current is actually ticked in VRM, not just set. Easy to miss. Also make sure SVS and SCS are both on, otherwise the voltage sense goes haywire with multiple charge sources.

If your external MPPT isn't Victron, forget it — third party kit on DESS is a nightmare, it'll just freewheel.

Really interesting thread — this is something I had to wrap my head around when I set up my motorhome emergency backup system last year.

One thing nobody's mentioned yet: when you're running DESS with an external MPPT via VE.Can or VE.Direct, the charge current limit sent by the Multi RS is essentially a suggestion to the external MPPT, not a hard cap. The external unit needs to be properly configured to respect the DVCC charge current limit in VictronConnect — it's not automatic.

Quick checklist worth running through:

• DVCC enabled in VRM/Cerbo
• "Limit charge current" ticked and set correctly
• External MPPT firmware is up to date (Victron are constantly patching this)

@JA_Solar makes me wonder — was your shepherd's hut MPPT a non-Victron unit? That changes things significantly because third-party MPPTs often ignore DVCC entirely.

Good point from @GrumpyBuilder — the propagation issue is the crux of it.

What actually works: set your charge current limit via the DVCC settings in VictronConnect/VRM, not in the individual MPPT config. DVCC is what lets the Multi RS coordinate with external MPPTs over VE.Can or VE.Direct properly.

Few things to check:

• DVCC on, SVS and SCS enabled
• External MPPT connected via VE.Direct or VE.Can (not just comms-less)
• BMS (if you've got one) is the charge current "master"

DESS will then respect the combined limit rather than each device doing its own thing. Had this exact issue on my garden office build with a Victron SmartSolar bolted on alongside the internal MPPT — sorted once DVCC was properly configured.

What BMS are you running? That changes the answer slightly.