EVCS Opportunity Loads and available solar

Been noticing something a bit odd with my setup lately and wondering if others have hit the same wall.

Running a Victron Multiplus-II with a Cerbo GX, and I've got an EVCS tied in as an opportunity load. The idea being it only charges the car when there's surplus solar — makes sense in theory. But in practice it seems to be pulling more than what's actually available, consistently dragging the batteries down rather than just using the excess PV.

From what I can tell the GX is calculating available power based on what the inverter can deliver, not purely what the solar is producing at that moment. So if the Multiplus is rated to export X watts, the EVCS treats that as fair game even if the panels are only doing half that on a grey afternoon (which, let's be honest, is most of the time here).

Anyone else found this? I'm on a modest 4kW array and the EVCS seems to think it's got a lot more headroom than it actually does — ends up being more of a parasitic load than an opportunity one.

Curious whether:

• There's a setting I'm missing in VRM or Venus OS
• Tweaking the minimum SOC threshold helps in practice
• Anyone's had luck with the ESS assistant config to tighten this up

Feels like the logic should be simple enough — only charge when solar > load + buffer — but clearly something in the control loop isn't quite right. Would love to know if this is a firmware quirk or just user error on my end.

@GrumpyWanderer had almost the exact same head-scratch moment last spring with my static van setup.

The behaviour I noticed was the EVCS essentially "fighting" the Multiplus for priority when solar was genuinely abundant — the Cerbo wasn't always quick enough to signal available excess before the charger throttled back.

What sorted it for me was tweaking the ESS minimum SOC threshold and making sure the EVCS was properly configured as a controllable load rather than just sitting on the AC-out bus being dumb about it.

Worth checking your DVCC settings too — if shared current sense isn't enabled, the whole system can't really "see" what's available holistically.

One thing I'd ask: are you running Node-RED on the Cerbo, or relying purely on the built-in ESS logic? Because there's only so much the stock firmware will do without a bit of custom automation layered on top.

Seen this with my garden office setup - the Cerbo's ESS assistant can be a bit slow to react when the EVCS ramps up demand suddenly. Worth checking your minimum SOC and feed-in excess solar settings in VictronConnect, as these directly influence how aggressively it'll offer surplus to opportunity loads.

One thing that caught me out: if you've got AC-coupled solar, the EVCS might be seeing "available" power that the Multiplus-II is already mentally earmarking for battery top-up. The Modbus TCP integration between the EVCS and Cerbo needs to be properly configured so they're genuinely sharing the same power budget figure.

Have you looked at the DVCC settings? Shared Voltage Sense can sometimes cause the system to be conservative about what it declares as truly spare capacity.

Had this exact frustration in my motorhome build last summer. Worth checking your Min SOC and Feed-in excess solar settings in the ESS — if the Cerbo thinks battery is below threshold it'll throttle the opportunity load before it even looks at available PV headroom.

Also, the EVCS communication lag with the GX can be a real pain. Mine was set to poll every 10 seconds which meant it was always reacting to solar history rather than what the panels were actually doing.

Dropping the polling interval helped massively. Victron's VRM portal lets you overlay the EVCS load against PV yield — worth pulling a few days of logs to see where the gaps are actually occurring before chasing ghosts.

Good thread. The bit that catches people out here is the EVCS current limit not being dynamically tied to available PV unless you've explicitly configured the solar charging current feedback loop via the Modbus TCP or MQTT bridge.

On my narrowboat I had to write a small Node-RED flow that polls the Cerbo's VRM local API every few seconds, calculates genuine surplus (PV output minus house loads minus a buffer), then pushes that figure back to the EVCS as the permitted charge current. Without that active arbitration layer, the EVCS just requests what the car wants and the Multiplus has to decide whether to supplement from battery or grid — it won't proactively throttle the charger.

@LesPhillips is right that ESS reacts slowly; the minimum ESS charge time parameter compounds this. Worth setting your ESS sustain voltage conservatively too, otherwise you'll be chewing into battery reserves before the control loop catches up.

@JohnBaker is right about the dynamic current limit issue — worth adding that the EVCS talks to the Cerbo via Modbus TCP, and if you haven't set the scan rate correctly you'll get a laggy response even if everything else is configured properly.

On my boat setup I had the same thing — solar available, batteries healthy, but the EVCS was throttling itself because the Cerbo wasn't updating fast enough to reflect actual PV output.

Check your services.conf and make sure Modbus TCP is actually enabled, not just assumed to be. Obvious thing but catches people out.

Also — what firmware is your Cerbo on? There were some ESS-related fixes in recent releases that specifically affect opportunity load behaviour.