ESS “Only critical loads” discharges battery when a non-essential AC-in load is added, despite DC PV

Been scratching my head over something similar with my van build — though obviously single-phase rather than three — and I reckon this rabbit hole goes deeper than most people realise.

I'm running a Victron MultiPlus-II with a Fogstar Drift 200Ah LiFePO4 and a Renogy MPPT feeding DC PV into the system. When I set ESS to "Only critical loads" I'd expect the battery to sit tight while AC-in is present, with solar doing the heavy lifting. But the moment I add any meaningful AC load — even something modest on the non-essential side — I watch the battery tick downward on the Cerbo GX display despite decent solar input.

What's puzzling me most is the sequencing. The DC PV should logically offset demand before the battery even gets a look-in. Is this a VE.Configure minimum discharge current thing, or is ESS making some sort of compensation calculation that's pulling from the battery to stabilise AC before solar can respond fast enough?

I've seen mention that the MultiGrid firmware handles this differently from standard MultiPlus firmware, which would make sense for three-phase setups like the one prompting this post — but even on a simpler single-phase arrangement it feels like the behaviour isn't quite what the documentation implies.

Has anyone managed to nail down whether this is a firmware issue, a config issue, or just ESS working exactly as Victron intended but explained badly in the manual?

Would love to know if tweaking the ESS minimum SOC or the AC input current limit makes any meaningful difference here. What are others seeing?

@Davo51 the key thing to understand here is that ESS "Only Critical Loads" mode still permits battery discharge to stabilise the DC bus, not just to feed AC loads. When you add a non-essential AC-in load, the inverter's internal consumption rises, and if your DC-coupled PV can't quite cover both the bus overhead and that parasitic draw, the BMS sees a deficit and the battery fills it.

What catches people out: Victron's DC bus priority logic doesn't distinguish why current is being pulled — it just sees a shortfall.

Fixes worth trying:

• Raise your minimum SOC threshold so discharge stops earlier
• Check your DVCC current limit settings
• Ensure your SmartShunt is correctly positioned — a misplaced shunt will give ESS completely wrong data to work from

What's your actual PV array size vs. inverter VA rating? That ratio matters enormously here.

@Davo51 had this exact headache with my T5 build. Worth checking your minimum SOC setting in ESS — if it's set too low, Victron will happily discharge past what you'd expect the moment AC-in wobbles or a load spikes.

Also, something that caught me out: DC-coupled PV doesn't automatically offset AC loads in all ESS modes the way you'd assume. The MPPT feeds the DC bus fine, but the inverter decision logic can still trigger discharge independently of what's coming in on DC.

Check your grid setpoint too — if it's not calibrated properly it'll essentially "hunt" and pull from the battery unnecessarily.

The Victron community forum has a cracking thread on this, and @DevonDweller is right that the mode behaviour is less intuitive than the manual suggests.

@Davo51 the bit that catches people out here is the AC input current limit interplay with the DC bus. Even with healthy PV on the DC side, if ESS sees a non-critical AC-in load pushing the AC input current above your configured limit, it'll actually pull from the battery to compensate — essentially using the Multi/Quattro's inverter logic to "balance" the AC input rather than letting DC PV offset it directly.

On my boat I sorted this by carefully setting the AC input current limit in VEConfigure to something realistic for my shore power connection, then letting the DVCC shared current sense handle the DC PV contribution properly. Check your /Settings/ESS/AcPowerSetPoint value in the Venus console too — if that's misconfigured it'll fight you constantly.

@ForestBoater raises a good point about the DC bus, but the bit I'd add from running my garden office setup is the DVCC interaction. If DVCC shared current limit is enabled alongside ESS, the Multi can still pull from the battery to maintain AC-out voltage stability even when you think DC-coupled PV should be handling it.

Worth checking in VRM whether your DC PV power is actually being reported correctly — I had a period where my Victron MPPT wasn't communicating properly over VE.Direct and the ESS algorithm was essentially blind to it, so defaulted to battery discharge.

Quick sanity check:

• DVCC enabled/disabled state
• VE.Direct cable seated properly
• ESS "minimum discharge power" threshold

Sometimes it's embarrassingly simple. Mine turned out to be a loose cable.