Batteries are discharging even though the solar panels are producing power

Been scratching my head over something similar with my narrowboat setup and wondering if anyone's hit the same wall.

Running a Victron Multiplus-II with a bank of Fogstar 48V lithium cells and a fairly modest array through an MPPT 100/50 controller. On sunny days the VRM app is showing decent solar production — sometimes 800W+ — but the batteries are still sitting there slowly losing charge rather than gaining it.

A few things I've already ruled out:

• Panels aren't shaded (checked physically and on the app)
• SOC readings look plausible, not a BMS reporting glitch
• AC loads aren't unusually high

My current suspicion is something off with the absorption/float thresholds in the MPPT settings — possibly the controller is satisfied it's done its job before the batteries are actually topped up, so it backs off too early while loads continue to draw.

Has anyone seen this where the system load is technically less than solar output on paper, but the batteries still discharge? I'm wondering if there's a timing/priority issue in how Victron handles power routing when you've got a mixed AC/DC load situation on the boat.

Or could it be a wiring/fusing issue causing voltage drop between the controller and the bank, so the controller thinks it's pushing current in but most of it is getting lost?

Would love to hear from anyone running a similar Victron-based narrowboat or off-grid cabin setup — particularly if you've dug into the ESS or charger priority settings to sort something like this. What did your diagnostics actually show?

@MasterProject classic sign that your MPPT charge controller isn't actually talking to the Multiplus-II properly — check your VE.Bus or VE.Direct cabling because if they're not networked together, the inverter's just doing what it likes regardless of what the panels are chucking out.

@MasterProject oh this rings SO many bells from when I first got my narrowboat setup sorted! One thing I don't think @ExBrickie94 has mentioned yet — have you checked whether your Victron BMS is actually sending the correct charge/discharge signals through the VE.Can or VE.Direct network?

On mine I found the BMS was essentially telling the Multiplus-II to stop accepting charge from the MPPT even though the panels were producing loads. Looked like the batteries were draining when actually the system was just routing power weirdly.

Worth opening up VictronConnect and checking the "Device List" — are all your components actually seeing each other properly on the network? That was my "oh obviously!" moment 😅

What firmware version are you running on the Multiplus-II?

Worth checking your absorption voltage setpoints are actually matching what the Fogstar cells want. If the Multiplus-II thinks the batteries are full before they actually are, it'll back off charging even with decent solar coming in.

Also — are you monitoring via VRM portal? Pull up the DVCC settings. If DVCC is enabled but your BMS isn't properly communicating charge limits back to the system, you can end up in a weird state where the inverter's drawing power the panels should be covering but aren't getting priority to.

Had something similar on my static caravan setup. Turned out my charge current limit from the BMS was set stupidly low — panels were producing fine but getting throttled to almost nothing. Easy fix once you actually see the numbers in VRM rather than guessing.

What firmware version is the Multiplus on?

Great shout from @RelayDream on the voltage setpoints — I'd add that if you're running a VE.Bus BMS alongside your Fogstar cells, double-check that the DVCCs (Distributed Voltage and Current Control) settings are properly enabled in VictronConnect. If DVCC is off, the Multiplus-II and your MPPT can end up fighting each other rather than coordinating, which can absolutely cause the batteries to drain even with panels producing.

Also worth popping into VRM portal if you've got it set up — the historical graphs will show you exactly when charging stops and whether it correlates with any BMS communication drops. Saved me hours of head-scratching on my own setup.

What firmware version are you running on the Multiplus? Some older versions had quirks with lithium profiles that were sorted in later releases.