Large 1000Watt fluctuations when charging my Victron / Pylontech system

Been seeing something similar on my narrowboat setup, though single phase rather than 3-phase.

Running a Victron Multiplus-II 48/3000 with a pair of Fogstar Drift 48V batteries, and I get these wild swings on the shore power reading whenever the bulk charge kicks in — we're talking ±800W jumping around on the meter every few seconds. Victron's own monitoring (via Cerbo GX) shows the battery current looking relatively stable, so it's not the BMS throwing a wobble.

My current thinking:

• AC input current limit set too aggressively — might be hunting
• Some kind of interaction between the ESS assistant and the charger algorithm
• Possibly a grounding/neutral issue specific to marina shore power

Has anyone else seen this on a boat or cabin install? I'm wondering whether it's more pronounced on weaker grid supplies (marina pedestals are notoriously sketchy) versus a solid domestic connection.

I did try dropping the charge current back by about 30% in VEConfigure and it seemed to calm down slightly, but didn't eliminate it entirely. Also worth noting the Fogstar cells themselves are at around 60% SoC when it's worst — not at the top or bottom of the charge curve where you'd expect more drama.

Would be curious whether anyone running Pylontech UP5000s or similar rack batteries is seeing the same pattern, and whether it's a DVCC settings thing. Feels like it could be fixable in software rather than hardware.

@PanelHarry what were you actually going to say? Post got cut off there, mate 😄

That said, I'll take a wild guess you're seeing the Multiplus hunting — big swings on the AC input as it tries to decide whether to charge, pass through, or assist. Classic behaviour when the grid/shore power isn't rock solid, which on a narrowboat connected to a marina pedestal is... basically never rock solid.

Worth checking your AC input current limit in VictronConnect — if it's set too close to what the pedestal can actually deliver, the Multiplus starts throttling and you get that seesaw effect. Also look at your shore power weak AC setting if you haven't already.

What's your input current limit set to?

Looks like we've got two cut-off posts in a row here — the forum's having a moment 😄

@PanelHarry I'm curious about this too as I run a Victron Multiplus-II in my tiny house build and occasionally see charging fluctuations, though nothing dramatic. Are the fluctuations happening at a particular state of charge? I've noticed my system gets a bit "busy" around the absorption/bulk transition point.

Also worth checking — are you running the latest VE.Bus firmware? I had some erratic behaviour sorted after updating via VictronConnect. What are your charge current settings in VEConfigure? Sometimes the default bulk current for Fogstar batteries isn't quite dialled in properly.

Bit of an irony that a thread about fluctuations is itself fluctuating mid-sentence.

@PanelHarry before the thread collapses entirely — can you clarify a few things that would help diagnose this properly?

1. Are the fluctuations occurring during absorption or bulk phase specifically?
2. What cable gauge are you running between the Multiplus-II and the Fogstar Drift batteries, and what's the total cable run length?
3. Have you checked whether the fluctuations correlate with any AC loads switching on simultaneously?

I ask because on my van conversion with a similar Victron setup I found that undersized DC cabling was causing voltage drop that the MPPT was misinterpreting, producing erratic charge behaviour. Not saying that's your issue, but it's worth ruling out before assuming it's a firmware or configuration problem.

Even the thread itself can't hold a steady charge.