Generator surging

Funny how adding a BMS suddenly turns your genny into a drama queen — had almost exactly this with my Victron Multiplus setup and a Fogstar Drift battery last winter.

The moment the BMS started doing its thing, the generator began hunting like it couldn't decide whether it was coming or going — classic surging, very annoying at 11pm in rural Pembrokeshire.

Turned out the BMS was sending charge current limit (CCL) signals that kept yanking the target charge amps up and down, and the genny's AVR simply couldn't cope with the rapid load fluctuations. It was basically playing tug-of-war with itself.

A few things that helped in my case:

• Slowing down the CCL ramp rate in the BMS settings (if your BMS exposes that — Fogstar's app does, just about)
• Setting a minimum generator load in VictronConnect so it doesn't try to throttle back too aggressively
• Checking the generator's governor sensitivity — cheaper units are notoriously twitchy

Worth checking whether your BMS is communicating over CANBUS or just doing it via voltage thresholds, because the fix is quite different depending on which route you're going down.

Anyone else had surging specifically on three-phase setups? Curious whether balancing the loads across phases makes it worse or better when the BMS starts throttling — feels like it could compound the problem rather than help it.

@MarineClare yes, the BMS current limiting at low SoC is the classic culprit — the Multiplus sees a sudden load rejection and the generator's AVR can't respond fast enough, so you get that horrible hunting surge.

What fixed it for me was enabling the "weak AC" input setting in VE.Configure and dropping the AC input current limit down to about 80% of the genny's rated output. Gives the Multiplus headroom to absorb those sudden impedance changes rather than passing them straight through.

Also worth checking your battery absorption voltage — if the Fogstar BMS is cutting charge current sharply at the top of charge, that's another surge trigger. Dropping absorption by 0.1–0.2V smoothed things out considerably on my cabin setup.

What generator are you running? Honda EU series handles this far better than the cheaper inverter units in my experience.

@MarineClare @IslandVanLifer worth checking your Multiplus charge current settings too — if the BMS is throttling back and the Multiplus hasn't got a proper ESS or DVCC setup talking to it, the inverter-charger can get quite confused about what the generator actually needs to supply.

On my narrowboat I had similar grief until I enabled DVCC in VictronConnect and let the BMS properly communicate charge limits through the Cerbo. Once the Multiplus was actually listening to the battery rather than just doing its own thing, the surging sorted itself almost completely.

What firmware are you running on the Multiplus? Some of the older versions were a bit agricultural about handling dynamic current limits from the BMS side.

@MarineClare this is so familiar — had almost identical drama on my narrowboat last season. The bit nobody mentions is the generator's AVR struggling when the Multiplus suddenly switches between charge and inverter modes during that BMS current-limiting dance. The genny sees what looks like a massive load swing and the AVR just can't keep up, hence the surging.

Worth checking your PowerAssist settings in VE.Configure — if the input current limit is set aggressively low, the Multiplus starts "borrowing" from battery to supplement the genny, which then causes the BMS to intervene, which causes the load swing... it's a lovely little feedback loop of misery, that one.

Logging via VRM or even a basic Bluetooth dongle on the Fogstar will show you exactly where the current spikes are happening.

@MarineClare had this exact scenario with my garden office setup — Multiplus II and a Fogstar Drift 200Ah. What sorted it for me was enabling the ESS assistant and setting a proper minimum SoC threshold so the BMS never got anywhere near its hard limits in the first place.

The generator surging was basically the Multiplus hunting for a stable load point while the BMS kept shifting the goalposts. Once the battery had a decent buffer, the whole system settled down.

Also worth checking your AC input current limit in VictronConnect — mine was set too aggressively and wasn't giving the generator enough headroom during bulk charge. Dropping it slightly made the genny run noticeably smoother.