Generator keeps cutting out mid-cycle - faulty AVR or something else?

Been through this exact nightmare with my backup setup on the narrowboat last winter. Nine times out of ten when a genny cuts mid-cycle it's one of three culprits:

1. Overloaded AVR struggling with a sudden load spike (inverter chargers are terrible for this — Victron Multiplus, I'm looking at you)
2. Fuel starvation — dirty carb jet or a partially blocked tank vent creating a vacuum lock
3. Thermal cutout triggering due to poor ventilation, especially in a confined space

Before you condemn the AVR, check whether the voltage is collapsing before the shutdown or after. If it's dropping then cutting, that's AVR territory. If it just dies cleanly with stable voltage right up to the moment of death, bet your money on fuel or thermals.

What generator are you running? Brand and kVA matters here. Cheap Chinese units (no shame, I've got one) have notoriously flimsy AVR boards that can't handle the power factor demands of battery chargers. The AVR replacement itself is usually £15-40 and a Saturday afternoon job if you're handy with a multimeter.

Also — and people ignore this constantly — check your earthing. A dodgy earth can cause all manner of mysterious gremlins that look electrical but aren't really AVR-related at all.

What's the exact failure mode? Does it trip a breaker, just stall, or kill itself via the low-oil cutout? Those details will narrow it down massively. Drop the model number and I reckon we can sort this properly.

@Renogy_Pro — worth adding a fourth culprit that catches people out: fuel starvation under load. On my shepherd's hut setup I had identical symptoms until I traced it to a partially blocked fuel filter that only caused issues when the carburettor demanded full flow under sustained load. Idle and light loads were fine.

Procedure I'd suggest before condemning the AVR:

• Check fuel flow rate with filter removed (compare against spec)
• Inspect fuel cap vent — vacuum lock is surprisingly common
• Monitor DC output voltage with a multimeter during the cut-out event

If voltage collapses suddenly rather than sagging gradually, you're likely looking at a mechanical fuel issue rather than AVR regulation failing. AVR faults tend to produce that characteristic output hunt before shutdown. Distinct signatures if you know what to look for.

Good points all round, but I'd add one that's bitten me personally on my narrowboat setup: carbon brush wear on the alternator end.

When brushes get low, you'll often see the genny run fine at idle or light load, then cut out exactly when the AVR demands higher field current — which is precisely mid-cycle during bulk charging. Classic symptom is intermittent rather than consistent failure.

Pull the brush gear and measure brush length against manufacturer spec. On most Chinese-origin gennies (Kipor derivatives especially), brushes are cheap and often undersized from the factory.

While you're in there, check the slip rings for glazing — a light sand with 400-grit sorts it.

@Renogy_Pro and @SuffolkExplorer have covered the main bases well; I'd just work through systematically before condemning the AVR itself, as replacements vary wildly in quality.

@DevonDweller carbon brushes — the silent assassin. Had mine go on the Lister-powered genny moored at Middlewich two winters back. Generator cutting out, me convinced it was the AVR, spent three days chasing gremlins through the wiring like some kind of electrical detective who'd lost his magnifying glass.

Turns out the

Has anyone ruled out governor hunting as a cause? On my tiny house setup I had a similar mid-cycle cutout that everyone assumed was AVR-related, but it turned out the governor spring had fatigued and the engine couldn't maintain stable RPM under load — the AVR was just reacting to the frequency fluctuation and tripping downstream protection.

Wondering specifically: does the cutout happen at a consistent point in the load cycle, or is it random? If it's consistent — say, when a compressor or pump kicks in — that points toward a different failure mode than what @DevonDweller and @PanelSteve are describing with brush wear.

Also, what's the actual generator make/model here? Diagnosis changes quite a bit depending on whether we're talking a Honda inverter unit versus a brushed open-frame type.