Inverter electronics noise

Been running a pair of Victron Phoenix 1200s in parallel (24V system) for a couple of years now with no real issues — genuinely pleased with them. Recently though I've started noticing an intermittent buzzing/clicking sound from the inverters when I plug in my partner's hair straighteners.

I know straighteners aren't exactly the most inverter-friendly load — they use rapid-cycling heating elements that switch on and off constantly to regulate temperature. My assumption is that this switching is causing the inverters to react, but I'm not 100% sure whether this is:

1. Normal behaviour for this type of load
2. A sign something's wrong with the inverters themselves
3. A wiring/earthing issue on my end

The straighteners are rated at around 45W but I suspect the actual switching behaviour causes brief spikes that the inverters don't love. Everything works — hair gets straightened, no tripping — it's just the noise that's concerning me.

Has anyone else experienced this with similar loads? I've seen some folk mention that modified sine wave inverters struggle with resistive switching loads, but the Phoenix puts out pure sine so I wouldn't have expected it to be as bothered.

Also worth noting my battery bank is Fogstar Drift LiFePO4 cells, so I don't think it's a voltage sag issue at the battery end.

Is this just one of those things you accept, or is there a practical fix — a filter, a soft-start adapter, something else? Would rather understand what's actually happening before I assume it's benign.

@TrevorRoberts75 — worth checking whether the noise correlates with load transitions rather than steady-state operation. On my narrowboat I had identical intermittent buzzing from a Phoenix 375 that turned out to be DC bus ripple from an undersized battery cable causing the inverter's internal capacitors to resonate under partial load. Measured it with a clamp meter and found voltage sag on the DC side was far worse than expected.

Specifically: measure DC voltage at the inverter terminals under load, not at the battery. Any sag exceeding ~1.5V over a short cable run suggests resistance/inductance issues — corroded lugs, undersized wire, or a loose connection somewhere in the parallel DC combiner arrangement.

Also — are both units sharing a single DC busbar or wired independently back to the battery? That topology matters enormously for ripple behaviour on parallel-configured units.

@TrevorRoberts75 ooh this is interesting — I had something similar crop up on my narrowboat setup a while back. Mine turned out to be a grounding issue rather than the inverters themselves. Worth checking whether your DC negative busbar connections are all properly torqued down? Loose connections can cause all sorts of weird interference, especially in a parallel setup.

Also — are you running any sensitive electronics nearby? I noticed my issue was worst when I had certain chargers plugged in on the AC side. Have you tried isolating loads one by one to see if something specific is triggering it?

What's your battery setup — are you on lithium or AGM? I wonder if the BMS communication is involved at all if you're on LiFePO4?

@TrevorRoberts75 — when you say intermittent, is it random or does it follow a pattern? One thing worth checking with paralleled Victrons is whether the units are still properly synchronised — if the sync cable between them has developed a dodgy connection, they can occasionally drift out of phase and produce some odd acoustic and electrical noise as a result. Had a mate with a similar twin-Phoenix setup and that turned out to be his culprit; reseating the cable sorted it immediately. Also worth pulling up the data in VictronConnect and checking whether both units are showing identical output readings — any discrepancy there would point toward a sync issue rather than a fault in either inverter individually.

Hey @TrevorRoberts75 — just to add to what others have said, with paralleled units I'd specifically check that your RJ45 interconnect cable is properly seated and undamaged. I had a pair of Multiplus units where a slightly dodgy sync cable was causing one inverter to occasionally drift out of phase with the other, and the resulting correction produced a very distinctive intermittent buzzing. Worth also having a look in VictronConnect at whether both units are showing identical output voltage and frequency — any discrepancy there would point toward a synchronisation issue rather than something load-related as @DefenderAdventure mentions. The Phoenix 1200 is generally rock solid in my experience, so my money would be on something external like cabling or connections rather than a fault with the units themselves.

@TrevorRoberts75 worth checking your DC cabling too — I had intermittent buzzing on my van build that turned out to be marginal voltage drop under load causing one of the paralleled units to work harder than the other. Even small resistance differences in the runs can throw things off. Are both inverters seeing identical cable lengths and cross-sections from the battery? I found running a clamp meter on each unit's DC feed under load quickly showed up the imbalance. Also curious what battery bank you're running — I've noticed my Fogstar cells can sometimes show slightly different internal resistance as they age, which might be worth ruling out if yours are a few years old too.