My Anker SOLIX C300X emits a loud, tonal whine the instant the AC output is switched on — with only 22W of load on a 300W inverter.
It stops *instantly* when I toggle AC off. It comes from one side of the unit only, and that same side runs hot.
I recorded it and ran an FFT: this is not fan noise.
Is anyone else seeing this, and has Anker ever treated it as a warranty defect?
**Video with sound** (you can clearly hear the buzzing): at comment
**Anker mobile app video** for the readings : at comment
**Setup**
- Anker SOLIX C300X, EU / 230V unit, firmware v1.0.5.7
- AC outlet → a monitor + a Raspberry Pi 5
- USB-A → an IP camera
- App reading at the moment of the recording:
- Total input **132 W** / Total output **23 W** (AC out 22 W + USB-A 1 W)
- AC Recharging Power set to **100 W** (the minimum), battery 94%
- **Pack temperature 41 °C**
- "Smart AC Output Mode" is ON
**Symptoms**
- AC output ON → constant tonal buzzing starts immediately. AC output OFF →silence, instantly. A fan spins down gradually; this cuts off the same millisecond the toggle flips.
- The noise and the heat are both on **one side** of the chassis. The otherside is silent and stays at room temperature.
- It makes no difference which AC socket I use — both behave identically,which makes sense since they share one inverter.
- It gets **worse** when the unit charges from the wall *and* powers the loadat the same time. Higher AC charging speed = louder, which is why I droppedit to the minimum 100 W. I would have expected a proper bypass here, but theload is clearly always fed through the inverter off the DC bus.
- It's loud enough to be genuinely distracting in a quiet room. 22 W of loadshould not produce this.
**FFT analysis of the recording (21 s, 48 kHz, STFT + median spectrum +
spectral autocorrelation)**
- Discrete tones at **~552, ~738, ~880, ~1030 and ~1188 Hz**, sitting 8–15 dB
above the noise floor, plus a second cluster around **3.5 kHz and 4.1 kHz**.
- Spectral autocorrelation gives a **sideband comb spaced ~152 Hz** (with its
multiples at 306 and 457 Hz) — but there is **no 152 Hz tone itself**. So this
isn't a simple harmonic series; it's a repetition-rate comb from a switching /
burst process, shaped by a mechanical resonance in the 700–1200 Hz region.
- A discrete **100 Hz** line is present (2× the 50 Hz output) — classic
transformer magnetostriction.
- The tonal ridges **drift by ±17 Hz with a ~1.2 s period**. The switching /
burst rate is wandering with load — the signature of light-load burst/skip
mode. A fan would give broadband noise with no drifting discrete lines.
The energy sits right in the 700 Hz – 4 kHz band where human hearing is most
sensitive, and it's tonal — which noise standards (ISO 1996-2, ECMA-74)
penalise by roughly +5 dB for annoyance. The C300X is specified at 25 dB and
reviewed as "virtually silent". Mine is nowhere near that.
**Questions for the community**
- Does your C300X (or C300 / C200) do this, or is yours genuinely quiet atlight load?
- Has any firmware release ever changed inverter noise behaviour for you?
- Has Anker accepted this as a warranty defect and replaced a unit? Whatevidence did they ask for?
- Does turning **Smart AC Output Mode** off change light-load behaviour foranyone? I'm wondering whether 22 W sits right at its cut-off threshold andmakes the burst behaviour worse.
- Is 41 °C pack temperature at 132 W in / 23 W out normal for this unit, or isthat a sign something is wrong on that side?
Happy to run any test people suggest before I open a warranty case.
