What is this high pitched sound coming from my UPS backup battery?

Question

I own two of these APC Back UPS Pro BX1500M units that were purchased new. Both of these units have always made this very annoying "booo-eeeep!" sound when plugged into the wall. I've owned backup batteries before and I've never heard this before. Everything works fine otherwise.

This sound stops after switching to battery. It also takes a little bit before the unit will start emitting this sound when plugged back into the wall. Performing a reset on the device by following steps to provided by the manufacturer will cause the sound to stop when plugged back into the wall but resume again after a few minutes.

I've recorded the sound and boosted it to be heard at a normal volume. I have to put my ear to the unit to hear it consistently over the HVAC in my apartment but it it clear as day across a room when it's quiet.

I have recently replaced one of these with a similar APC Back UPS PRO BN1500M2 which does not have this issue. I have owned these for about two years and this sound has always been consistent. I worked around it by placing the units in areas there the sound would not be audible.

I can confirm that this is NOT the sound of any of the relays switching to or from the battery.

What is this sound?

See the following link for the recording I mentioned above:

https://drive.google.com/file/d/1lxMCApKv-wQnd9s6JOufWFPzX_l1f8v0/view?usp=drivesdk

I'd like a confirmation from someone who is electrically inclined and some kind of explanation as to why it's audible. I've been around all kinds of electronics my whole love and this is the first time I've heard this particular sound.

Here is a reference from another APC owner and a response with the possible culprit:

This noise comes from the battery charging circuit, and it's not necessarily abnormal. The newer APC UPS models use a "switching" or pulsed approach to charging the battery, and depending upon the pulse frequency, you might hear a faint hiss, buzz or squeal. (If any of these noises are really loud, that would strongly indicate a malfunction.)

https://forums.apc.com/spaces/4/back-ups-surge-protectors/forums/general/11892/high-pitched-squelching-noise-from-back-ups-xs-1500-bx1500lcd

Answer 1

Happens quite often these days that a product is functional, except one little bummer of a "feature" that makes it unusable by design, like LEDs that dim via a 200Hz PWM. How did that pass quality control? No idea.

The usual causes for this kind of sound are: magnetoscriction, aka "coil whine" which occurs as variable magnetic fields cause inductor/transformer core material to shrink and expand slightly... and piezoelectric effect, which causes Class-2 ceramic capacitors to also expand and shrink as voltage across them varies. The PCB will propagate these vibrations and act as a loudspeaker.

This is why switching frequencies are normally kept out of the audio band, above 25 kHz.

Since the noise sounds like a chirp, there's varying frequency involved. Probably a converter that ramps up or down in frequency as it starts up or shuts down, or triggers some feature like current or voltage limiting. Maybe it does that when it decides the battery is charged and it should stop charging. Since it makes no audible noise most of the time, it must usually operate outside the audio band, except when it doesn't.

Now, take an insulating plastic rod, like the body of a BIC ballpoint pen.

Glue that to the butt of a plastic or paper cup.

You now have a Less-Lethal stethoscope! Stick the rod on something, and if that something is vibrating, sound will come out of the papercup. Now open your device (remember it has lethal voltages, don't sue me if you zap yourself) and stick the plastic rod on transformers, inductors, big ceramic caps, etc. See if you find the culprit.

If it's the transformer, then you're not in luck. If it's ceramic caps, you could replace them with ones with flexible terminations designed to prevent the capacitor's piezoelectric vibration to make the board flex and act like a loudspeaker.