What is this high pitched sound coming from my UPS backup battery?
Happens quite often these days that a product is functional, except one little bummer of a &quot;feature&quot; 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 &quot;coil whine&quot; 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.