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I've one of those small batteries used to charge mobile USB devices like cell phones when there's no accessible power outlet around. It's a Cellularline FREEPSLIM3000 model HY-L-3000.

Last time I tried to charge it, I heard an unusual sound, like a tenuous whistle. I've uploaded a recording here, it's a 12-second audio file file called "powerbank whistle.m4a". The sound can only be heard while charging the battery, not while discharging.

I tried to measure the peak frequency with my phone and if the app I used is accurate (no idea about that) there are two frequency peaks, one at 8k and one at 16k Hz.

Now, my question is obviously not about what should I do with my battery or whether there is a warranty on this specific product, but what electric component of the device could produce this sound?

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Buck converter

The power bank takes 5V, and converts that to about 3.7V to charge the internal batteries. It almost certainly used a buck converter to do that. You can read more about how they work online, but the important thing is they have some capacitors and inductors inside which have a high frequency current passing through them. Either:

  • One of those components has started to vibrate at the frequency of that current, and that creates a noise. Most likely it is an inductor, and Assuming it didn't always do that, it probably means a part of that component was fixed in place and can now move, which may or may not mean that component is on it's way to failure. Or
  • The operating frequency of the buck converter has changed from >20KHz (which is too high to hear) to a lower frequency you can here. In which case there is something wrong with it, and it probably won't last all that long.
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