Whistle while charging small cell phone power bank

Question

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?

Answer 1

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.