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?
Best Answer
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: