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My Nokia cell phone is charged with a very lightweight compact charger similar to this AC-15 (can't find the one I actually use).

While the battery is close to empty the charger works silently - just heats up a little. Then, as the battery charges it starts to produce silent yet audible chirps and the more the battery charges the more time these chirps take and less time the charger is silent - something like one chirp with duration of one second, then one second of silence. Finally as charging stops and the phone says "charging complete" the charger starts producing a continuous tone of the same frequency as those chirps. When I unplug the charger the sound fades within about two seconds.

What component of the charger circuitry produces those chirps and why and also why would chirping pattern change as battery charges?

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    \$\begingroup\$ I'd guess that the charger has a switched mode power supply and that the sound you're hearing is made by vibrations of the coils. It's a common problem and there's no easy solution for a consumer. \$\endgroup\$ – AndrejaKo May 13 '11 at 9:06
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As long as the battery is nearly empty, your charger produces its nominal current and runs constantly at a frequency that is likely around 100 kHz > 15...20 kHz = your audible range.

Once the battery is nearly full, the charger goes into burst mode. This means that it switches at 100 kHz, stops switching entirely, and then produces some more cycles at 100 kHz (i.e. the next burst), stops again, ... (repeat).

What you hear is not the main switching frequency, but the frequency of the bursts. You can think of this like an amplitude modulation between 0% and 100% with an inaudible carrier of 100 kHz and a signal of the burst frequency, likely of some kHz and in the audible range.

The reason this is done is that you are able to greatly reduce the standby power consumption of a charger using burst mode. (A nearly full battery looks to the charger like "almost standby".)

For further reading, you can search for keywords like "burst mode" and "switch mode power supply".

The components producing the noise are the transformer, and maybe the EMI filter chokes.

It could also be the case that the charging circuit inside the cell phone modulates its current consumption and thereby imposes an audible modulation onto the switch-mode power supply inside of the charger device.

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    \$\begingroup\$ High dielectic constant capacitors can squeak too, due to the piezoelectric effect. \$\endgroup\$ – Martin May 13 '11 at 9:54

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