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Does charging an already charged battery of cell phones/laptops consume electricity?

Whenever my cell phone becomes fully charged while charging, it asks me to remove the charger from the socket to save energy.

I was wondering why would a charged battery consume energy/electricity from the socket?

NOTE: My phone is a Nokia 7210 Supernova (old model, I know!)

archilius
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The charger has to have some "housekeeping" circuitry active in order to detect that the phone is attached and react to the state of charge of the battery. Some of the best will go to sleep and periodically wake up to check to see if there's a device that needs charging attached. So it's not delivering charge to the battery, but it is using power to keep some of the circuitry alive. There are also parasitic and efficiency losses in the circuitry that converts the AC voltage to the DC housekeeping supplies.

John D
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