Many of the times we left our cell-phone on charging for the whole night, it will be fully charged after a while. What happens with the battery and the excess energy we add? After sometime we also notice some charger emitting a different pitched sound after it has fully charged the battery. Is this just a mechanism that kicks in to redirect excess energy? Where that extra energy go we add to our phone after it get full charged. How much it would be harmful to us?
ANSWER :-
Once the battery is fully charged it will not accept any more energy (current) from the charger, since all the energy levels that were depleted when empty are now at their highest level. For example in a Lithium ion Battery when all the ions have arrived at the proper electrode the resistance to more current becomes very large, but not infinite since there will be some electron mobility.
It is Ohm's law that reigns at this stage: I=V/R
The charger ideally would stop charging if it finds infinite resistance between its + and - outlet voltage ( as it is when plugged in the outlet but not the mobile),but it will waste some energy heating the battery because the battery will not have infinite resistance, and it will waste some energy in the transformer circuit heating the charger, because it also has some resistance. It is best to unplug the charger when not in use.
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