If I leave my cell-phone charging the whole night, it will be fully charged after a while. What happens with the battery and the excess energy I add? Also, I noticed my 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?
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.
Let us assume that your battery charges at a rate of 100 ma. When it is full the plates in the transformer of your charger will be saturated as they are made for this specific purpose. When the plates are saturated there is no magnetic field to cut and the charger is 'off'. If you leave it long enough the magnetic field will appear again and the charger will be 'on' again. This may account for the difference in noise. If you open a charger you may see a transformer and a diode , the regulator consists of the transformer plates saturating at the amperage for which the transformer was made. It is always wise to switch chargers and all electrical items not in use to the 'off' position.
Well actually it is oviously not normal because of too much current entering to your battery cell that produces those sounds. The battery cannot accept anymore current because it has already reached its highest point (i.e., fully recharged).