Mobile Phone(electricity supply)problem

I would say it is nothing to deal with the life of batteries, but rather, i think your mobile phone maybe too old that the detection system for the battery level is out of order.

First of all, normally mobile batteries are using electrolytic cells (usually Nickel-cadmium battery). The exact machenism for producing electricity is actually a series of redox reaction. To make it simple, you can simply tread it as a charged capacitor. The mobile phone actually measures the voltage across the cell to determine the electricity supply , which is reflected by the no. of 格s on your mobile phone.

Now, when you are talking via mobile phones, the signals are actually trasmitted through air by means of EM wave production. It is simple to understand that excess energy is required to produce EM wave, so it is easy to understand that the electricity supply drops after you have talked via mobile phone for 20 minutes.

But the interesting point is the voltage across the battery won't increase without charging it. This imples the electricity supply reflected by the mobile should not increase after you didn't use it for 15 minutes. If this really happens, the only conclusion is the mechanism of measuring the voltage across the battery is out of order.

Afterall, you have used your mobile phone for 5 years, it is normal to see that happens.

P.S. For your reference only, the exact redox reaction inside the Nickle-cadmium cell is:
2 NiO(OH) + Cd + 2 H2O ↔ 2 Ni(OH)2 + Cd(OH)2
This is a reversable reactionm, which represents the charging and discharging process. The alkaline electrolyte (commonly KOH) is not consumed in this reaction.
According to wikipedia:
Power/weight 150W/kg
Charge/discharge efficiency 70%-90%

Hope the above information helps =)