Why does the supply voltage in my mobile drop when I plug in the charger?

[tiger]

My phone is a Sony Xperia.

For example, at any given time, the battery voltage is reported as 3.915V. Now, when I plug in the charger, the voltage is reported to be 3.810V. When I plug out, the voltage increases again to 3.915V.

Why does the reported voltage decrease when I plug in the charger? Don't you think it should increase a bit, if not remaining constant?

[malcolm]

in all honesty contact Sony as they have the licence to the Manufacture of the phone and supplier of the said battery and charger

[IrvS]

It's quite possible that plugging in the charger puts a resistor into the circuit
to protect the meter from charging current.

[Joe]

I might guess that the meter circuit was designed with an assumption of current flowing out of the battery. While charging, current is flowing into the battery.