Remove back cover by applying pressure and sliding up towards the top of the phone.
Remove the battery by lifting from slot on the right hand side.
Use Multimeter to test the voltage of the battery.
Put the black lead into ground slot of multimeter and the red lead into the V(ohm)mA slot.
Place the red and black leads to the two outside metal plate, each on opposite sides, on the battery as shown.
Check that the voltage displayed on the multimeter matches closely to the voltage defined on the battery. Normally 3.7 V.