Normally, mobile phones have a voltage rating of 2.4V to 4.8V. Be careful while connecting a substitute power supply, as even a slight increase in the applied voltage can damage the phone.
Some phones go dead due to a shorted RF power amplifier. If a battery is connected to such a handset, it may suddenly get fully discharged and become dead.
Thus it is advantageous to verify the overall loading of the handset before connecting an external power supply or battery. For the purpose, you can use an ohmmeter. The battery terminal of the handset reads 5 to 50 ohms in one direction and 1 kilo-ohm to 150 kilo-ohms in the other direction. If a wide difference is noted, the circuit is either open or shorted.
The article was first published in September 2003 and has recently been updated.