a 5 V mobile phone charger from a 9V battery using a voltage divider circuit. Can...

Voltage dividers don't work very well at high currents. What you want is a 3 terminal voltage regulator such as a LM7805

You would be better off with 3 AA or C cells, which will give you about 4.6 volts, which will probably run most chargers. That way you don't waste 4 volts.

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put one 80 ohm 1 watt resistor in series with the 9V. At charging current 50mA,it maintains 5 volt output.