-> Look at it this way: The battery in my cell phone (Siemens M35i)  
-> is 3.6 Volts, 500 mAh. Or 1.8 Wh. That's enough for 3 to 4 days  
-> of average operation. A 500mW generator could charge the battery  
-> in less than 4 hours. So 500 mW is plenty for a cell phone. 
  
It depends on the conditions. Cell-phones automatically increase their 
transmission power when the local cell site is difficult to communicate 
with, for example if it is distant, or if there is a lot of stuff in 
the way that reduces the signal strength. So phones use up batteries 
faster under these difficult conditions than when communication is 
easy. A phone that is capable of operating under very difficult 
conditions (which most of the little new phones are *not* capable of 
doing) may transmit several watts of power. 
  
                               dow 
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