My mate phoned me and said, as per the title, that if you turn 3G off then the phone lasts much longer - he's on the phone all day with his work and he claims it's made a big difference.
Obviously you'd neeb to turn it on again if you wanted to do some surfing but when the phone is just sitting idle it's probably worth a go if battery life is an issue for you.
I have noticed my battery drains quickly overnight - I live in an area with v.poor 3G signal, while at work its full 3G (city centre) and lasts say 10 hours without dropping a bar...
It's pretty poor that you need to disable 3G on a 3G phone just to get decent battery life.
i think this has always ben the case, it certainly was with nokia's first 3G phone the 6630.
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Phones ive owned:
Nokia 5110
Ericsson T20
Nokia 3310
Nokia 8210
Samsung A300
Nokia 7650
Nokia 3650
Samsung P400
Nokia 6600
Nokia 7610
Nokia 6630
Nokia N70
Nokia N91
LG K920
Nokia N95 (current)