fossyant said:My Father in law has got totally hissed off with his i-phone - eats the battery in a day...can't turn it off completely (eh.....?) and the GPS is always on....... apparently this is common...... any i-phone experts here.....
biggest advice of all would be to hook it up to iTunes and do a software update, make sure its not running an earlier firmware/OS this is free but takes a little time to down load and then install, some of the earlier firmwares were a bit sore on battery.
next is usage! if its never off, mine lasts about a day and a bit (iPhone3G) but with "normal" use ie AS A PHONE it lasts days 3/4/5 dependant on use.
another point is that i live in a fairly remote area where 3G signal is non existent, the GSM radio chip uses next to nothing in terms of battery. but once i go to the city, even with no use on the 3G network the battery seems to drain, its pigging ridiculous.
HOW TOs

to put it into standby immediately press the button on the top right. to switch off COMPLETELY press and hold the TOP RIGHT button and the HOME button on the front, a prompt will appear saying "slide to power off" -simples
to switch off GPS go to SETTINGS -> GENERAL -> Location services, set this to "OFF" and that will switch off GPS.
another tip is to set the screen lock to as short a time as you are comfortable with and set the brightness as low as you can. and switch off auto brightness, as it tends to set the brightness as it pleases and usually a BIT brighter than I like, YMMV though
ALSO try setting the email update fetching to "only when i open MAIL" I think its set to once an hour by default, or even every 5 mins, can't remember, but that would drain the battery especially on 3G
oh and turn off wifi is he doesnt need/use it as it will try and pick up/search for every wifi network/home modem it passes, and going down the street, that could be a LOT, again another drain.
hope some of that helps