There has been many reports that enabling the numeric battery feature on the iPhone actually kills the battery life quicker. Some people have reportedly found a massive increase in battery life after turning off this simple feature. Especially those with jailbroken phones who use SBSettings to turn on the feature.
Some people oriignally thought the battery died quicker, but thought it was a psychological thing and that you thought it was as you could see the percentage go down. However I’ve now turned off the numeric battery for good and hope I get better battery life as currently it’s not lasting a whole day with light use.