1. Is your iphone jailbroken? Does it a have SBSettings?
2. Just in case it is a no to both, jailbreak it with spiritjb, jailbreakme or greenpois0n depending on your firmware. Go to cydia and download SBSettings (SpringBoard Settings)
3. Once it's done respringing slide your finger over the status bar. You'll see a small inset. Click on 'more' then on 'system options'. Turn on numeric battery.
Hope this helps!