Magel282 is your device fully charged?

i found that mine was reporting being fully charged when it was actually only at 88%

also have you enabled power management for the WIFI? Have you reduced the output from 100 mw to 10 mw?

if you are using the wifi continuously for 12 hours then 12 hours is actually pretty good for any device. If not, then check how often you are checking for emails etc.

My phone easily lasts a full day including 3-4 hours of browsing over wifi and another 3-4 hours of playing games on it.