This does NOT make sense...v551 battery (of course)

So I bought my v551 on Feb. 10. Had crappy battery life. With less than 20
minutes talk time it died in 36 hrs. No bluetooth, no headset. But I tried
conditioning the battery with multiple full discharge/charge cycles and my
battery life seemed to be getting better. Lasting anywhere from 3-5 days. In
fact, on its last charge it lasted from Saturday, March 5th at 7p.m. to
Thursday, March 10th at 11p.m. with 2:15 hrs of talk time. This was looking
great! That was over 120 hours! (Most of the 2 hrs of talk time was Saturday
night just after it charged.)

So I charged it Thursday night and didn't take it off the charger until
9a.m. Friday morning. With 3 minutes of talk time (checking my voice mail
only) it is now already beeping Low Battery. Only 51 hours! With no talk
time!

And furthermore, it went straight from 6 to 0! When I looked at it this
a.m., it still showed a full charge at 6 bars. And now it shows 0! I'm used
to it not lasting for any time at all at 2 bars, but usually it does make a
temporary stop at 4 bars.

I made no changes to the settings. No bluetooth. No change to the
brightness. No change to the screen timeout. Nothing. I dont' understand!
What gives? Is it the phone or the battery? I just sent in my rebate form
last week too because it was doing so well. And officially I am now over my
30 days! (Just by 1 day or 2)

I just don't understand why it lasted longer when I talked on it vs when I
didn't. And with no change to settings or where I'm using it (work and home)
why did I get 120 hours last time and only 50 this time?