That's the thing though, I never let my battery get lower than 20%, and let it trickle charge as well, whenever I could, until recently because my battery doesn't last long and I can't charge my phone all the time.
My problem is the overheating of the battery, even when I first got my device. With normal use (I don't play games on my phone), my phone would heat up so much that I had to put my phone down, in order for it to cool down first. Let's not even talk about how hot it got while it was charging. With certain firmware updates, I've noticed that a few would make my phone heat up much more than others, so when I see that my phone would heat up too much, I'd downgrade to a more heat-friendly firmware. One of them (B634, if I'm not mistaken) used to heat up so much, I'd get a popup to tell me that my phone was overheating, and my phone would shut down.
So that is the only thing I can think of that would mess with my battery.
I still have my 4 years old HTC Desire S with it's original battery, and that lasts longer than my P6.