I have two batteries (one from my first Pre Plus and the other that came with my Pre 2).
I use the Pre+ as a charging slave and switch between the batteries if I happen to run one down.
Here's the thing, the battery that came with the Pre+ lasts all day, depending what I do. The battery that came with the P2 lasts a few hours.
The difference is pretty dramatic, from 2x to 4x.
Lesson: if you are suffering from horrendous battery life, it might be worth it to buy a replacement. Could be the battery itself...