I use
Insider Pro.
It has the most extensive battery log ever, and never sucked anything out of my battery.
I recharge my L520 once every THREE days with normal use with Insider Live Tile and lock screen active.
It is also much more than just a battery app.
For instance before Insider Pro I used Battery Sense, and that app DID shorten my battery life to about two days.
Insider Pro also has a free version, fully functional, but with an add included.
So actually you only pay if you don't want the add.