Really strange battery drain

Hello there,

I've a new G4 since last friday and I'm experiencing a really strange battery drain.

Let's say I will charge my phone to 100% and than leave it idle, it will start discharging really slowly with all syncs active (for example last night it lost 2% in 8 hours). That's cool.
The problem is that as soon as I start using it again it will drain really fast, like 1% every 2 minutes. What is strange is that after start using it it will drain fast (1% every 10 minutes) even if i will let it idle again, so screen off and same operations as the 2% lost during 8h in the night.

At the moment I'm in Germany and I've experienced same drain ratio under good and bad cell signal and wifi. My model is H815. I'm not rooted so I cannot see good statistics in BBS.

I've already tried a factory reset.

Thanks everyone fo the help