My just bought Samsung galaxy s5 battery drain quickly

I have purchased a Samsung Galaxy S5 from the store. I open it and I begin by charging it for the first time, but without turning it on. The battery was 61%, so I waited until 100%. After that, I turned on the phone and proceeded to set it up. The version of Android was 5.0 Lollipop (I don't know how this is possible as I thought I need to update it myself from 4.4.2 to 5.0).

While I was entering in the information for the first time, the battery suddenly fell from 100% to 95% in just 5 to 10 minutes, during the first steps where I had to create a Google account and set up my phone options.

I heard rumors that 5.0 Lollipop drains the battery quickly so I downgraded to 4.4.2 KitKat with the hope that it'll fix the battery problem. I downgraded successfully without error.

I have the same problem—the interesting thing is that I didn't download more than 5 apps. I only have Skype, Facebook, Messenger, WhatsApp, Viber and Instagram. All the other apps are from the phone itself.

Whenever I go to WhatsApp to check messages or chat (or anywhere else), the battery falls 1% after 3 minutes. All of the NFC options, location options and other unnecessary options are turned off. The brightness is also on low. In 15 minutes of use I notice that the battery falls from 80% to 65% or 60%.

I have already done a factory reset twice, while I was downgrading to KitKat 4.4.2.

I'm a new user of a Samsung Galaxy S5. Is this normal or abnormal? What is the problem and how can I resolve it?