Asus India — Asus zenfone 5. Subject:- very poor battery backup.

My phones battery backup is very poor. If I made the battery 100% charge, hardly I can use it for 2 and half or 3 hours.
But if am keeping it idle battery won't get low. My phone is in warranty period. I went to a service center in Bangalore seven times with the same complaint. The fact is still they can't identify what is exact problem. They told me they have changed battery. And software is a new version now but the problem is not yet resolved.
If this is the situation what is the need of a cellphone. I need to keep it for charge all time.
The service center I went is ' F1 info service'. They made me to go there seven times still the same problem.
I need a resolution for this.