Why battery in HTC desire 820 goes down so fast?

A

AC Question

Hi,
I purchased HTC Desire 820 mini (dual sim) - made in china phone. I have a problem with it's battery life which does not exceed even 15 hrs when only using for phone calls. I kept the data off and the power saving mode is ON. Upon fully charge (100%), it remains only about 12 -13 hrs. Is my product is faulty or is it normal?