My phone quickly drains battery ev3n when fully charge and in standby.

My phone discharges so fast even after fully charging and not being used.analysis says battery in good condition.phone has no downloaded apps but just the apps that came installed withe phone.all settings are in power saving mode, eg.bcklight 30%, screen tmeout 15secs, silent, no vibra..etc..how do I fix this.? FYI phone is 4 years old.