Zenfone 2 ZE551ML switches off suddenly despite 20 percent battery remaining

My Zenfone 2 (Z551ML- 4GB RAM) turns off at 20% battery life! I tried turning it on but no luck. I need to connect the charger before the phone will boot up. But when I turn on the phone with charger connected the battery shows as 20%. I have tried to calibrate the battery but still no luck! Any suggestion regarding this will be very helpful... Thanks a lot in advance...