1. Psychological: believe it or not but after we receive any upgrade for our device,  we get curious about the new changes and to explore those new changes/features our phone usage increases and as a result we notice that battery might be draining too fast.
2. Software & Hardware calibration: it is very important that your hardware & software understand each other and cooperate to provide best performance, normally after major upgrades smartphones take some time to make adjustments with hardware & software compatibility.
3. Performance: with each upgrade smartphone manufacturers try to increase performance of old smartphones which leads battery to drain faster. and i have seen lots of developments in this upgrade.

Solution

1. After this upgrade screen brightness has been improved, and screen looks much brighter than before, i felt i can easily use phone by reducing brightness up to 50%. and in my case display was eating most of the battery, so after brightness was reduced to 50% with auto adjustment. i felt battery life increased.
2. Battery calibration is very important, this is how i calibrated my battery. keep using phone until battery drains fully & phone switch off automatically, now insert charger and let it charged in switched off mode till battery reaches 100% . it is very important you repeat this step every time when you need to charge your phone. follow this process for at least one week. after that you can charge battery by your way.