Battery Drainage suddenly increased !! Iphone 6

Dears,

Suddenly my Iphone6 started to suffer extremely fast battery drainage.

I NEVER used any other charging device than the ORIGINAL charger.

Battery drainage is astoundishly fast even when the Iphone is left with all applications turned off, as well as wifi, and no phone conversation.

This happened from one day to the other, without explanation.

Someone can help me? Go to Apple Store? Any trick I could learn?

Thanks to the community and please APPLE INC. help us!!!!

I do have IOs 8.4 installed.

Best, Claude