why my battery is draining fast

[Raffaelo1]

Hello
I bought my siny z3
From cex UK
It was working generally fine
One day my battery qas drained fast
I took of from charging
100% i went to sleep
When i woke up it was 30 %
I saw battery usage
And show me the android system and Android ios are drained my battery
I replaced the battery as well in cex because of my warranty but still same
.anyone.HELP ME OUT
Please....

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