Battery problem

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http://forums.androidcentral.com – Hi. I'm having a problem with my s4 battery. If it is 100% charged, the first 20% of the battery last about 8-9 hours. However, after the first 20% (when it is only 80% left) the baterry drains so fast. The remaining 80% last only about 16 hours. By d way I wasn't using the wifi, gps and any apps that time. No text, calls and the features of the s4 (hand gesture, etc) are deactivated and the brightness is already set in the lowest level. Is this normal or not? I hope you can help me with my problem (HowTos)