Recently updated my S7 to Oreo and here is my observation on battery performance:
Drains 100-90%: With in 1 hr. Drains 90 - 40% : It will take 7-8 hrs after using mobile moderate to heavy (Youtube, Twitter, Calls, Camera & Video calls etc). Drains 40 - 15% : It will take 2-3hrs
Finally I am getting 12 - 13hrs of Battery and I did not notice any difference when compared to Noughat
Thank you for reaching out to the Samsung community! We would like to inform you that battery usage depends upon the application running in the device. The estimated battery usage time may vary depending on your device settings and operating conditions.
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