How can I fix the issue like battery fluctuation in my iPhone 6s ?
I face most of the time my battery perecentage get down even without any usages?.
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I would suggest to check your battery health: Settings>Battery>Battery Health (beta) .. then tap on it, and read the maximum capacity percentage. If it is lower than 85%, your battery is close to reach the critical point of its life, and could have unpredictable behaviors.
In this case, have it replaced at your most convenient Apple Store or Authorized Service Provider: Apple - Find Locations - Location Selector
If the battery health is good, than you could have something running in background, or check the apps that are using your battery.
Read also this support guide: Batteries - Maximizing Performance - Apple
Regards
Giulio