Apple has addressed a minor issue for some users of the iPhone 6s and 6s Plus where the battery percentage doesn’t update. This is most likely a software issue and not a hardware issue. If you don’t mess around with your time zones then chances are you will never see this problem. If you are traveling quite a bit and changing your time zones then this is where you may encounter seeing your battery percentage not updating. The best thing to do is to restart your iPhone and go to Settings > General > Date & Time and make sure that “Set Automatically” is turned on.
If you are not changing your time zones or the above method does not fix your issue then you may want to contact Apple or take your iPhone into an Authorized Service Center for iPhones such as Connecting Point. Although Apple stated they are investigating the cause, our guess is that Apple will address this issue in the upcoming iOS 9.3 update which is in beta right now.