Talking to Apples’ official support site, it admitted on iOS 9.2 battery problem both iPhone 6S and iPhone 6S Plus. The mentioned malfunction causes the device battery level to freeze or neither battery drain nor not charging displayed in the charge level.
Just to clear, the phone batteries still functioning, it just iOS does not report properly the log.
As stated on the support website, Apple suggests a temporary fix.
First is to restart your device, then hover to settings, go general tab then date & time. Make it sure that it is set to automatically (It should be turned on).If this happen again and you did not change the deice time zone, you can contact Apple Support.
For the meanwhile, Apple admits that they are fully aware with the issue and now on the process of investigating the cause and now working for a permanent solution.
Interestingly support document from Apple blames the issue directly on the device time zone shifts:“If you change the time on your iPhone 6s or iPhone 6s Plus manually or change time zones when traveling, you might notice that the battery percentage doesn’t update.”
But after some investigation with the issue from official support communities of the company, the dominant thread discussing the recent bug does not factor in phones time zones. The thread says that the bug originated long time ago and dates back 18 September.