To avoid any problems in the future, you have to check your IPhone battery before buy it. For IPhone 6s Models produced between September and October 2015, their batteries would have some problem in the near future. According to Apple company announcement, all users should replace their batteries at your nearby shop in area avoiding problem.
In order to know whether your battery should be replaced or
not, you have to do the following:
After that you search for Serial Number, if you see the
following letters in Fourth or Fifth place, your IPhone battery should be
replaced.