If you have a bad battery in your iphone you may experience the phone turning off suddenly, very short standby time, acting funny when charging such as making the connection noise repeatedly or the screen going on and off repeatedly. There can be many other problems caused by a battery. The problems listed above are just a guide and are not 100% indication of a battery problem.
We sell only Top quality batteries for the iphone 6S. This means you get a battery that will hold a great charge and is very unlikely to be faulty.
For optimal performance, calibrate your newly installed battery: Drain battery below 10%, then charge uninterrupted to 100%.