this is another common issue. this is normal wear and tear over time, but can be exasperated by not using the proper charger and cord that comes with the phone. also, some batteries use a smart feature that learns charging behavior from the user. these are factors at play here with these types of issue.
first you could try to re-calibrate the battery by completely draining it until it turns off (sometimes this is inhibited by a smart setting to turn off when low power is detected in settings) from there, youll want to let the phone charge to 100 percent before turning back on. see if this helps (not typical with todays li-ion batts)
if this fails, youll probably want a new battery. here is a link to replacing and to the part youll need. make sure to math the model number for correct specs:
Samsung Galaxy Note5 Battery Replacement
if any further assistance is needed, post back here.