Check what you got running in the background.
Turn it off with power switch and restart it.
BE REALLY SURE THAT THERE IS NO BATTERY INTENSIVE STUFF RUNNING IN THE BACGROUND
If this won't seem to help,return it,you have a warranty.
Good advice from Stefan. I had the same problem with my almost new 7+ and Apple advised me to reset and restore from backup to see which app was sucking up my battery. I found out the "HiYa" telephone app was using my battery like a thirsty camel drinking water in the Sahara. I removed that app and now my 7+ battery will last for 3 days in between charges.