Its perfectly fine to use a battery case all day long, the iPhone doesn't charge when it reaches 100% and when the power does go down it will charge again
So don't worry its ok to use a battery case and wont effect the iPhones battery, just make sure you take your phone out of the case once every month and let the phones battery drain and then give it a full charge using your apple charger, if you do that your battery will stay healthy! Merry Christmas