How old is the phone? Do you know how to close all the background apps. Do you know how to prevent a program from running in the background. How kind have you been to your battery? Did you regularly let it go down to almost zero Or did you stop using when the battery turned red? What do you use the phone for? What is the signal like where you are most of the day?
The phone is less than 3 months old. I do not know about the 'background' programs or how to find or turn them off. I shut my phone down each nite and charge it each morn. Lately - like I said - the battery doesn't even last a half day.
It is not unusual that you need to charge a smartphone each night, but it is not normal to last just a half day.
Is this a brand new phone, used phone, rebuilt phone? Which iPhone model?
First, go to Settings/General/Usage/Batter Usage
There you will see a list of apps come up that indicate what % of battery usage is used for each app or servide. You can look at the last 24 hours or the last 7 days. What are your top 5 and how much % are they each using. Also tell us what is stated in Usage and Standby at the top.
Now go back to Settings/General. Just below Usage is Background App refresh. Look at the apps there and turn off (gray) any app you do not need to run in the background. Remember you may no longer get alerts if you don't allow background refresh for some apps.
Now go to the home screen with your app icons. Double click the home button. Swipe up each app page until everything is closed. Press the home button once. Now Press and hold the sleep/awake button at the top (side if iPhone 6 or 6 plus) until the turn off slide comes up. Fully turn off the phone, wait 15 seconds, then press the sleep/wake button again until the Apple logo comes on.
Report the apps you saw in the first step, and see if above helps. You also did not mention your signal levels in areas you are at every day. Low signal and signals switching between LTE and 3G can use up a lot of battery.