Excessive Battery drain on iPhone 15

There 3 reasons that is most common:

• App using battery resources when viewed at Setting > Battery. Post a screenshot that includes the Graph as well as the top apps when you scroll down. Apps listed with Background Processes are using your battery whether the app is open or not.
• Poor Battery Health. When the Maximum Capacity noted at Settings > Battery > Battery Health & Charging falls below 80%, it is time for a replacement.
• Fringe area of cell coverage. Especially noticeable on 5G where the range is shorter and more power is required to maintain the connection.

If you go to Settings>Battery and check the app using battery during the time period you describe, is it just one app, what % of battery use? Are you using data, and if so, are you on wi-fi or cellular, and what the cellular signal look like? There are a number of reasons a battery can trend downward quickly, and I've named a few, but it is hard to say about your device since you did not provide any background of usage, apps, etc.