my phone battery goes down quick

Salutations alexandratas, 

Your iPhone X running iOS 11.3 or later includes results from a built-in diagnostic tool that can tell you if there is a hardware issue with your iPhone battery. Please take a look in Settings > Battery > Battery Health. Here what your iPhone displays information about your battery’s capacity, peak performance, and whether your battery needs to be serviced.

The following article, iPhone Battery and Performance, under section ‘Battery Health’ has more information and also includes some screenshots if you need examples of what you’re looking for. 

If all appears healthy there, let’s next explore usage of the battery. The following article goes over what you’ll see in Settings > Battery as far as usage - Check the iPhone battery health and usage

View your battery usage information

Go to Settings  > Battery.

Information about your battery usage and activity appears for the last 24 hours and up to the last 10 days.

• Insights and suggestions: You might see insights about conditions or usage patterns that cause iPhone to consume energy. You might also see suggestions for lowering energy consumption. If a suggestion appears, you can tap it to go to the corresponding setting.

• Last Charged: Indicates how fully the battery was last charged and the time it was disconnected.

• Battery Level graph (in Last 24 Hours): Shows the battery level, charging intervals, and periods when iPhone was in Low Power Mode or the battery was critically low.

• Battery Usage graph (in Last 10 Days): Shows the percentage of battery used each day.

• Activity graph: Shows activity over time, split by whether the screen was on or off.

• Screen On and Screen Off: Shows total activity for the selected time interval, for when the screen was on and when it was off. The Last 10 Days view shows the average per day.

• Battery Usage by App: Shows the proportion of the battery used by each app in the selected time interval.

• Activity by App: Shows the amount of time each app was used in the selected time interval.

• Note: To see battery information for a specific hour or day, tap that time interval in the graph. To deselect it, tap outside the graph.

Thanks.