I'd recommend taking a look at your Battery usage. Go under Settings> Battery and device care> Battery> Then tap on the graph of your usage. If anything looks out of the ordinary, like a app using more Battery then seems reasonable, you may have found the issue.
If everything seems normal there I'd recommend trying good gaurdians (it's an app on the galaxy store) and running Galaxy app booster and checking the Battery tracker to see if there is any suspicious amount of usage.
If neither of those work you can try going under developer settings and reducing background processes and turning on "don't keep activities."
If none of those work, then your Battery may simply be going bad. Battery's naturally degrade and you may need to replace it. Samsung announced that Batteries+ would be offering official Samsung Battery replacements so maybe give that a look.
