If you leave different apps running and you have alot of them running itll drain the battery. When your done with an app close out of it with the button on the bottom left of your phone it should be the three line thing. In there you can close the apps.
I have a Galaxy S8 Active and recently my battery has been draining very fast my husband got a whole new S8 active and it's still didnt fix it I dont want a new phone
Check what could be draining battery on the phone. Go to settings>device care>battery, then tap on battery usage. You will see list of apps, services and the percentage next to them. Also the graph above shows some info as well. See which app in the list has more % taken out of the battery.
There were cases where Google Play service could eat up some battery after it was updated. Periodically google updates Google Play service, some updates are good, some aren't.