You can use the Device Maintenance features in the Galaxy S9 and S9+ to prevent your battery from quickly draining. There are a few ways to control the battery settings in order to optimize the battery life by going to Settings > Device maintenance > Battery.
The Battery Usage section allows you to view details of the battery usage by item. For example, the Power saving mode allows you to extend battery life by selecting a power saving mode (OFF/MID/MAX). And the estimated remaining battery life for each mode is shown.
The App Power Monitor feature puts apps to sleep so they do not use battery power when it is not being used.
And Unmonitored apps allow you to select apps to exclude from being put to sleep by the App power monitor. And you can configure the advanced battery settings under More Options > Advanced settings
