Option 1: Turn Off the Low Battery Alert
You have two options for dealing with those annoying "Battery is low" notifications in Android 9.0 Pie: You can either make them less intrusive, or outright disable them. I'll cover both method, starting with instructions for turning it off.
Open the main Settings menu and select "Apps & notifications." Choose "see all X apps" (X will be the number of apps installed on your phone) and select the three vertical dots in the upper-right corner. Choose "Show system" to reveal the system apps in the list.
Once selected, find "System UI" on the list and select it to open its App Info page. Choose "Notifications" to reveal a list of all the different categories of notifications created by the System UI app.
Find the checkbox next to "Battery" and simply tap it to disable notifications. Select the back arrow in the upper-left corner to save your choice. When done correctly, you will see a message at the bottom of this menu stating "1 category deleted," indicating you turned off battery alerts.