There are printer settings that tell the printer to shut off after a certain duration of being idle. To disable these idle shut-off settings, please follow the directions below.
Make sure your Brother QL-810w printer is connected to your computer via the connection cable provided by the manufacturer.
1. Connect the printer you want to configure to the computer if it isn't already connected.
2. Start the Printer Setting Tool.
For Windows 10 / Windows Server 2016 / Windows Server 2019
Click [Start] - [Brother] - [Printer Setting Tool].
For Windows 8 / Windows 8.1 / Windows Server 2012 / Windows Server 2012 R2
On the [Apps] screen, click [Printer Setting Tool].
For Windows 7 / Windows Vista / Windows Server 2008 / Windows Server 2008 R2
From the Start button, click [All Programs] - [Brother] - [Label & Mobile Printer] - [Printer Setting Tool].
The main window appears.
- Check that the printer you want to configure appears next to [Printer].
If a different printer appears, select the printer you want from the drop-down list.If only one printer is connected, it is unnecessary to make a selection because only that printer is available.
3. Click on "Device Settings" (#3 as shown above).
#2 Printer
Specifies the printer whose device settings are to be configured.
If only one printer is connected, it is unnecessary to make a selection because only that printer will appear.
#3 Settings Tabs
Contains settings that may be specified or changed.
If 
appears in the tab, the settings in that tab cannot be specified or changed. In addition, the settings in the tab will not be applied to the printer even if [Apply] is clicked. The settings in the tab will also not be saved or exported when the [Save in Command File] or [Export] command is executed.
#6 Parameters
Displays the current settings.
#8 Apply
Applies the settings to the printer.
To save the specified settings in a command file, select [Save in Command File] from the drop-down list.
The saved command file can be used with the mass storage mode to apply the settings to a printer.
Basic Tab
#1 Auto Power On
Specifies whether the printer turns on automatically when the power supply cord is plugged into an AC power outlet.
Available settings: [Disable], [Enable]
#2 Auto Power Off (AC/DC)
Specifies the period of time that passes before the printer turns off automatically when connected to an AC power outlet.
Available settings: [None], 10/20/30/40/50/60 minutes
#3 Auto Power Off (Li-ion) (QL-810W/QL-820NWB only)
Specifies the period of time that passes before the printer turns off automatically when powered by the rechargeable Li-ion battery.
Available settings: [None], 10/20/30/40/50/60 minutes
[Auto Power Off] is disabled when connecting with wireless network, wired network (QL-820NWB only), or Bluetooth® connection (QL-820NWB only).
#4 Eco Charging (QL-810W/QL-820NWB only)
Specifies to switch to Eco Charging. The service life of the battery can be extended by setting it to charge to [80 %].
Available settings: [100 %], [80 %]
To get the longest life from the battery, select [80 %]. Although the number of pages that can be printed on a single charge will be less when you select [80 %], you can get a longer life from the battery.
4. Within the "Auto Power Off (AC/DC)" drop down, make sure this is set to "None".
5. Click "Apply".
The above help articles are derived from Brother's help articles located through the below links:
"Using the Communication Settings (for Windows)"
"Using the Device Settings (for Windows)"