In the old days, you would buy a UPS, those tend to work well when the product being powered also accepts information to say the mains power has been removed so it can successfully shut down.
The BL printers do not have this.
You may be better suited with a large capacity portable power station.
The bigger ones have mains sockets to hook up your 3D printer.
You would run the printer off the power station and leave the power station plugged into the mains.
This will then run your printer until all charge has been depleted.
You can also use it for many other purposes outside of the home/office when required. Something not possible with a traditional UPS.