If your comp is plugged into the wall, it shouldn't fall in battery power while using; if so, Contact your tech people (manufacturer, retailer, etc) - Nothing to do with HB.
You may need to fiddle more with the power options to make sure your harddrives, ethernet adapter, etc don't turn off after a certain period of time. I'd make sure none of the manufacturer's installed software is causing any issues either.
That's what I'm saying: "My PC is plugged into the wall e.g. it shouldn't fall below 20% power since its plugged IN" => "The power saver is set to go to power saving mode once it falls BELOW 20%" e.g. it shouldn't fall below it...
If it somehow went to power saving mode the battle.net desktop app should have been logged out as well which it wasn't. Am I right?
I had just woken up went I wrote it sorry if It didn't make sense before..