Once it goes into 'desktop mode' the battery is completely disconnected from the playback circuit. Power comes fully from the 5Vdc external power.
So should significantly save battery cycles.
This has changed from Mojo 1.
Yes this is a nice feature, but it's still annoying that you have to connect a second USB cable to the dedicated USB power port and there's no way for it to run off a single USB cable for both power and data.