It would really be ideal if that's the way it was. But in my opinion the battery pack is just a charger. It's not an additional battery which can be accessed by the phone for power.
Why do you think it couldn't charge the phone while the phone's in use? And if you're saying it wouldn't be able to actually keep it at 80% while it's being used, I agree, but it would be charging all the while until it's finally back at that percentage.
Again, and this is just my opinion, the battery pack exists for one reason only. To charge the internal battery. When it's depleted then you would see the internal battery start to decrease from 80% or 100% depending on the settings.
So as I said earlier, the phone would be in a constant state of charging, which I think would degrade the internal battery as time goes on.
Posted via the Android Central App