I've actually done that after depleting the mophie battery and having about 20% phone battery left and woke up 4-5hrs later and the phone battery was full but the mophie had only charged about 75% (3 out of 4 light dots)
Another thing that disappointed me about my mophie case is its not water resistant. Which ruins the point of having a water resistant phone in some instances. Don't get me wrong it's a good case, I just expected more for the price of it.