  The Page curve plotted by the typical random state approximation is not
applicable to a system with conserved quantities, such as the evaporation
process of a charged black hole during which the electric charge does not
radiate out with a uniform rate macroscopically. In this context the
symmetry-resolved entanglement entropy may play a significant role in
describing the entanglement structure of such a system. We attempt to impose
constraints on microscopic quantum states to match with the macroscopic
phenomenon of the charge radiation during black hole evaporation. Specifically,
we consider a simple qubit system with conserved spin/charge serving as a toy
model for the evaporation of charged black holes. We propose refined rules for
selecting a random state with conserved quantities to simulate the distribution
of charges during the different stages of evaporation, and obtain refined Page
curves that exhibit distinct features in contrast to the original Page curve.
We find the refined Page curve may have a different Page time and exhibit
asymmetric behavior on both sides of the Page time. Such refined Page curves
may provide more realistic description for the entanglement between the charged
black hole and radiation during the process of evaporation.
