If a co-located battery charges from the grid and then discharges later in the day, are the discharged units counted as PPA revenue and billed to the customer?
No. That would only happen if the battery charged from the solar and increased the net generation consumed on-site. You also can’t have a PPA attached directly to a battery. Although, you can have a PPA attached to a solar system where the battery can impact that PPA by storing (and then on-selling via the PPA) solar PV output that would otherwise be wasted or sent to the grid.