GoodWe · GW5000D-NS
Can I add a battery to a GoodWe GW5000D-NS?
Not through an evidenced native battery interface on this DNS inverter. GoodWe's manual identifies GW5000D-NS as a grid-connected PV inverter, so retaining it points to a separate AC-coupled storage system.
Do you need to replace it?
Not necessarily. Replacement is one architecture option, not a conclusion BecSpec draws merely because the inverter is solar-only.
Backup
Backup depends on the new battery inverter/controller and electrical design.