What is the charging time of a 20kw battery?

Question is incomplete. Charging time depends on battery energy storage capacity, initial charge condition, charging current, and battery efficiency of conversion (type of battery).

You are referring to high energy storage battery, which can supply 20 KW for few hours. You will need to refer to battery manufacturer for charging such a huge battery. They will be in better position to give you a proper charging regime, depending on the overall storage capacity of battery, its voltage and current limitations.