A 6m boat with 3 POB reported they had a battery problem and required assistance.  The boat was 600m south of Whale Island. An all stations alert was transmitted and a good samaritan who had a spare battery offered to assist. Sea conditions were good with 10kt wind from WSW and a calm sea. At 1244hr the good samaritan arrived alongside the boat and transferred the spare battery. At 1323hr the boat was safely back in port. Many thanks to the good samaritan.