Steve: We have Mastervolt AGM batteries which were fitted by the factory when the boat (a 331) was built. They are now 8 years old and appear to be a s good as new. According to the Mastervolt on board battery monitor they still charge to 100% capacity. They hold their charge through the winter without dropping below 13 volts without being charged as i don’t like leaving the boat connected to shore power for long periods We use the boat quite a lot and have been down to South Brittany most summers for a twelve week cruise. I have always ensured that the batteries are never discharged below 55% of capacity which i think helps with longevity. We too make extensive use of the autopilot when sailing and the fridge is on constantly. We find we can anchor for three days before needing to recharge i.e. we are down to 55% capacity. Domestic battery capacity is 550 amp hours.