They are always on a charger at the dock or completely charged up every day while out cruising. They never are allowed to get below 50% discharged. Between the wind, solar and my wife's use of the microwave and hot water shower (which requires generator time) we get plenty of charge time. My last set of 4 D's lasted about 6 years and when they tanked it was quick. One day they were fine and the next the battery alarm was going off at 2am. It took three weeks to ship new ones into Mexico so every night one of us would have to get up and start the generator for an hour. It was like having a baby again with night feedings. What a PITA. What is your procedure for load testing you batteries if I could ask? I'd like to give mine a test.