My new boat to me came with 2 AGM batteries which are 5-6 years old. Have not had any issues with them but they were always on a charger till now. For now I have left them on the boat and plan on charging them about once a month over the winter. Will these survive a winter under these circumstances? I am also thinking of adding a third starting battery next year and wondering if a flooded battery be compatible with the AGMs on a 3 bank charger? What are the advantages of AGM batteries given their initial cost when compared to deep cycle batteries? Thanks