I have to agree with emals. I buy the cheapest marine batteries possible ($79). Usually last five or six years. I only use one battery at a time for the whole day then switched to the other one next time out. I start the boat with one battery. My thinking on this is if the battery is starting to weaken I'll know by the sound of the start. If one battery goes bad I replace both at the same time. The information that you guys are giving shows a lot of knowledge but for the average person like myself it gets to technical.