Age is only one issue with decay of a battery. How you store it , charge it and "cycle" it can effect its decay greatly! Always disconnect when not in use and always fully recharge it on a proper slow charging battery charger. I like to use a 2amp/10amp overnight with an auto shutoff so it cant overcharge. I will sometimes set it to 40amp for a couple hours when I can watch it as it cooks the sulfates off the plates when it heats up but watch that it doesnt push out the acid. When stored it needs to be in a good place not to freeze and preferably on a "trickle charger" or "battery maintainer" type charger fully disconnected from anything but the charger.. I have gotten extra years from even cheap deep cycle batteries doing this..
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