Join Date: Feb 2010
Location: Narragansett Bay
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Re: REALLY Dead Battery ???
Sounds like you may have left a light on, but even if everything is shut off, there remains a parasitic load from many electronic devices that can slowly drain a battery. You wouldn't expect parasitic load to kill off a healthy battery, but being 9 years old could mean it had very little of it's original amp hr capacity left. The 12.7 volts you saw does not indicate capacity.
As some have mentioned, you could try to recharge it. There are two risks. First, she takes a charge, but really has very little capacity and you find yourself without a battery, while sailing. Second, damaged batteries can be an explosion danger, particularly when charging.
As others mentioned, nine years is a very good useful life. You're replacing soon one way or the other.
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In the harsh marine environment, something is always in need of repair. Margaritas fix everything.