I bought a used boat in 2013. The boat was built in September of 2001. I was in the engine room topping off the water in the batteries and noticed the batteries had install date of 9-1. Maybe it is just a happenstance that the batteries have the same install date as when the boat was built or perhaps they were replace in September of 2011? I am thinking there is no way batteries could last 15-1/2 years. ? The Marine batteries are still putting out a strong current. What do you think?