Mac, your battery is dead when the voltage
is below 11 volts. The average battery {lead acid) is full charged at 12.72 volts and 50% discharged at about 12.2 volts. The better maintained a battery, the longer it will last. I would explain all that with a book written here but I think your further inquiries would allow many others to respond and for you to further inquire about solar or other charging and also batteries you would like to use. Remember the responces you will get are ideal!You will need to weigh out their input and your need to justify your $ output to the advice you get.Seriously, you can have some fun here learning about the different batteries and solar charging. When it is all said and done, you decide what is best for your budget and boat. You have started the thread, I don't think you will be dissapointed.r.w.landau