Originally Posted by rydog
Okay that makes sense. Now I would run the system until it dies and still have my starting battery but when I run the audio battery my boat eventually alarms out (because of the lowering voltage?) so I have to turn the boat on and charge. So I can't run my battery dead because the alarm will go off and I have to charge.
Yes, you need to add a second battery or more.
As MLA writes, deeply discharging a battery significantly reduces battery life so having more batteries that are depleted less will be cheaper in the long run. (assuming you properly maintain the batteries with a full charge from shore power at the end of the day)
If you are idling the boat just to play the stereo, this IS hard on the alternator since the alt is only putting out maybe half its rated amperage. Low output on top of a heavy draw from a depleted battery AND stereo draw is a recipe for alternator replacement.
Fuel cost idling a boat just to run the stereo will quickly mount to more than additional batteries.
Finally the safety factors of carbon monoxide poisoning or some drunk stumbling onto the throttle or jumping into the drivers seat should be considered.
(I have seen ALL of the above happen in the Cove)