08-15-2011
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Senior Member
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Join Date: May 2007
Posts: 454
Rep Power: 6
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The rate that batteries charge decreases as they "fill". It's not usually practical to charge them above 80% by running the motor (unless you are motoring along for many hours anyway). As mentioned above 50% is as low as you should draw them down before starting a recharge. Therefore the useful range is about 30% of total capacity (from 80 to 50%). In this case about 150 amp hours is all you should count on.
Even if you run your motor at 2,000 rpms you will likely get around 35 amps from your 50 amp alternator (40 if you are lucky). I think the alternator ratings are based on some sort of ideal conditions that just don't exist in an engine compartment on a boat. You would need about 70 additional amps from your batteries, which means you are done after 2 to 2 1/2 hours.
A genset is the way to go here. Don't size it to run at max rating all the time. You wont be happy.
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