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Re: Alternator advice, dealer recommendation
If that is 240AH combined, total capacity, as opposed to two 240AH batteries?
then in theory you only want to draw down 120AH maximum (50% cycle) and preferably more like 75AH (30% cycle). Split the difference, say you want to recharge about 100AH when you run the alternator. Conventional wet cells are usually supposed to be cahrged at some 20% of their capacity, which would be 48A (let's call it 50A) charge rate, so a 50A alternator or anything higher would do the recharge, in two hours.
More power could could them faster, but might also boil off more electrolyte and exceed charge rate recommendations, although there's lots of debate on that.
A bigger alternator, rated higher than 50A, would have more reserve power and run cooler, so that's a good idea, but once you exceeded 75A I'd call it simply excessive and unneeded.
So...a 60A or 70A alternator probably would work very nicely for you, right in line with your expectations.
746 watts is equal to one horsepower, so a 60A 14.4V alternator putting out about 850 watts actually puts out a little over one hp. With inefficiencies and friction, that might consume 2-3hp from your engine when it was running at full power.
Proper belt tension will help reduce some of the losses, but you'll also find folks have installed manual switches to lower alternator power consumption, i.e. to cut it back to "low" power, when or if you think you'll need the engine for propulsion.
And again--if your batteries are charged up, the alternator will simply not consume that much power, it will only be "sipping" regardless of how powerful it is.
If I may suggest, before you replace the alternator, look at properly matching alternator speed to engine speed. This is done by selecting the right pulley size for the alternator, which may mean having one custom made for $100. (A simple lathe job in a machine shop, but nothing simple is cheap anymore.) Ideally you want an alternator that puts out good power during engine idle speeds, but isn't over-revving at full cruising speeds in a storm. Alternator makers have spec sheets. It might say "60A alternator, puts out 20A @500rpm, 30A @1000 rpm, 40A @ 1200rpm, 60A @1500-3000 rpm, not for continuous duty above 3500 rpm."
So if the pulley ratios have your alternator running 1:1 with your engine speed, you'll get damned little power at 500rpm, and you'll need 1500rpm to get the rated 60A. With a 3:1 ratio you'd get full charging power at idle--but burn out the alternator at high engine speeds. You need to make sure whatever pulley you use puts you somewhere in the middle ground, so you get good power at "normal" speeds. Different alternators come with different stock pulley sizes available and different output curves, and picking the right one can literally double your alternator life--and cut your charging time in half.