Originally Posted by hellosailor
Then there's the idea of running the engine at low speed to charge the batteries. WRONG! Let's say the engine has an idle speed around 800 rpm and a cruising speed around 2600 rpm. If the alternator was properly matched up (pulley size & output curve) then the alternator will not be putting out signficant power at low rpms. At 1000 rpm it may be putting out 20-25 of the rated power, at 2000 spm it may be closer to 85% of the rated power. (Numbers drawn from a hat but good enough for examples.)
You need to know how the system has been designed, or have an ammeter in circuit to see what the alternator is putting out, because they are usually designed to put out something near rated power at something near typical cruising rpm, and all you are doing at lower rpm is wasting fuel.
Part of this is cheap and sloppy design (who notices?) and part of this is money, because an alternator that can put out full power at low rpms costs more. And an alternator that can put out full power at low rpms while also not burning out at sustained high rpms (i.e. trying to claw off a lee shore for eight hours) is more expensive again.
I had done that in my new set up and the performance of the new alternator and new setup was measured with different RPM. I have not here the list but I found out that probably the most efficient relation between charging and fuel consumption was between 1350 and 1500RPM and that about 2000/2150RPM the max charge was obtained. The cruising speed on my boat is 2400/2600RPM and the engine max RPM are 3600. Of course all systems are different but this can give a general idea. The alternator has 120A and is not a particularly good one but all the rest is maximized, I mean I have a good regulator and a bank charge distributor.