With my 100 amp Balmar with smart regulator I can return 25 amps to the battery bank in about an hour of motoring, or less, if the bank is depleted to 60 percent or so. (batteries accept charge much faster when discharged; a battery bank at 95 percent capacity would take hours to achieve 100 percent from the same regulator.)
For that reason, on say a week-long cruise, a yacht's batteries are never recharged to 100 percent. The sweet spot is to maintain somewhere around 50-80 percent charged. Then, when the engine is turned on, the alternator output is initially maybe 80 amps. You watch on the battery monitor as the output drops as the bank recharges. You watch as the charge percentage rises from, say, 50 to 75. then turn off the engine.
A rule of thumb makes such recharging easy: when the alternator output drops to 15 amps, stop charging. From then on the charging rate becomes slow and you're just wasting diesel fuel.
The justification of a more powerful alternator with external regulator is speed of charging of discharged batteries. The stock 55-amp Motorola does fine, it just takes longer. Its internal regulator cannot provide variable charge rate.
A 100-amp alternator does place additional strain on the alternator belt. But a factory single-belt system is fine, if it is aligned and the belt is new.
Such an alternator uses one horsepower for each 25 amps of output. If the bank is low, it can really lug the engine at startup.
For this reason, some fit a cut-off switch so you can start the engine and engage the alternator later, after warm-up. Or disengage it for full engine power if fighting a current or such.
A Balmar 100-amp alternator can also be detuned to, say, 80 amps. This is done if full power puts too much strain is placed on the belts or the engine.
For cruising, a powerful alternator combined with an external regulator and a Victron battery monitor means no worries about energy management.
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