Practically speaking, recharging time aboard a vessel at anchor, using the main engine alternator, is at a premium. If we are to meet the needs of that 102 amp hour draw, than two 8D batteries of about 225 amp hours each would provide us with a total of 450 ampere hours. If you like to figure by the seat of your pants and skip some of the number crunching, your boat's total battery capacity should be at least four times the amperage demand between periods of charging. OK, we've determined our needs, purchased and installed appropriately sized battery banks and now we must be concerned with recharging them. For our discussion we will limit ourselves to the primary source of battery recharging, the engine driven alternator.
All alternators have an amperage rating.
Unlike the controlled test conditions the manufacturer used to arrive at their figures, actual marine use could be compared to a torture test. We're sticking this alternator in a hot engine room and asking for all the amperage it can give. Most automotive alternators will melt down quickly in a hot engine room attempting to meet the demands of a high capacity, depleted deep-cycle battery bank. To avoid meltdown and system failure, certain specifications should be met by your heavy duty alternator choice. Try to find an alternator manufacturer that rates it's alternators at a full continuous load in temperatures of up to 200 degrees.