His bank of batteries is a large bank of four group 31 TPPL (thin plate pure lead) AGM batteries made by Odyssey. The bank is 400Ah in total but can also accept about 320A of charge current on the low end of acceptance, in bulk mode and over 1200A on the high end..
Contrary to popular beliefs and/or misconceptions about batteries and alternators it is the battery that "takes" or "accepts" current from the alternator. The alternator does not "force" current to the bank. The alternator only follows what the batteries want, up to it's limits. In this case the batteries, in bulk mode, or what is commonly referred to as constant current mode, will take far more than any alternator that could be fit to this engine can, or could deliver.
To be more specific an Odyssey PC 2150M 100Ah battery can accept anywhere from 0.8C or 80A of charge current, to 3.1C or 310A of charge current when charging in bulk mode. That is a HUGE amount of "accepted" charging current. Contrast that to most deep cycle wet cell batteries that take just 0.25C or 25A per 100Ah of capacity. Think about it like this, a 400Ah bank of deep cycle wet batteries will take about 100A of charging current and this bank of Odyssey batteries will take as much as 1240 amps of charge current. Big differences in the loads the alternator can and will see.
With a bank of four PC 2150M batteries, which this boat has, they can literally accept 1240A of charging current. Unfortunately there is no charging system available for a sailboat that can supply that kind of charging current. If there was, you could literally charge from 20% of capacity to 80%, or replace 60% of your banks capacity in TEN MINUTES. Wow....
Like any AGM battery these TPPL batteries can take massive amounts of charging current tapping out just about any stock alternator, and exceeding in acceptance, the output of just about any alternator that would fit onto this engine. This is why I feel it is important to use a serpentine kit and do a proper installation that can handle these batteries.
Over the years I have seen many stock alternators burned up by the high acceptance rates of AGM batteries, and Odyssey's are no different. They really need to be fed with a charging system than is designed for it and can handle them.
With the owners stock crank and water pump pulleys sized for a 3/8" v-belt he was really limited to about 80A of charge current at a maximum, in an alternator output, before the belt would begin to wear quickly.
We discussed the options and decided on an Electromaax 140A alternator and the Electromaax serpentine pulley kit was a good fit. We could have gone bigger but it would have meant a bigger HP bite out of his relatively small propulsion engine. 140A is a good fit..