Ok, here's some more info as requested:
I do not know the size of the alternator. I don't believe it is very large, and it is certainly not one of the high-output alternators a lot of people have as aftermarket equipment. It dates to at least the last time the engine was rebuilt, if not earlier, which was circa 2003. The engine is a Perkins 4-108.
The alternator is wired to the combined terminal on the battery combiner switch. The wire is probably 0 gauge. The batteries consist of two 110Ah Group 27 wet cell batteries (IIRC). Each battery is wired separately to the battery combiner, but I always run them with the combiner set to "both", so they're treated as one bank.
I do not know the depth of discharge in terms of Ah since I do not have a battery monitor (it's on the list, but I currently need to spend money on more pressing repairs/upgrades). The battery voltage was about 12.4V the first time I began charging, and over the course of the next couple of days, it ended up around 11.5V, despite my efforts at charging. I do have an ammeter and it showed nearly no draw except for when the fridge was cycling, which was 9A for only a short duty cycle (other things on were GPS and wind instruments, together they don't even draw enough to register on the ammeter). As an example, one time the battery voltage was about 12.2V with no load and about 12.1V when the fridge was running, then when I started the engine it went to about 12.3V with no load and 12.1V with the fridge running. Over the next hour or so it climbed up to about 12.5V with no load but would still drop to 12.3V or 12.4V when the fridge kicked on. (the fridge runs frequently with very short on-time. this is not a marine fridge, it's a normal 120V minifridge running on an inverter. yes, I'm measuring it's current draw on the DC side of the inverter. getting a proper marine fridge is one of those higher priority items I need to spend my money on rather than a battery monitor).
When I got home, I plugged in the shore power, and my 40A charger was able to get the voltage from ~11.5V up to ~13V instantly. It stayed in bulk charge mode for 3-4 hours. I don't know how long it was in absorption mode, but it was full by the time I woke up the next morning, so less than 8 hours. I'd expect my alternator to put out at least 40A, so I'd expect it to be able to raise the voltage similar to the shore power charger.
I'll do some more testing of the connections and wiring with a DMM this weekend and I'll report back. From what you guys are saying, I strongly suspect the regulator is shot. The regulator is internal to the alternator, btw. I've never heard of a failure mode where it would output just a little bit of power, I always thought they were all or nothing kinds of devices. Thank you for your help!
1979 Gulfstar 37 Laissez Faire
Last edited by rmeador; 09-09-2011 at 10:56 AM.