Completely seized alternator!

[Stu Jackson]

I think I may drain the batteries to a similar level, while safely at the dock, and try running the engine to see what happens. I'll slowly add loads as suggested.

I don't understand any confusion about this.  Your internal regulator is seeing the need for maximum alternator output, and without any alternator temperature sensing, is providing just that, coupled with a dying house bank that is always asking for maximum input.

[Sailing Amok]

Stu, the confusion is with regards to a battery bank that is only down 40ah accepting 40 amps of current from the alternator. It's still a pretty full bank. Don't batteries only accept that kind of charge when there down around 50% or 60% SOC? As I understand it, Ken had suggested that maybe there was an issue with one of the other loads. So I'm thinking I'll create a similar situation and start turning individual loads on and off. At the dock, rather than in the middle of Lake Superior, in case I end up with a burnt alternator.

[Stu Jackson]

Aaron, that's not quite how it works.  Even with a modestly depleted bank, and certainly a larger one of 400 ah, the initial load has been documented at 50-60A, sometimes however for short periods of time as the charge amperage tapers off based on battery acceptance - the fuller it gets the less it will take.  However, with an internally "challenged" damaged battery bank, that bank is always going to "asking" for as much as you can throw at it with essentially unlimited acceptance.  Regulators know nothing about alternator temperature and only work on the electrical signals from the battery sense wire which controls the field current and thus the alternator output.  If the requested input exceeds your available AO or size, the alternator is working full out.  Small case alternators are not made for continuous output beyond 50-60% of their rating.

[Sailing Amok]

Thanks Stu, I don't think this battery bank is "internally challenged" as it is performing as expected. I didn't realize that the bank would accept that much charge with so little of its total capacity used, but I guess even a small % of a large bank is still a lot of space to fill, so minimal resistance for the alternator to push against. Though, my understanding was it's only the lower 80% of a bank that will take charge that quickly. We were well above that. From your explanation, it sounds like these big banks will drive an alternator hard, even above that 80% SOC. For now I'm just going to keep a cheap spare alternator aboard, until I can afford to upgrade to something larger with external regulation. With our fridge broken, we're barely using any power anyway, and things have been fine the last couple weeks. Alternator running cool. Our solar has us back to full charge by noon every day on the hook. I've still got some investigating to do with the fridge, and suspect that may have something to do with the load on the alternator during the one experience.

[KWKloeber]

Aaron

down 40ah accepting 40 amps of current from the alternator. It's still a pretty full bank. Don't batteries only accept that kind of charge when there down around 50% or 60% SOC?

I am clueless about your bank. 
How did you determine those numbers?
What is your theoretical total AHs and what was the battery voltage during the episode?
The alternator output on a dumb regulator depends strictly on the absorbing resistance, which is also related to the battery voltage/SOC.   What batteries and brand are they?

-Ken 

[Sailing Amok]

I am clueless about your bank. 
How did you determine those numbers?
What is your theoretical total AHs and what was the battery voltage during the episode?
The alternator output on a dumb regulator depends strictly on the absorbing resistance, which is also related to the battery voltage/SOC.   What batteries and brand are they?

-Ken 

Hi Ken, the house bank is 4 6v Trojan T145 so 12v at 520ah. The "down 40ah" was based on the Xantrex Link. It's an older model link, don't recall the model off the top of my head, and I know from reading one of RC's articles on his site that ah counters aren't super precise. Honestly I didn't check voltage that morning, just looked at the ah counter. Also had a few basic loads on (radio, nav instruments) plus the malfunctioning fridge, which seems to be drawing 7.5 amps rather than the 5 or so it should be.