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Old 23-11-2018, 15:03   #1
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Alternator Problem

I have twin Cats, 3208 TA, 425 HP. When I bought a year ago, it had the stock, original alternators, 55-60 amps.

I put in a new house bank, 1350 amp hours, and talked to my mechanic about replacing the alternators with a higher output.alternator.

We put in 2 new 90 amp alternators, and all seemed good. Now, after 10 months, 1 alternator went bad. The shop said one of the rotors broke apart. They could not give me a reason.

So my question is: The stock alternators lasted 20+ years, the replacement, 1 bad after 10 months. The only difference is that they are now charging a 1350 bank, instead of the original 800 Ah bank. Is this just an unforeseeable event, as the alternator shop said, or some other potential cause. Overheating, etc?

Your thoughts are appreciated.

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Old 23-11-2018, 15:17   #2
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Re: Alternator Problem

Are they externally regulated with a temperature sensor on the alternator? If not, take a look at the Balmar regulators.

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Old 23-11-2018, 15:20   #3
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Re: Alternator Problem

"one of the rotors broke apart" - can you describe better or do you have photos? There are a few things alternator rotors do not like, one of them is overspinning which can happen if the pulley on the alternator is to small and thus the rpm of the alternator becomes to high when the engine is running at high rpm. Another one is the nut of the pulley not properly torqued which can cause the shaft to break.
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Old 23-11-2018, 15:40   #4
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Re: Alternator Problem

It doesn't matter how big your house bank is. Really. It could abe a zillion amp hours.

What does matter is how low the bank is when you start charging with the alternator(s).

Given ANY daily energy use, a larger bank simply means one of two things:

1. You can go longer without recharging (i.e., more days at anchor without starting the engine)

2. You do not change your recharging habits (i.e., bigger bank but still charge every day or every other day as you used to do)

In Case 1, you are charging against a more depleted (SOC) and the regulator will require the alternator to put out more amps.

In Case 2, you are charging against a higher SOC and the regulator will not require the alternator to put out as many amps.

You can do the math to figure out, based on your daily use, what the difference between an 800 ah vs. 1350 ah bank will do (1350-800=550 / 2 = 275 usable). Not a tremendous difference, if you daily load is 200 it gives you one mroe night on the hook.

Enlarging your bank as you did should not make much difference in alternator output demand, regardless of which case you used in your operations.

Assuming you installed the alternators' regulators properly as mentioned, my guess is you just got a bad alternator. It happens.

Good luck.
Stu Jackson
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Cowichan Bay, BC, (Maple Bay Marina) SR/FK, M25, Rocna 10 (22#) (NZ model)
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Old 24-11-2018, 09:57   #5
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Re: Alternator Problem

Are they standard internal reg?

Get them modified for external reg and get a pair of balmar mc 614 regulators with alt temp sensors.

Your batteries and Alts will thank you.
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