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post #1 of 6 Old 01-02-2011 Thread Starter
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Load on one single alternator

Happy new year everyone.
I need to bother you for an advice on a repeated problem I am encountering.
When we installed a new Yanmar engine in 2009 I asked for a 2nd
alternator to be installed; the mechanic bolted an "L" shaped plate to
the front of the engine and fitted a 120A alternator on it. Already
twice since installation, two of the bolts that hold the plate in place broke off leaving a stub inside the engine. This happened even after I replaced the bolts with a more appropriate tensile strength. Anyway this is my problem to sort out....

The original reasons why I installed a 2nd alternators were:

1) as a redundancy
2) not to overload the main engine alternator (80A) by charging the
full batteries bank (6 x 100Ah AGM)

I am now forced to make a "plan B" at least until I resolve the mounting problem. I will keep the 2nd alternator as a back-up, remove the belt and making work only if the main alternator fails (I still achieve objective 1).
I understand that this way it will take longer to recharge the batteries.

The question I have for you is:
Is it risky to ask the main alternator (80A) to charge all the 6 batteries?

Keep in mind that when cruising I probably run the engine every second
day and that all my loads are 12V.
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post #2 of 6 Old 01-03-2011
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Enough charge?

Enrico - I got a lot of information from a solar site on charging different types of batteries. (no affiliation)
Deep Cycle Battery FAQ
Your 120 Amp alternator should be able to handle the load with appropriate controller and maybe temperature control for the alternator and batteries.
There are so many issues to consider. I've just been poking around solar and battery manufacturer sites gathering what I can and still learning much!
With 600Ah capacity and AGM it looks like you could charge up to 150amps but I would check with your battery manufacturer to be sure. Lower charge current would just mean longer engine runs assuming you don't overheat you system components. I would personally just run the 120a alternator and use the 80a as a spare to keep things simple. A good mutli stage controller with temperature compensation is very important in my opinion as well.

s/v Lucky Duck
Catalina 36
Sandusky Ohio
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post #3 of 6 Old 01-03-2011
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There shouldn't be any danger in using the stock alternator to charge your house bank, it will just take longer. The regulator will not allow the alternator to "overload" itself, and you could go one step further and get a programmable regulator that allows you to set parameters about how much of a load will be put on it.

The mount that was built for your 120 amp alternator must not be designed properly. Vibration more than likely is breaking the mounting bolts. I would consult someone else about having a different mount made so that you can continue to use the second alternator, there is no reason why you should have to remove it. I know I have seen commercially made after market mounts for Yanmars, I'm sure a search will come up with something that might work for you.

SV Laurie Anne

1988 Brewer 40 Pilothouse

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post #4 of 6 Old 01-03-2011 Thread Starter
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Thank you Baker and John from your quick reply.
I agree that fixing the bracket problem is the utlimate solution but it's good to know that in the mean time the main alternator is sufficient.
Best regards,
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post #5 of 6 Old 01-04-2011
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Enrico, the only real danger to your alternator is heat. Regardless of the regulator, if it is called on to put out full power for long durations it may overheat and fail. You can prevent that by using an external regulator that is programmable (to less than full load) and has an optional temperature sensor (to monitor alternator temp) and by making sure there is adequate ventilation, i.e. dual fans on the alternator or even a blower or duct aimed at the alternator to ensure it gets good airflow.
An inexpensive infrared "non-contact" thermometer, $20-30, is a great way to actually check the temperature of the alternator and verify that it is working cool enough.

If your main alternator is internally regulated, actually an automobile design, then it probably cuts back very quickly, so it will protect itself but...auto designs often have just one fan and are intended to have gobs of cold air coming through the engine compartment. They also charge deep cycle batteries VERY slowly.
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post #6 of 6 Old 01-11-2011
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The genious that thought of this did not use a 80/120 amp alt that are physically the same?It takes a little online work but they are.If you want do it forward as far as keeping dollars go buy 2 70's era 110 alts and make a dual bracket for them.Do this yourself and learn to cut and weld.Price for the non-marine alts $50x2.........Price for doing it yourself and learning?
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