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Old 18-09-2020, 09:39   #1
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Upgrade Alternator?

Doing some routine maintenance on my Tayana 48 CC, I noticed a slight crack in my Hitachi 12V-55A alternator. Was about to have it welded when a buddy suggested I also have it serviced and upgraded to higher capacity. I had already been thinking about getting a spare to have onboard. Am now thinking about purchasing a second to use as the primary and keep the original as a backup (once repaired). Any suggestions or tips on what I might want to upgrade to? Are there any configuration or other electrical issues to consider or do I just bolt the new one on and go (providing it mounts correctly and such)?
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Old 18-09-2020, 10:05   #2
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Re: Upgrade Alternator?

Should be bolt in ready. If I were spending the money is upgrade to an externally regulated unit and derate to what you need for output.

I have a 100. Would like to upgrade to a 160 and derate down to 120 or so for longer life
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Old 18-09-2020, 10:24   #3
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Re: Upgrade Alternator?

Here's a good option for a 100 amp alternator. As you can read in the description, MaineSail custom builds these for heavy duty boat use. You will need a Balmar external regulator. This combination will Balmar regulator will give you a much faster charge and will likely extend your battery life by charging them properly.

For safest and best charging get both the battery and alternator temp sensor options.

Be sure to use the belt regulator setting in the Balmar regulator to take some of the load off the belt. 100amp will wear a 1/2 belt quite quickly. Surprisingly, using the belt regulator to derate a larger alternator has very little effect on charging rate. Here's a long but excellent discussion of all things "regulator"
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Old 18-09-2020, 10:29   #4
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Re: Upgrade Alternator?

You will have to check in the engine manual what the maximum PTO is for your engine (Power Take Off). The crankshaft & waterpump bearings are the limiting factor.
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Old 18-09-2020, 11:04   #5
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Re: Upgrade Alternator?

The decision on upgrading alternator all depends if you plan to need and be able to utilize it to charge batteries. Do you need a bigger alternator? Will your batteries accept the higher charge rate? Do you normally run the engine just to charge the batteries now?

I have a 100 amp alternator now. If i changed to a 55 amp it wouldn't really matter. For me it would work the same going the other direction.
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Old 18-09-2020, 12:11   #6
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Re: Upgrade Alternator?

Originally Posted by sailorboy1 View Post
The decision on upgrading alternator all depends if you plan to need and be able to utilize it to charge batteries.
Exactly. But I should add that it gives you the OPTION to be able to charge the batteries much more quickly, even if you don't use it often. In other words, reduce your motoring time. There are other scenarios, like you've been at anchor a long time and your generator or solar or whatever wasn't able to recharge, so you had to run the main. Again, minimizing that run time, as well as putting some extra load on the main, is probably good.

But my real reason for posting is to make sure your wiring is up to the task of carrying the extra current. There are cable size calculators on line, plus make sure the connections are good all the way back to the battery bank. I didn't do this and had a bit of a smoke show recently. All is good now, but I should never have assumed the existing wiring was adequate. Long story, lesson learned.
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Old 18-09-2020, 12:44   #7
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Re: Upgrade Alternator?

Originally Posted by Espiritu Buho View Post
Doing some routine maintenance on my Tayana 48 CC, I noticed a slight crack in my Hitachi 12V-55A alternator. ...............................

Don't know if the Maine Sail links include this.

Hitachi/Yanmar Alternators: (by Maine Sail)

Some alternators though, such as those made by Hitachi and found on Yanmar diesels, are dumber than a pound of beetle poop. Actually, to the alternator, they are pretty smart but to your batteries and the speed of charging they are flat out stupid. Why?

Hitachi alts with dumb regulators, and some others, limit voltage but also reduce voltage based on alternator temperature. This is a self protective feature installed in the internal dumb regulator to prevent the alternator from cooking itself. Remember voltage is the pressure that allows more current to flow. So, if we reduce the absorption voltage, then we also reduce the current the alternator is supplying.. Any battery at any state of charge simply will not accept the same current at 13.4V that it did at 14.4V and as a result the alternator will run cooler. What do you suppose this does to your batteries over time.......?

The problem is that when cold you will get 14.3V to 14.4V out of the Hitachi but as the alternator heats up the dumb regulator begins to reduce the CV/voltage limit based on the alternators internal temperature. It is not uncommon to find a Hitachi alternator at 13.4V when hot. This is REALLY, REALLY DUMB....

If you have a dumb regulator, and notice the voltage dropping, it is likely a temp compensated dumb regulator. Get rid of it or plan to buy new batteries more often.

If you have a temp compensated alternator or a Hitachi alternator on a Yanmar you really are in dire need of external regulation if deep cycling a larger battery bank.

This is from:

and these, too:
Stu Jackson
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