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Old 07-12-2010, 08:42   #1
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Is a 100 amp Alternator Too Large ?

We have a Yanmar 3GM30. It currently has a 55 amp alternator. I would like to replace it with a 100 amp alternator.

Will this cause any overloading issues?

Thanks
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Old 07-12-2010, 09:03   #2
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What size battery bank do you have?
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Old 07-12-2010, 09:13   #3
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short answer nope. put it on.
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Old 07-12-2010, 09:16   #4
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Can you afford to lose about 5 or 6 Propulsion Horsepower to the Alternator?
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Old 07-12-2010, 09:21   #5
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Bright Eyes, short answer is not always the best answer. You want to size your charging system for maximum efficiency. Too small an alternator will take a longer time to charge, and too large of an alternator is a waste of expense and efficiency. It will depend on what type of batteries you use and what size your bank is and if all of your house batteries are combined into one bank or split up.

Determine banks acceptance rate.
Max rate bank can be recharged.
Average between 50% and 80% of full charge.
Wet cell acceptance rate 25% of total 20 hour AH rating.
3, 8Ds = 660 AH would take 165 amps.
Gel cells 40% of 20 hour AH rating.
3 Gels = 600 AH = 240 amps.

AGMs Will accept 100%
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Old 07-12-2010, 09:28   #6
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Originally Posted by Waterwayguy View Post
Bright Eyes, short answer is not always the best answer.
You also want to look at what you're asking your engine to do. You've got 24 HP pushing a 37-foot boat. That's not a lot of power to spare. Ask yourself how your boat would respond once you've only got about 20 available HP.

I'm not sure I'd want to be your engine at that point.
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Old 07-12-2010, 09:30   #7
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And,

normally a 100 amp alt. needs to have a double belt system otherwise you'll be going through belts pretty fast. Unless you have a large battery bank (e.g. 1000 Ah) it would be over kill.
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Old 07-12-2010, 09:33   #8
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normally a 100 amp alt. needs to have a double belt system otherwise you'll be going through belts pretty fast. Unless you have a large battery bank (e.g. 1000 Ah) it would be over kill.
That too, additionally Yanmar has a maximum recommended alternator so that you don't wipe out the FW pump bearings.
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Old 07-12-2010, 09:39   #9
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When installing a bigger alternator, always install a three-stage regulator that will control the amount of charging realitive to the needs of the house and starter batteries. Otherwise you'll be quick charging your batteries all of the time which will shorten their life.

Also consider installing a monitor. This device will prove invaluable for your electrical system as you can indentify the charge rates and electrical usage. This will also allow you to keep your batteries above 60% full which helps battery life and will never require the alternator to go to full charge mode.
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Old 07-12-2010, 09:45   #10
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That too, additionally Yanmar has a maximum recommended alternator so that you don't wipe out the FW pump bearings.
Yanmar sells two alternators for this engine without any additional mods required. The larger unit is 80 - 90 amps (sorry, not on the boat right now).

Also keep in mind that the actual output on any alternator is significantly less than its rated output.

Fair Winds,
Mike
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Old 07-12-2010, 09:55   #11
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I have the same engine and a 100 amp alternator (400 ah house bank) on a single pulley, other than changing the belt every 150 hours or so it's fine.
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Old 07-12-2010, 10:24   #12
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i liked my 80 amp alternator o nmy ericson-- btw-- doesnt make the boat push slower. was recommended by my yannie distributor/dealer-- works wonderfully well.
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Old 07-12-2010, 10:25   #13
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Del's right.
A single belt isn't good for more than about 80 A.
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Old 07-12-2010, 10:26   #14
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I put an over 100 amp alternator on my Perkins 4108. I did break a lot of belts and had to add a second belt which was an engineering hassel and is still not completely right. When my engine is run hard it is on the verge of overheating (Have worked on solving this for years). The alternator kicking in when the batteries are low and the engine is at full speed sometimes puts it over the top and we start overheating. The alternator definitely puts a big load on the engine, when the batteries are down. When the batteries are well charged, the alternator is a minor load. One can hear the engine lug down when the alternator goes on when the batteries are low. Installed a switch in the alternator field circuit so I could turn the alternator on & off manually. Plan to change it to a rheostat so that I can adjust the alternator load. There are lots of times when I don't want to reduce engine power by running that big alternator.
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Old 07-12-2010, 11:14   #15
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Yanmar sells two alternators (with brackets and cover) for this engine without any additional mods required. The larger unit is 80 - 90 amps (sorry, not on the boat right now).

Fair Winds,
Mike
If you need more power this would be the best solution! IMHO. You'll also have a backup system.
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