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Old 29-04-2010, 14:54   #1
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Alternator Limitation

Hi everyone,

I have a question about my preferred setup since I am not the most tech when it comes to Alternators :

I have a perkins 4.236 with a 100 Amp Alt and Balmar regulator.
6 8D gel batteries 180 ah each ( so I have a 1080 Ah bank??) based on what I read I would need a 300+ Alt

I don't want to install a larger Alt on a small diesel but want to add 400W of solar panels and I am going down to Mexico next year.

From your experience, would the 400W solar compensate for the Alt?


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Old 29-04-2010, 16:09   #2
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I wouldn't call the Perkins 4.236 a "small diesel engine". That's a pretty heavy duty lump of iron, in my book. That engine will be happy driving about the biggest alternator you can find, I believe. Diesels like loads.

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Old 29-04-2010, 16:27   #3
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The size of your alternator is dependent on the size of your batteries (you don't want to charge flooded cell bateries at more than 25% of their amphour capacity) and how long you want to run your engine to recharge. With AGMs you don't need to worry about the charging limitation.

You have a huge battery bank.

If you are going to be living aboard "on the hook" I would do the following and why:

Living aboard will use 100-150 amp hours per day depending on how efficient your refrigeration is.

Upgrade your alternator to a 200 amp, double belt, large frame alternator. This will let you add back what you use in a day in less than an hour of engine running.

If you have room to install 400 watts of solar panels, do so. These will produce about 1/3 of their wattage in amphours each sunny day or about 130 amp hours. This will keep you in energy balance if the sun stays out.

When it is cloudy for a few days, run your engine and let the 200 amp alternator recharge your battery bank.

If you want to save money and don't mind running your engine longer, just install the solar panels.

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Old 29-04-2010, 17:47   #4

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Between our 400 watts of solar and a wind generator we could do without an alternator. Fridge runs 24-7 I sew onboard during day listening to radio and watch movies on dvd ot tv at night. After 4 days of continuous sail am still hitting 14 volts daily around 3 o'clock. Were running 8 , 6v golf cart batteries for house. Don't make water.
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Old 29-04-2010, 19:21   #5
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I'm trying to figure this out. If you're running a Perkins 4.236--that's 100 HP?-- you're probably in the 48-50' range? And I'm guessing, from the way the question is framed, that you haven't got a generator on board. So what are you doing with such a HUGE battery bank? Is this a former lightship? Surface support vessel for deep-submersible ROVs? Running an 8-person spa off the inverter?

Seriously, rather than asking how to support 8 4Ds, a better question might be why keep such a huge bank on the boat. You've got more than 1,000 lbs of battery there. Even on a boat your size, that's a lot of weight.

Do yourself a favor and figure out how much it's going to cost to replace that bank on a regular basis.

I have a 420 Ah bank that I'm able to keep charged with 260 watts of solar backed up by 200 watts of wind. During the summer months or at lower latitutes, I really don't need the wind power unless I'm doing an all-night sail. I run a separate refrigerator and freezer, and have no problem meeting all my power requirements. Admittedly, my energy budget may be lower than yours, but I think the place to start is by working out your energy budget rather than asking how to keep an oversize battery bank charged.
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Old 29-04-2010, 22:52   #6
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Thanks for all the feedback. The Perkins is 85HP the boat is 54'.
The current setup (was on the boat when we got her) is an 11KW generator, 110V fridge, 110V water maker, 3 AC heat pumps... well you get the picture.

Everything is coming off including the generator.

The new setup will have a coolblue fridge, 12V watermaker and 4 100W solar panels.

we will be living on the anchor in Mexico for a year or two and go from there. I feel with the cost of converting the engine to double pully system and the cost of a 200 amp Alt I better put 2 more panels since I have the space.

I am looking to hear from prople that live on the anchor if the solar can take care of the electrical system.
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Old 30-04-2010, 01:48   #7
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I have 250 watts of solar and 480Ah of cheapo truck batteries.

I can run a fridge 24 / 7 in the tropics and have never had to worry about power.

I should mention that all my house lights and cockpit lights are LED.
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Old 30-04-2010, 08:09   #8
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more than adequate

Originally Posted by wingover View Post
The new setup will have a coolblue fridge, 12V watermaker and 4 100W solar panels..
400 watts of solar is plenty. Adding a larger alternator would be pure overkill at that point. You're going to have excess power as it is.
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Old 30-04-2010, 09:32   #9

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Look into an "MPPT" controller for the solar panels, it will buy you about 10% more efficient charging from them, i.e. effectively making your 400W panels into 440W. (More precisely, keeping them at 400 rather than wasting 40W of that as a conventional controller would.)

And if yo have the room for it, I would suggest installing TWO offset 100A alternators, each on a single belt, instead of one alternator with dual belts. Dual belts are a continual maintenance nightmare, the tension always changes and one winds up carrying the load--then bursting, and you need to change and adjust belts again. If you can install opposed alternators (preferably with a pillow block) then there's less offset load on the engine and bearings, you have a fully redundant system if one fails, and you don't have to monkey around with dual belts.

Just with integrating the regulators.<G> Or chosing one for that setup.
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Old 30-04-2010, 09:56   #10
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Actually your battery bank is adequate, not overly large, for a serious cruising boat, so if it is already there you have a good start. But much of your question can only be answered knowing what your usage is going to be on a daily basis and and then plan your system to be able to replenish what you will use each day. Calculate a most use situation, add another 10 to 20% and then move forward from there. Solar panels don't charge at night, on very overcast days and their charge is greatly reduced by clouds, shading from the rigging, etc. So you need to come up with an average per day charge that they will put back into the batteries. Alternative charging for days on end when your panels might not be putting out much at all needs to be available. 100 amp alternator is on the low side. You will at best need an external regulator to allow the alternator to put out the maximum charge for the longest period of time. Don't look at this from an add solar panels approach, look at this from a total system approach and make your decisions based on all of this information. Good luck

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