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Old 27-05-2008, 12:17   #1
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Alternator selection

I'm looking at a repower on my boat, and am not sure what size alternator I should select.
I have 4 AGM group 31 house batteries and a 90 Watt solar panel. The electronics on the boat include VHF, Radar + C70 chartplotter, spectra watermaker, and I might install an SSB.
Can anyone suggest what size alternator might make sense, or point me in the direction of some good information on the subject?
A 40 amp alternator is the standard option on the engine I'm looking at, and I'm wondering if that will be enough.

Thanks.
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Old 27-05-2008, 14:16   #2
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Do you have refrigeration? How you use the boat can make a big difference too. The idea is to not let the batteries fall below 50% of capacity then charge them back to 100%. This would be a perfect situation. The other part is to have enough battery capacity so you can fit within those first goals. If you add a lot of batteries you eventually make a situation where you never can get them all charged to 100% again.

A bigger alternator can put out more power but a smart regulator will cut the power output of the alternator to match the charging profile of the batteries. Batteries accept charge based on type and by the percent capacity. A low level battery can take more charge than one nearly full. That means as you get up to 80% full the larger output alternator may not be putting out as much as it could because regulation as throttled it back. The bigger alternator only uses full output when your batteries are low and your need for power is great and even then only a short period of time. Charging up a cold plate in a fridge while making water when the batteries are low would be a great time to run the engine with a bigger alternator. AGM batteries have a lower internal resistance and can accept more charge faster so could perhaps take advantage of a larger alternator longer but it's not a huge difference. The key is if the increase in alternator cost is going to come back with savings for you. This is one of those "it all depends" questions.

You need to determine how you will use power and when you will recharge the batteries and how long that will take based on the charge profile for your batteries. You have both solar input, alternator input, and shore power input. You may find a 70 amp alternator does more bang for the buck than a 150. You may also find that the 40 amp does the same job in only a slightly longer period of time. In your case the 150 amp alternator will never be able to cut your time to recharge as a function of the increase in size alone. It won't let you reduce engine time by 75% because if you added 150 amps all the time the batteries would fry quickly. Large alternators are really great when you plan on using the generated power right then. Alternators do not run for free. They convert engine power to electrical power and not at perfect efficiency.

If you look through the electrical postings in the forum you can find a large amount of details on this subject. Electrical use aboard is an interesting and important topic. DC power is not something most people understand well if at all. It does not work like the AC power in your house. A lot of details come into play and you need to step back and look at the total system to get a plan that suits your style of using electrical power use and to understand the costs and burdens required. When you understand the bigger picture you can better see where the alternator fits into the the overall plan.
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Old 27-05-2008, 21:25   #3
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Assuming you've bought good quality AGM's and plan to re-charge them when they fall to 50% charge...you should buy sonething in the 100-100amp range for bes efficiency of charging a 450a/h bank which is about what you have. I am assuming a full time at anchor basis and discharging/recharging between 50 and 80% except for an occasional 100% charge.
So...you will be charging from 225AH's remaining to 360AH's and letting your solar panel add its approx 25-35AHs a day at a slower rate. This will let you run the engine for just a bit over an hour daily at full output putting in 125amps or so and then another 25 from the panels (consrvatively). The 150ah's is more than plenty for most cruisers with refrigeration and if you don't have refrigeration you can charge every couple of days instead of daily. Be sure to get an alternator with a good 3 stage regulator to take care of those AGM's as well as a similar regulator (mppt is best) for your panel if you don't already have one.
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Old 28-05-2008, 12:20   #4
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Thanks for the information.
I forgot to mention that I do have refrigeration, I'm not sure how many ah it uses per day, but it's a small fridge.
So is it a simple formula, insofar as a 40amp alternator would put no more than 40 amp hours back into the batteries every hour that I run it?
The standard alternator on the engine is 40amp, it would seem like I ought to get a larger one.
Balmar say that a "rule-of-thumb is to size your alternator at approximately 25% to 40%" of your batter capacity. So that would appear to suggest I need 100amps at least. I do have a Balmar three-step regulator on the boat.
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Old 28-05-2008, 12:48   #5
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Quote:
I forgot to mention that I do have refrigeration, I'm not sure how many ah it uses per day, but it's a small fridge.
Odds are it use more power than anything else and maybe everything else combined over a 24 hour period.

Quote:
So is it a simple formula, insofar as a 40amp alternator would put no more than 40 amp hours back into the batteries every hour that I run it?
No, it is a formula that will put out more amps at first and as the voltage rises in the cells then tapper off to a low level charge. Getting the last 10% in is usually hard to do with an alternator since you are a sailboat and not likely to be running for long hours. There is a proper charging profile associated with each batter type. Your Balmer docs should have information on this and show how to set the profile for your AGM batteries.

I think the overview by Camaraderie is about as close as we can get for a rule of thumb because the rule depends on how you actually use power. No one is typical but the key thing is if you understand how it works you can alter your use so that it can better match your overall system.

One item worth adding is a battery monitor. It can measure amp hours in and amp hours out. It is a true gauge for batteries just like your fuel gage. Voltage level alone is not that exact. It can compute how much power you have used and can display the amp hours currently in use. It becomes the best possible tool for learning just what is going on with your batteries. It also helps you manage when to charge and you can see the readings as things happen. You don't have to be a math wizard to use the tool and provides the raw data to determine the health of the batteries as well as the size of the battery bank that would be optimal for your use. Too many batteries and you never get them recharged and not enough and you drain them too low and they die fast.

The basic 12 volt boat fridge runs about 5.5 amps when it is on and a rule of thumb is it is running 50% of the time. The would mean 66 amp hours consumed per day. An anchor light may take .5 amps. Watermaker about 5 amps. A microwave oven on high is about 100 amps (nasty things those microwave ovens).

If you go through the boat and count up the amps and when they run you see how your use pattern may be smoothed out and where most of the power is going. Your exact numbers will give better detail than rules of thumb. Not all refrigeration uses the exact same amount of power nor do they use it in the same time periods.
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Old 28-05-2008, 14:51   #6
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Thanks, I think I'm getting a clearer picture. If I understand correctly I just need an alternator to provide as much power as I use, rather than one based on the overall ah capacity of my AGM battery bank (approx 400) so it will come down to how often and long I need to run the engine to compensate for used power.
The issue is I am looking at repowering with a new Beta Marine engine, and the standard alternator size is 40 amps for the 25hp. The highest powered optional alternator for that engine is 70 amps. To get a 100 amp alternator I'd need to go with a bigger, higher HP (more expensive) engine.
So ideally i'd stick with the 25hp engine, and try to keep my power usage low enough so that I don't have to run the engine more than and hour and a half a day. It would seem, with my fairly high capacity AGM battery bank, that I could hopefully charge it up to 80-90% daily (more efficient charging) without (hopefully) discharging it below 50%.
My ignorance about the systems on the boat is due to the fact that I just bought it recently, and haven't moved it closer to home yet. I don't have the manuals handy, just receipts, and the survey.
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Old 28-05-2008, 15:10   #7
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It sounds like you have the idea based on your reply. It's the bigger picture not the details that are the problem. You now seem to have it.

On a 25 hp engine spinning a very large alternator detracts from your power at the prop as well. Going to a larger alternator than 40 may make sense but of course not if you have to upgrade to a bigger engine than you need.
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Old 28-05-2008, 15:30   #8
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Paul, watermaker, 5 amps. Really?
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Old 28-05-2008, 15:47   #9
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My watermaker's manual claims it draws 8 amps. It's a Spectra 200c. It says it makes 8 gallons an hour. If that holds true I can't see running it for more than an hour every other day on average.
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Old 28-05-2008, 15:54   #10
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[QUOTE My watermaker's manual claims it draws 8 amps. It's a Spectra 200c.][/quote]

Okay, I thought they would draw much more. Sorry Paul, didn't mean to sidetrack the thread.
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Old 28-05-2008, 17:54   #11
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First you need to check with your engine manufacturer to determine what is the maximum amperage alternator that your engine can accommodate.
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Old 28-05-2008, 19:16   #12
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To add to David M's comment, there is normally a non electrical limit to what each engine can handle. In addition to to sapping horsepower there is the torque to turn it that has to be transmitted through the belt. If the alternator is too big, the belt either has to be doubled or even tripled, to prevent the side loads on the water pump bearings from causing their destruction, (the water pump pulley is normally the idler in the drive sequence with the alternator). That means you should pay attention to the engine manufacturers recommendation.

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Old 28-05-2008, 19:37   #13
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Don't Let the Tail Wag the Dog

If I'm reading sgtPluck correctly, he has a 400ah bank, and will have significant demands on his charging system, needing to run a refrigerator and watermaker, in addition to the smaller daily draws.

sgtP, if you're ordering the engine anyway, why not specify the maximum size alternator the 25hp motor will accommodate (70amp)? At less than 20% of the bank's capacity, the 70 amp alternator certainly isn't overkill, but putting a smart regulator in charge of it will maximize its effectiveness, and in this particular application seems to be the most practical choice, since thinking beyond this requires the ordering of a larger engine.

Going from 40 to 70 = the cost difference of the alternator; going to 100 = the cost difference of the alternator plus the cost difference of a larger engine. Going up in engine size just to shorten your real-world charging cycle by a few minutes doesn't seem to be a good trade.
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Old 29-05-2008, 05:12   #14
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Quote:
In addition to to sapping horsepower there is the torque to turn it that has to be transmitted through the belt.
A 150 amp output from an engine requires almost 2.5 horespower of input to spin the alternator. That means 10% of the engine is driving the fan belt. The loads on large alternators are indeed quite high as noted. You can do the math:

Volts * Amps * 0.00134102209 = horsepower

The 70 amp would be 1.1 HP and the 40 amp would be 0.64 HP.
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Old 29-05-2008, 05:14   #15
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Quote:
Originally Posted by Pblais View Post
A 150 amp output from an engine requires almost 2.5 horespower of input to spin the alternator...
With belt slip & electrical inefficiencies it's even worse than that.
I'd expect to use more like 3.5 HP to generate 150A @ 14V nominal.
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