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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #1  
Old 08-30-2008
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Battery charging

OK folks, It's been a long time since my college physics, so how about a little help.

How many horse power does it take to generate 100 amps of 12 volt current?

Can a 120 Amp alternator deliver 100 amps over an extended period of time. I mean like all day?

Thanks in advance.
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Old 08-30-2008
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1200 watts= 1.6 HP but with some conversion loss call it 2HP.
Dunno about all day output. Would definitely need excellent ventilation and I would think that it will wear out quickly. Maybe someone else will know. What are you thinking about doing?
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Old 08-30-2008
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1 hp ~ 750 watts, and a 12 volt system is actually 14.4 volts on the alternator, so a 100A system woulod be 1440 watts, just under 2 hp. Figure closer to 2.5hp with system losses and friction.

A 120 amp alternator will not be rated 120A continuous duty, you need to find the continous duty rating and IF it is adequatley cooled (dual fans, good airflow, no excess paint, good air temperature) it might be able to run at 100A for extended periods. Better to ask the manufacturer for their numbers, or to buy one that has been rebuilt with a heavier diode pack and heatsink to take the continous load.

Somewhere around 100A you also push the limits of what one v-belt can handle, and either you need twin belts (problematic to keep the load balanced) or a ribbed belt, and to think about the offset load on the engine bearings.
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Old 08-31-2008
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A way to solve the belt problem is to use a cog belt.

A cog (timming) belt doesn't require tension to carry the load.

You can get matched v-belts from Gates.

You might look to the aircraft or industrial welding industries for you altenator and use 24 or more volts.

Rick
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Old 08-31-2008
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HS...I don't understand your calculations...pls. elaborate. To convert from watts to horsepower, multiply by 0.00134. This gives approx 1.6 HP required.
Allowing for losses I rounded up to 2HP. In my mind it does not matter what voltage the alternator produces since the idea is to deliver 1200 watts to the batteries as the takeaway from the batteries will be at 1200 watts from whatever is going to do the draw. The alternator is supplying power rather than simply amps or volts and the battery is really storing watt/hrs rather than amp hours since a 12V 100ah battery is 1200 watt/hr battery.
A 120amp alternator at 14.4V supplies 1728 watts so it actually delivers much more than required by the load. i do agree that the continuous rating should be checked out with the mfr. but I am more interested in your math.
Where am I going wrong on this view?
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One hp is generally converted to about 750 watts. (746 actually). Using 0.00134 may be more precise but that's also a lot more numbers to carry in your head.
The voltage does matter--big time--because you are dealing with watts (actually volt-amps not just watts, and there's more loss in the conversion) so anytime either the voltage or amperage changes, so will the total wattage. An alternator power system is generally set at 14.4 V not 12V when the system is nominally rated at 12V. A nominal "12V" battery is 12.6 volts or 12.7 volts depending on the chemistry when it is charged, and the charging system will be running at 13.8-14.4 volts in order to overcome charging losses, so if your alternator is running because you've decided to charge the battery and apply power other systems, it will probably be running at 14.4 volts. Anything less is a bonus, you can't presume it will have cut back to a lower voltage level "yet".

From 12V to 14.4V there's a difference of some 20%, so running "the same" 100A output at 14.4V will produce some 20% more wattage, and in turn consume some 20% more horsepower form the engine.

120A @ 14.4V is indeed 1728VA, not 1728 watts. I've no idea what the conversion factor is for an alternator--it can vary widely. But "easy math" says at 750 watts per hp, 750 x 2 = 1500 watts for two hp, remainder 228 watts, that's another one-third hp, roughly.

2.3"ish"HP at full output, ignoring some decimal points AND more important, ignoring the power factor in converting from VoltAmps to Watts. (That can range 40-90% in various applications, it is not ignorable.) And again, ignoring friction losses, which will be real with something that big belted up tightly. I'd just round that up to 2.5, if I couldn't spare 2.5hp to run it--I wouldn't worry about whether it was really going to be 2.2 or 2.3 or maybe 2.7 if the power factor and friction losses were higher than expected. (Unless the engine "as installed" has been on a dyno, do we really have such a precise faith in the hp ratings for the engine anyway?[g])


Rick, if you go to cogged belts, don't you need to get "more stuff" in order to add a tension adjuster since you can't just swing the alternator anymore, and the layout dimensions become more critical in order to get the cogs and new cogged pulleys to all fit?

Gates may supply matched belts, but in point of practice, it is still harder to match and maintain two belts than one. I'd rather use one ribbed belt. (And even plain v-belts are getting damned hard to find!)

Last edited by hellosailor; 08-31-2008 at 11:15 AM.
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Old 08-31-2008
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Here's what Balmar has to say on both of these subjects:

"What horsepower load will I put on my engine with a new alternator?

Typically, when an alternator is working at full output, it will require approximately one horsepower for every 25 amps it produces. As such, a 100-amp high-output alternator will re up to four horsepower to operate.


Does belt choice affect alternator performance?
Certainly, belt quality will have a dramatic affect on both alternator performance and the life of the belt itself. We find that high quality belts, such as the Top Cog belt by Dayco or the Green Stripe belt by Gates, will provide the best performance and longest life possible.
Keep in mind, the width of the belt limits that belt's horsepower capacity. As a result, any belt -- no matter what quality it might be -- will fail before its time when the alternator load exceeds the belt's capacity. Once again, if the belt is narrower than 7/16", the maximum amperage load we can safely carry is 80 amps. If the belt is 1/2" wide, 110 amps is our upper limit. Any alternator in excess of 110-amp output will require dual belts, or multi-groove flat belts."



P.S. What on earth are you running on a sailboat that would require that type of load off your alternator for "extended periods" of time or "all day"???
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Last edited by Maine Sail; 08-31-2008 at 07:47 PM.
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HS & Halekai...thanks for the additional detail...some loss! Good info on the belts...I've had good luck with gates myself.
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