EcoModder Forum alternator hp draw

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 04-25-2012, 04:12 PM #1 (permalink) mikehallbackhoe   Join Date: Mar 2012 Location: weaverville, california Posts: 126 honda crx - '84 honda crx 1300 90 day: 55.83 mpg (US) Thanks: 0 Thanked 4 Times in 4 Posts alternator hp draw I found this formula to calculate how much hp is required to run your alternator. amps x volts = watts 745.7 watts = 1 hp hp x 15% = hp loss hp +hp loss = total hp used example 55 amp x 14.9v = 819.5 watts 819.5 watts / 745.7 = 1.098 hp 1.098 hp x 15% = 1.26 hp
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 04-25-2012, 04:33 PM #2 (permalink) Administrator     Join Date: Dec 2007 Location: Germantown, WI Posts: 10,542 CM400E - '81 Honda CM400E 90 day: 39.62 mpg (US) Daox's Grey Prius - '04 Toyota Prius Team Toyota 90 day: 49.53 mpg (US) Daox's Insight - '00 Honda Insight 90 day: 64.33 mpg (US) Swarthy - '14 Mitsubishi Mirage DE 90 day: 41.67 mpg (US) Thanks: 1,971 Thanked 2,108 Times in 1,288 Posts Dunno where you got that but most alternators are not 85% efficient from my reading. They're more like 50-60%. So, you should really multiply by 1.5 to get your final hp loss to the alternator. Of course, this only calculates the max hp loss to the alternator. You need to find the current draw in normal driving to get an accurate number, and of course that'll vary depending on the electrical load. __________________ Current project: Heating the manual trans with engine coolant
 04-25-2012, 04:44 PM #3 (permalink) ...beats walking...     Join Date: Jul 2009 Location: Tucson, AZ Posts: 6,116 Thanks: 175 Thanked 1,458 Times in 1,077 Posts ...also, don't forget that ELECTRICAL POWER is needed to energize the FIELD WINDING to create/control the magnetic field that's generating the output voltage...that's another "LOSS." __________________ • 2014 Toyota Prius 1.8L eCVT • 2009 Pontiac Vibe 1.8L/SFI 4A • 2004 Pontiac Vibe 1.8L/MFI 4A • 2011 Chevrolet LTZ Cruze 1.4LT 6A • 1971 Dodge Charger 318 3A • 1970½ Plymouth AAR 'Cuda 340/6BBL 4M • 1968 Dodge Charger 383 3A • 1967 Plymouth Barracuda Formula S 383 4M • 1965 Plymouth Barracuda Formula S 273 4M
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Quote:
 Originally Posted by Old Tele man ...also, don't forget that ELECTRICAL POWER is needed to energize the FIELD WINDING to create/control the magnetic field that's generating the output voltage...that's another "LOSS."
And you would want then to figure what percentage this is of the power your engine generates in order to accelerate your vehicle or hold steady state, no? If at freeways speeds you only needed say 30hp but your alt is drawing off 1.5 or more, you have maybe at least 5% going to power generation instead of vehicular mobility, no?
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 04-25-2012, 06:19 PM #5 (permalink) mikehallbackhoe   Join Date: Mar 2012 Location: weaverville, california Posts: 126 honda crx - '84 honda crx 1300 90 day: 55.83 mpg (US) Thanks: 0 Thanked 4 Times in 4 Posts if the efficiency rate is 60 % that would make maximum draw on a 55 amp alt 1.53 hp. now of course you would only be using that much if you were placing a heavy drain on you battery. under normal driving, the alternator would not be using much hp. but these numbers help us to see why 10% better mpg is possible with an alternator delete. when you figure an engine doesn't produce much hp under 2,000 rpm, lets say 15 hp in my 1300 cc crx, 1.5 hp would make that 10% very plausible.
 04-25-2012, 08:39 PM #6 (permalink) Master EcoModder   Join Date: Dec 2011 Location: Boise Idaho Posts: 842 Thanks: 39 Thanked 89 Times in 69 Posts or another way, normal driving you are drawing 15 to 20 amps for fuel pump, computer and ignition. which ain't much.
 04-25-2012, 08:40 PM #7 (permalink) Drive less save more     Join Date: Jul 2011 Location: Vancouver Island, Canada Posts: 1,189 Dusty - '98 VOLKSWAGEN Beetle TDI TEAM VW AUDI Group 90 day: 60.42 mpg (US) Thanks: 134 Thanked 159 Times in 132 Posts Good information, my car uses a 120 amp alternator so my losses are large, approximately 3.5 hp which when cruising on the hwy with the alternator cranking to recharge a low battery, my losses could be as hi as 25%. Yesterday I split my 30 watt panel in two(it was 2 panels on a hinge) and put 1 panel on my Beetle's huge dashboard. The panel will keep my battery fully charged and aid the alternator when the car is in use. I am pleased with the benefits of it. __________________ Save gas Ride a Mtn bike for errands exercise entertainment and outright fun __________________
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Quote:
 Originally Posted by ecomodded Good information, my car uses a 120 amp alternator so my losses are large, approximately 3.5 hp which when cruising on the hwy with the alternator cranking to recharge a low battery, my losses could be as hi as 25%. Yesterday I split my 30 watt panel in two(it was 2 panels on a hinge) and put 1 panel on my Beetle's huge dashboard. The panel will keep my battery fully charged and aid the alternator when the car is in use. I am pleased with the benefits of it.
lets say your starter draws 500 amps for 4 seconds when starting.

your alternator would need to produce 120 amps for 16 seconds to recharge it.

then it draws the same 10 to 15 amps every other cars takes.

 04-26-2012, 12:11 AM #9 (permalink) Drive less save more     Join Date: Jul 2011 Location: Vancouver Island, Canada Posts: 1,189 Dusty - '98 VOLKSWAGEN Beetle TDI TEAM VW AUDI Group 90 day: 60.42 mpg (US) Thanks: 134 Thanked 159 Times in 132 Posts Assuming my battery is fully charged from my solar panel when I cranked the starter you would be correct ! Which is why I have the panel, 15 watts of 14.2 volts at 1 amp is very close and maybe the same on most days as a battery tender junior, which charges at 14.2 volts & 750 ma. __________________ Save gas Ride a Mtn bike for errands exercise entertainment and outright fun __________________
 04-26-2012, 08:25 AM #10 (permalink) Master EcoModder   Join Date: Oct 2011 Location: ellington, ct Posts: 790 Thanks: 30 Thanked 89 Times in 70 Posts I think running without the alternator for shorter trips is a very good idea. Using a portable PV array to help keep it topped off is also a good idea, but, I wonder if it's worth the cost and trouble? I think it may be if you remain parked for long enough stretches to provide a substantial charging. How long that period of time is, I have no idea. Depends on the size of the array and the strength of the sunlight. I think a better idea is active management of when the alternator is used and use of deep cycle batteries. A simple way would be to toggle it on with the brake light signal. An accurate voltmeter would tell you when your charge was getting low enough to need to go to full time alternator use.