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Old 07-08-2019, 06:13 PM   #1
Mini-Skoolie
 
Join Date: Jan 2017
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Running air con from alternator - math?

Hi guys,

Running the numbers on if it is possible to run my roof mounted air conditioner from my alternator while driving.

The Math Bits…..


Bus has a 185 amp alternator. So, theoretically, alternator is putting out 185 amps x 12 volts = 2200 watts.

A two thousand watt inverter has a low-ball 85% efficiency (Aims inverters advertise they are 95% efficient).

So, the output from my inverter hooked up to the alternator could be around 1,887 watts (2,200 watts x 85% efficiency).

The distance between the alternator and the inverter is (a not ideal) 25 ft – (inverter is under the bed at the rear, engine at the front).

Using 2/0 gauge wire, I can expect a loss of between 3 and 10 % over that 25 ft distance (depending on which wire guide I use) – So, a drop to between 1,698 and 1,830 watts out of the inverter.

My Attwood Air Command air conditioner advertises 1,530 watts running – I’m looking to install a soft start to overcome the advertised 4,830 watts startup).


So, based on my math, my alternator should be able to run my air conditioner, right?

Anything I am missing?

Would I be better off running it direct or through my battery bank first?

Anything I could do better (aside from the ‘run a genny while driving’ solution) ?


Cheers

Douggy
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Old 07-08-2019, 06:31 PM   #2
Bus Nut
 
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I have thought of this, waiting for someone else to try it. It has been said it will not likely be enough cooling while moving.
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Old 07-08-2019, 07:12 PM   #3
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I suspect you quoted peak alternator output. They don't typically operate at the firewall; basically they're running to power operational needs and charge the batts, so excessive demands on it are likely to burn that alternator right out.
Beefier ones may be had for a cost. Have you hit the PowerBall recently..?
Christopher (AKA Cadillac Kid), our rezident mechanical, electrical, and A/C guru, will undoubtedly amble by presently to present you with the correct answer!
The soft-start capacitor IS a must-have!
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Old 07-08-2019, 11:31 PM   #4
Mini-Skoolie
 
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Thanks HazMatt,

I didn’t think about the draw on the alternator being too much for it. I understood that there may be a lower output at idle, but not much below the rating of the alternator.

I suspect I need to dig a bit more into the alternator output at different rpms. Looking at larger alternators, the price sure does get up there once you start looking at 250 amp+.
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Old 07-08-2019, 11:56 PM   #5
Bus Nut
 
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I installed a second alternator, running parallel 2/0 cables from the new alternator to the house batteries. The new alternator is rated at 350 amps. The chassis alternator is rated at 160 amps. I have a battery isolator to allow the house and chassis battery banks to be tied together, controlled by a switch. Both banks are flooded lead acid batteries. We have a Coleman Roughneck Mach III air conditioner. It pulls about 9 amps (A.C.) when on HIGH. This translates to about 110 amps at the batteries. When at idle, there is enough power to run the air conditioner.


When sitting in the driveway, using shore power ... the EMPTY 33 feet of cab area can get to reasonable temperatures (80-85 degrees) in a 95 degree day in the Texas sun. When driving down the road, the engine pushes a lot of heat through the firewall and the bus can maintain about 89 degrees in that same full-sun 95 degree situation.


The end result is that the Mach III (13,500 BTU cooler) can make the bus bearable. The addition of a few fans makes it a bit more comfortable. If you want REAL comfort, a second air conditioner may be needed.


Note: The bus has three coats of white elastomeric paint on the roof, with factory insulation and only the metal floor in the last 20 feet of space. The first 13 feet have an insulated floor installed. The rest of the body is flat black. Yes black.


Take all this with a grain of salt and a glallon of Tequilla! Your mileage may vary.
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Old 07-09-2019, 06:53 AM   #6
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ive done it.. even with the dashboard air in my carpenter bus, before I fixed the engine heat into the cabin issues.. i would run my 12000 BTU inverter portable A/C (2 hose with the hoses out a bus window and sealed around).. I have 2 AGM house batteries. , a 1500 watt inverter (sine wave go power) and a 200 amp alternator..



going down the highway my voltage at the inverter was still 12.9 which keeps the batteries up.. at idle I was discharging the batteries... at mid RPM.. around 1400 on the bus tach I was back to 12.9 at the inverter..



my alternator is a Bosch SB200 and is designed to output its maxx at a relatively low RPM (remember the spec sheets on alternators list it in aslternator RPM which is typically going to spin much faster than the engine)..



my A/C on high pulls around 1000 watts.. my dash air pulls 35 amps at the condensor and 15 amps on evaporator-1 and 18 amps on evaporator-2. so i was pretty much maxxed out.. fortunately I had no need for the electric A/C at night so I never ran this way with the lights on.. i suspect i wouldve not made even then..





as for A/C, the best way to cool a moving bus is with an engine driven air-conditioner... theres plenty of threads on that..
-Christopher
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Old 07-09-2019, 08:05 AM   #7
Mini-Skoolie
 
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Great info and thanks for asking the question because I'm sure it will come up for me eventually also. I may like to point out also that all these added alternators and AC compressors do have a HP penalty as well so just keep that in mind if you plan to climb any hills while keeping the interior a cool 68. I am sure there's a more accurate measurement available but what I've heard repeatedly so that its stuck in my head is ~50hp per alternator or compressor and keeping in mind that those few units I've seen with factory air had 2 compressors each running an AC unit so that's at least 100hp needed just for comfort systems and some of buses barely break the 200hp rating as it is. Hopefully a factory AC bus would be upgraded but since here we're usually dealing with an aftermarket solution its an important consideration whether that 5.9L is up to that task of pushing the bus AND keeping it cool simultaneously.
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Old 07-09-2019, 08:18 AM   #8
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Has anyone considered taking a generator apart and powering it's drive gear with a pulley or pto from the engine while driving? Not sure where you'd mount it down there. I know some rigs have more room than others in the engine bay.
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Old 07-09-2019, 09:40 AM   #9
Mini-Skoolie
 
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Actually I have been half-considering just getting the reefer unit off a box truck. It's a small diesel engine and cooler unit all in one so no demand on the drive engine at all. In addition, it shouldn't be too hard to mate it with an alternator for added charging capacity of house batteries. Of course they're usually crazy loud.
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Old 07-09-2019, 10:30 AM   #10
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Reefers have onboard alternators.. truckers have used the Reefers to charge their dead tractor batteries to get started at times..



I forget what the capacity of a reefer is but its high... however they are generally deisgned to run at lower temperatures than room temp..



as for Horsepower you are definitely not pulling 50 horsepower per A/C compressor or alternator.. an enhanced sanden SD7 compressor pulls somewhere around 12 horsepower if its pumping at its maximum capacity and load.. which chances are its not... so 2 of those running would be a max of 24 horsepower.. again thats Maximum.. I dont have any charts on alternators handy...

-Christopher
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