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Old 05-14-2017, 05:52 PM   #1
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Default Under The Hood vs Onan - A Question!

I was watching an advertisement for one of the big A's, and they were saying that on extremely hot days, the a/c at the dash just might not be enough - so throw on your Onan and the house a/c. This got me thinking about the Under The Hood Model - will it operate the a/c while running? I am really not knowledgeable about all these systems, but ask the Group who really have the knowledge. Ron
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Old 05-14-2017, 06:22 PM   #2
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I was watching an advertisement for one of the big A's, and they were saying that on extremely hot days, the a/c at the dash just might not be enough - so throw on your Onan and the house a/c. This got me thinking about the Under The Hood Model - will it operate the a/c while running? I am really not knowledgeable about all these systems, but ask the Group who really have the knowledge. Ron
Unlike the Onan, the underhood "generator" is actually just a high power 12 volt alternator which can't directly power the coach AC. To accomplish this, the second alternator + the coach batteries support a high power inverter (12VDC in - 120VAC out) that will power heavy load 120 Volt devices like the coach AC, a microwave, induction stove etc. If the coach batteries have an amp hour capacity that's >200ah Lithium or >400ah AGM, with the 2000 watt inverter typically employed, for short periods, the AC can run with just battery support.
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Old 05-14-2017, 06:52 PM   #3
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Second alternator will cost less, take up less space and weight.
Onan generator is more fuel efficent but noisy if not modified.
My Promaster came with a 220amp alternator which would be plenty to augment a battery pack while running an A/C occasionally. I have a 120amp battery to battery charger set up for this purpose along with an Onan generator for overkill purposes.
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Old 05-14-2017, 07:08 PM   #4
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Onan generator is more fuel efficient but noisy if not modified.
What modifications will reduce the db level of an Onan generator?
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Old 05-14-2017, 07:18 PM   #5
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Let's clarify a bit:

The underhood generator will run the a/c as long as you want it to.

However, during the time the underhood gen is on, the vehicle engine is also running. That's not an issue if you are driving, only becomes one when you are parked up/camped.

How long the a/c will run once the vehicle engine is off depends on what battery capacity you have, but is not unlimited.
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Old 05-14-2017, 07:25 PM   #6
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Quote:
Originally Posted by Ron J. Moore View Post
I was watching an advertisement for one of the big A's, and they were saying that on extremely hot days, the a/c at the dash just might not be enough - so throw on your Onan and the house a/c. This got me thinking about the Under The Hood Model - will it operate the a/c while running? I am really not knowledgeable about all these systems, but ask the Group who really have the knowledge. Ron
The answer is MAYBE!

Booster's underhood gen setup would for sure. Davydd's system likely would as well. Some of the non DIY setups might be able to handle it if the batteries are already fully charged before running the A/C.

The "death spiral" has been discussed here. It occurs when the alternator (part of the underhood gen setup) can't supply enough amps to keep the batteries charged when running large loads like the A/C. They slowly get depleted until the system would shut down.

The temperature, humidity, battery state of charge would all factor in.
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Old 05-14-2017, 07:37 PM   #7
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The answer is MAYBE!

Booster's underhood gen setup would for sure. Davydd's system likely would as well. Some of the non DIY setups might be able to handle it if the batteries are already fully charged before running the A/C.

The "death spiral" has been discussed here. It occurs when the alternator (part of the underhood gen setup) can't supply enough amps to keep the batteries charged when running large loads like the A/C. They slowly get depleted until the system would shut down.

The temperature, humidity, battery state of charge would all factor in.
I don't think there are any MAYBEs about it. With the engine running, the alternator will support the inverter and AC until the coach literally runs out of gas. There is no battery death spiral incurred unless engine operation is being controlled by Voltstart.
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Old 05-14-2017, 08:11 PM   #8
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I was thinking extreme heat, extreme humidity, 80% discharged batteries at the start, heat quickly de-rating the alternator, stuck in traffic etc. You'd need net positive amperage getting to the lithium modules to avoid a shutdown.
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Old 05-14-2017, 08:44 PM   #9
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Most of the time the AC will cycle on and off allowing the alternator to help the batteries catch up, unless you have inadiquate insulation. This would be evident if your dash AC can't keep your unit cool. I have dual pane windows and run sun screens outside and insulated panels on the inside in the cab area where they are not dual pane.
I ran my system on the alternator, along with the solar panels I didn't loose much battery capacity while I ran the AC in the desert on only the alternator and solar.

I don't normally run on the alternator this way as it isn't fuel efficient but only as a backup system if the generator craps out or if I want to top off the batteries which hasn't happened yet due to the solar panels keeping them topped off even with the fridge on 24hrs. If you allow your batteries to get down to 20%, you are not paying enough attention to them.
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Old 05-14-2017, 08:51 PM   #10
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What modifications will reduce the db level of an Onan generator?
I covered the outside box with a layer of neoprene/lead sheet/neoprene. (lead works better than MLV)
Added a second muffler
Used vibration aresting hangers for mounts instead of the hard mounts.
The undercarriage and surrounding area is covered with sound proofing foam over a ceramic coating.
The interior of my rig is multiple layers of soundproofing.
Its not easy but I was able to quiet it down significantly.
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