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Old 05-14-2017, 06:52 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, 07:22 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, 07:52 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, 08:08 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, 08:18 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, 08:25 PM   #6
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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, 08:37 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, 09:11 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, 09:44 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, 09:51 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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Old 05-14-2017, 09:53 PM   #11
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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.
Sure, if you are starting the sequence with 80% discharged lithiums the equation changes but even with this worst case scenario, unless Voltstsrt is involved, battery shutdown is unlikely. The 280 amp Nations alternator will still deliver 150 amps under hot conditions at idle which will support the AC and at least keep the batteries treading water. Interestingly, in this unique scenario you describe, the clumsy Roadtrek multiple BMS arrangement has an advantage over the single BMS Silverleaf system because with an Etrek you can mitigate the charging demand by shutting down to a single lithium battery.

However, if Voltstart has been implemented, it doesn't matter how many amp hours your battery(s) has available. At some point, they will be deader than a door nail. IMO, if you want to run the AC off the grid, it makes more sense to just fire up the engine and let it run, effectively delivering the batteries from discharge.
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Old 05-14-2017, 10:10 PM   #12
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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.
Man, that's a lotta work! How many visits to the chiropractor did this project involve

I do wonder about one thing. There are third party providers of generator exhaust extensions that route the exhaust up past the top of the coach. They make a point of claiming that their pipe dimensions produce no significant increase in back pressure that could detrimentally affect the valves. A muffler by definition creates back pressure and I wonder if an additional muffler mightl have long term effects on the valves.
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Old 05-14-2017, 10:57 PM   #13
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After hearing all the complaints about noise generators make inside the Travato,the vibration and the limited clearance, I put allot of effort in researching ways to engineer a better tighter installation, reduced the noise and vibration. I installed a resonator, not a muffler, its straight through so no back pressure.
Those extensions are a great DIY project as most of it is PVC drain pipe with an air gap so no back pressure. Do a search and there is a lot of DIY innovation to choose from. You might be able to make your own that will pack better for the limited storage class b 's have.
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Old 05-15-2017, 02:11 AM   #14
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Sure, if you are starting the sequence with 80% discharged lithiums the equation changes but even with this worst case scenario, unless Voltstsrt is involved, battery shutdown is unlikely. The 280 amp Nations alternator will still deliver 150 amps under hot conditions at idle which will support the AC and at least keep the batteries treading water. Interestingly, in this unique scenario you describe, the clumsy Roadtrek multiple BMS arrangement has an advantage over the single BMS Silverleaf system because with an Etrek you can mitigate the charging demand by shutting down to a single lithium battery.

However, if Voltstart has been implemented, it doesn't matter how many amp hours your battery(s) has available. At some point, they will be deader than a door nail. IMO, if you want to run the AC off the grid, it makes more sense to just fire up the engine and let it run, effectively delivering the batteries from discharge.
I think you are over estimating the 270/280 amp alternators output hot, especially if they are installed like 99% of them are with a Balmar regulator.

They are rated at 180 amps hot idle per the specs I just looked at, but that is at 2400 rpm alternator speed. Our Chevy with the 280 amp version and their pulley is at 1800 rpm alternator which is 40 amps less right there. So start at 140 amps. When it gets hot, the Balmar will turn it down to 50% field, so figure 70 amps. The 270 amp alternator will net about 165 amps average hot with a Balmar so about 60% of rated. 60% of 140 amps is 84 amps, which probably won't run the AC very well without some battery help. The hotter it gets, the worse it gets, and at idle you have no front cooling air coming into the engine compartment.

We have a 280 and a 250 amp in our Chevy, and have our Ample Power regulator set to deliver about 180 amps hot at speed. They drop to about 120 amps hot at idle, running without any turndown, so 60 amps apiece. They will be at 200+ degrees running at that output at idle. 120 amps would run the AC, but you have to remember that this is two high output (530 amps worth) alternators running in parallel.

Even though we have much more capacity than almost any other charging setup, and can keep the alternators moderately cool compared to others, running them long periods at idle speed doesn't appeal to me. IMO, it will be hard on a lot of things due to heat and load. We can even switch to a lower turndown on the regulator and get 280 amps at speed with 180 at idle, but it will overheat the alternators at idle in about 10 minutes at that output, unless I have the auxiliary cooling fans on to put more air through the radiator, which will give more time.

IMO, it really doesn't make sense to run the alternator to run the inverter to run the coach AC when you can just run the dash air, which is likely larger output than the coach air, even at idle.

I think this again points out how the running the AC off batteries and engine generators is absolutely inferior, from a capacity standpoint, to an Onan. The Onan has it's own downsides, but being able to run AC for long periods of time, without killing the batteries at all (and even charging them) makes it much better an option for those that need AC for more than a small amount of time.
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Old 05-15-2017, 04:17 AM   #15
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You guys are dredging up all kinds of scenarios.

In a practical sense, OP, the underhood generator will run your a/c, though you may need to run the engine too.
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Old 05-15-2017, 05:06 AM   #16
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You guys are dredging up all kinds of scenarios.

In a practical sense, OP, the underhood generator will run your a/c, though you may need to run the engine too.
How do you run the underhood generator without the engine running, as that is how it is powered? In the practical sense is exactly what we are talking about. At idle, it is very shaky as to if the underhood generator will be able to handle the coach AC, especially if you batteries aren't totally full and you are using any other coach power. Add low idle speeds and hot weather and you are talking a very difficult situation.

I think you are are talking about the inverter needing the engine running when trying to run the AC, which can happen. Even a 2000 watt inverter (at least some of them) can run the coach AC, so that isn't really the issue.

Roadtreks use the underhood generator, and the system goes into a battery killing death spiral of power loss, as they don't run the system continuously, and have a limit on number of cycles it will run.
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Old 05-15-2017, 03:02 PM   #17
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How do you run the underhood generator without the engine running, as that is how it is powered? In the practical sense is exactly what we are talking about. At idle, it is very shaky as to if the underhood generator will be able to handle the coach AC, especially if you batteries aren't totally full and you are using any other coach power. Add low idle speeds and hot weather and you are talking a very difficult situation.

I think you are are talking about the inverter needing the engine running when trying to run the AC, which can happen. Even a 2000 watt inverter (at least some of them) can run the coach AC, so that isn't really the issue.

Roadtreks use the underhood generator, and the system goes into a battery killing death spiral of power loss, as they don't run the system continuously, and have a limit on number of cycles it will run.
my 2000 watt inverter runs my couch AC.
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Old 05-15-2017, 05:31 PM   #18
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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
If you're talking about running the a/c off the inverter while driving down the road then the answer is yes. I have a Roadtrek Zion with AGM and underhood generator and I ran the coach a/c while driving as we toured the national parks in Utah last July. I turned on my inverter and ran the coach a/c on high while I drove because it was triple digits almost everyday for 3 weeks and I had my kids in the back. The dash a/c couldn't keep them cool enough. The underhood generator kept the AGM topped off the whole time. Hope this helps.
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Old 05-15-2017, 06:23 PM   #19
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The rear windows are one of the sources of high heat/cold transfer.
Easy dual pane windows:
Apply ceramic tint on the inside then make a template of the window area including the lip area, cut a piece of UV resistant plexiglass and install with double sided tape followed with a bead of clear silicone around the perimeter. With the lip and tape you will end up with about 1/4" air gap.
It makes a huge difference and will prevent condensation plus you can see through it. I was able to do the same to my side windows since they are fixed. The only windows that get condensation now are the cab windows which I have made insulated panels. Also, drill 1/16" holes in a couple of the corners to equalize pressure.
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