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Old 03-05-2011, 11:17 AM   #1
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AC on inverter project feedback requested

I had posted that I was thinking about setting up a battery plant under the benches in the dining area in the wasted space between the wall and drawers, I have a supply of telco rated VRLA rated for indoor use, so installing the batteries no issue.

I needed a use for them as the coach plant is 2 8D.

The thought sprang up...one roof AC on inverter.

Many folks run the gennerator while driving to run the roof air to keep cool, so about a gallon of fuel per hour consumed.

If one gets the temperature correct on shore power then a single roof unit should be able to keep things good enough.

The roof air is about 12 to 15 amps, so 1800 watts, and places like Harbor Freight has inverters rated 2000 cont, 4000 peak for about $160.00 or so on sale.

At $5.00 gallon, and 1 gallon per hour, 32 driving hours pays for the inverter.

Not wanting to make too complicated, the install is simple enough.

Batteries under benches with shore powered trickle charger, 12 volt power from isolator.

Single switch control, turns on inverter.

Power from inverter is ran to ac load center, powers relay that acts as ATS, disconnects front roof AC from load center at breaker and connects to inverter.

Only wiring modification is simple relocation of single wire, so returning to factory a 5 minute job.

Has anyone else done this?

Anyone have any thoughts or suggestions?

The batteries to be installed are 70 amp VRLA * 4 or more, the load per battery will be high, so battery run time without engine recharge is from 90 minutes full load (1800 watts) on 4 batteries to possibly 3 hours if I use 8 batteries.

Granted the AC is not constant duty, it will by cycling so the load will be on and off, and there will be engine support from the 160 amp alternator.

The engine alternator can fully support the system but I prefer to limit the alternator part and allow the battries to discharge more.

Our planned road trips are not likely to be more than a 4 hour drive, so the aproximate run times seem to be in range of the need.

So since this is in the "what about this project" stage, I am looking ant any and all common sense feedback.

Tony & Lori
1989 Country Coach Savannah SE
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Old 03-05-2011, 02:16 PM   #2
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Have you checked the amperage when the compressor starts ... I have a friend that dry camps in a travel trailer ... he bought a small portable generator thinking he could run his air conditioner ... the startup draw was considerably higher than the steady state draw ... I don't remember the exact detail ... all I know was that he had to go back and buy a bigger portable generator.

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Old 03-05-2011, 08:01 PM   #3
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Was thinking about that, one model is 2KW continous with 4KW peak at $160.00, the next size larger is 5 KW, 10 KW peak at $499.00.

Both units have standard plug in interface, so the batteries, relay and controls can all be designed and built, the inverter can be added after everything else is completed.

The generator is rated at 7.5 KW and can run both with room to spare, so the 2 KW unit may be ok, will need to go dig up the books.
Tony & Lori
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Old 03-05-2011, 08:45 PM   #4
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Is your dash a/c broken?
Won't it keep the front of the coach cool?
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Old 03-05-2011, 09:08 PM   #5
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Why can't you run the AC using the generator?
Most RV batteries live a long and useful life, some are murdered.
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Old 03-05-2011, 09:41 PM   #6
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Originally Posted by garym114 View Post
Why can't you run the AC using the generator?
Doesn't want to run the generator which he thinks burns 1gal/hour. Burn rate is more like .3 gal per hour depending on load.
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Old 03-06-2011, 04:14 AM   #7
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The inexpensive invertors have a lot of loss of power when changing the power from DC to AC so this will be a factor. Also an isolator to charge the batteries will consume more power. If you use a constant duty solenoid in place of the isolator there will be no voltage drop other than resistance that is normally in the wire.
I had a travel trailer with a no name a/c unit on it and it took over 20 amps to start it. Think it used 13 amps to run it. My Coleman unit on the MH will start on a 15 amp circuit.
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Old 03-06-2011, 12:39 PM   #8
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Yes, the dash air is broken, coach has front and back air, configured for R12, the coolant is low so no operation, instead of refilling with expensive stuff I prefer to convert to R134, when I discover the correct data for capacities that task will be done. Still the size of the cabin coupled with hot local environment may require more BTU than dash air can handle.

Yes, I could run the generator while driving, the factory spec is between .5 gal per hour at 1/2 load to a little less than 1 gal per hour at full load, so depending on how the ac units are running fuel consumption could be upto a little less than a gallon per hour, 1 gallon per hour was used for rough easy math although I know it will be less, the math does not need to be exact here.

After some research in the manuals and memory from last summer, I believe a good quality 2K/4K inverter should do the trick, we have 50 amp service in the coach, but I built a 30 amp twist lock cord for driveway use, it will run both roof AC for a little while before it trips the 30 amp breaker which converts to 3600 watts which is less than the peak output of the 2K/4K unit.

Since the inverters are converting from low voltage to high voltage, and power is constant with power losses involved in the inverter, it is clear that the inverter will need over 150 amps to produce full power, thus upto 8 batteries to be installed and the inverter right next to them to avoid power losses in the supply lines from the battery, supply line to the alternator will be undersized to provide some current limiting factor, this not determined yet.

A continous duty selenoid would remove the 0.6 volt drop across the isolator, this part of the design is still in the air.

One other concern is the charging system, given the trips are not often the home charging will consist of a floating or trickle charge, but for the weekend trip where the batteries need to be charged in a shorter period of time then a device with higher rate of charge may be needed, this too will need to be determined.

With the alternator rated at 160 amps it is capable of running the system, but having the isolator will help keep the different power systems in the coach isolated, the 0.6 volt drop should not be an issue.

As I stated in the OP, I happen to have some AGM/VRLA batteries and some space to install them, the "factory" system is ok as is, and although I think having the batteries and a place to install that was great, I needed something to use them for.

Modifying the factory system to have backup or split circuits was not concidered as it was not needed.

Given the planned trips should not involve more than four hours driving and the cost of fuel, one could charge the battery plant on shore power where energy is cheap, then use this energy instead of fuel to get a comfortable ride. I am not a "green" person, this will result in less polution, but the drive here is that I am "cheap", if I can spend a little money to result in a greater savings down the road then I am going to have a long look at it.

If one needed to purchase the batteries outright then it would not be a viable option, but in this case it is finding a need to use what is "in the way" in the shop.

This is just a "something to do" project that could result in better ride and money savings, currently in the "design phase", where all of the different parts are selected then grouped together into a system and the system performance determined.

So please continue to provide feedback relating to capacities and limits of equipment as all of these are reminders of things that need to be calculated in the design and I will try to not take anything personal.
Tony & Lori
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Old 03-06-2011, 04:48 PM   #9
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A couple of things.. First: are you thinking of running the roof air to save the cost, in gas, of running the dash air.. You are fooling yourself.

Yes, if you run the dash air it will cost you fuel (MPG) however if you run the roof air, off batteries, which are being charged by the motor home's alternator, that too is going to cost you MPG's. ultimately MORE MPG's. since there is conversion loss in converting the mechanical energy to electrical, converting 12 vdc to 120vac, and converting it back to mechanical.

So you are better off running the dash air.

Second thought:

Though 1800 watts running sounds right.

2,000 watt inverter will not start it, I don't think a 4,000 will either, a 10,000 perhaps, more may be needed.

In short.. You need at least 3,000 watts minimum.
Home is where I park it!
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Old 03-06-2011, 04:59 PM   #10
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I use a Harbor Freight inverter. I don't know if this will matter on your project, but the output is a modified sine wave. My microwave oven cooks much more slowly on inverter power. Good luck, and please keep us informed on your plans. We all learn from ideas like this, HarveyP
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Old 03-06-2011, 05:12 PM   #11
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Lets not forget. If the air pulls 2000 watts At 110 volts that is 18.18 Amps;;; When it goes through a 12 volt inverter now 2000 watts puts a 166.6 amp load on the batteries;; Needless to say the batteries will not last Ever 10 minutes. We had a friend/fellow rver. Who Used to brag about his generator had less then 20 hours on it over a (10) ten year poriod, We could never understand Why he was draging/carring That big useless thing around 85,000 miles and not use it. It might use 1/2 gallon per hour. Good batteries cost $130.00 each And may last 2 years if you use them that hard. Oh you would need at least 4 of them. Go figure ;; Life is good.
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Old 03-06-2011, 07:38 PM   #12
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Fuel saving not the goal.

THe primary goal is to not save fuel, if the project included the purchase of batteries, controls inverter and labor then one would be nuts to try as the generator is designed for this and the cost of the project would buy a lot of fuel.

We currently hav 8 70 ah batteries, the 3 hour rate is 19.83 amps, the 4 hour rate is 15.55 amps, so running battery only we should get 3 hours.

Currently the dash air system is not functional, and after it is fixed I doubt that it will keep the cabin cool in our hot summer.

The trips we have planned do not involve longer periods of driving time so being able to run off battery for that time would reduce the need to run the generator. Cost for shore power a lot less than the cost of the fuel.

We may need a larger inverter to compensate for starting currents, I will need to measure the starting current to see where it is, then factor in headroom for operation on a hot day.

We have a commercial quality ac power line equalizer that we could use to clean up the inverter and also output regulated 120 VAC, this may assist the inverter for compressor startup, it is a plug in device, so testing should be easy.

Looks like the first step is going to be actual measurements to see what the compressor requires.
Tony & Lori
1989 Country Coach Savannah SE
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Old 03-07-2011, 10:04 AM   #13
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I live in Phoenix and the dash air will keep us comfortable even when it is 110' outside as long as we do not get caught in alot of traffic.
I just completed a conversion on our MH from Freeze12 to 134a. At idle it blows 45', which is the same as on Freeze12.
A rough estimate of the charge weight for a 134a conversion is 2/3 of the amount of R12. You will need to change the oil in the system, the o-rings on the fittings, and as a maintenance item, the drier. If you do not have a high pressure cut-off switch in your system this should be added. The rubber hoses may work if in good condition. The hoses designed for 134a have a plastic barrier in the hose so the 134a molecules do not seep thru. Hoses that have been used with the mineral oil for R12 are thought to also inhibit seepage of 134a.
If both of your roof top units tried to start at the same time this could creat a huge load for the invertor. I would go with repairing the dash air first.
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Old 03-07-2011, 10:54 AM   #14
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a/c on inverter

i work for a major trucking company we tried this to have a/c when parked we used a 3000w 4500 surge inverter and six gel battery's that were equal to eight lead acid battery's worked well until we pulled the batteries down to below 50% on a steady basis we could run the a/c for eight hours ,mind you a smaller space I'm not familiar with the batteries you are using so don't know on them . the one thing we had to have was a 270 amp alternator because you still need to charge your other batteries and any load on the coach and the recharge time varies with temperature and condition of the batteries.
but if you start out with fully charged batteries and wire in a switch to start charging around 12.3 or so going down the road i think you have a good shot at this working

good luck frank,cricket and harley the dog

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