Inverter question

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Rif

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Hello All,

I am new here but this looks like a great place to ask my question.  I am in the process of converting a 1993 Chevy G30 Mini-School bus into a camper.  I had the idea of running a 110v 5000 BTU AC of an inverter.  I see many posts about the AC/inverter issue.  I believe the AC I have in mind has a 500watt output.  At that rate is a 110/12v inverter on my 2 stock batteries acceptable while the engine is running?  Am I missing a lot of basics in my thinking.  Please advise!

Thank you,

Rif
 

Just Lou

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First off, Rif, welcome to the forum.

I'm sure someone will be along soon to address your concerns with the electrical needs for your conversion project.

I will take the easy way out and answer your (yes or no) question.

YES, there are some basics you are missing in addressing your AC/DC needs.

You will need to do a search (both here on the forum and the internet) to learn the basic relationship between AC and DC power requirements.

(i.e. ....the Air conditioner I have in mind has a 500watt output...) is a meaningless statement.

Good luck, lou 
 

John From Detroit

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Well, first air conditioners do not have "Watt" outputs, they have "BTU" outputs  Watts are on the input and 1500 seems to be about the minimum with as much as 4,000 starting (Very brief, most circuit breakers have a slight time delay but when selecting an inverter you need to know this)

Is this ok on an inverter while driving with engine running?  Depends on the alternator  You want about 500 watts (50 amps) for vehicle needs, and if the alternator has the amprage (IE: 200 amps or more) you can do it.

But with the alternator off it'll kill your batteries while waiting for a Dominios pizza  (30 minutes or less)
 

Rif

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Thank you both for the welcome and the information.  The AC that I was looking at to get an idea of power requirements was a 5000 btu room AC, under the specs of it description is states "Cool Watts = 575" and "Cool Amps = 5.5"  So from what has been stated I guess the 575 in watts input.  This sounds to me like a relatively light load and perhaps can be handled by a $150 - $200 inverter.  I must however have a look at my alternator specs and see if it has the amp power you mentioned above.  Your comments are more than welcome, and if you have any general advice on how to approach the ac/dc power needs in a camper of my size, that is also appreciated.  Great forum!  Thank you.

Rif
 

Jeff

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Gif:

I am not sure how big your bus is but 5000 BTU's sounds pretty small to cool an RV. IIRC typical roof top a/c's are 10,000/15,000  BTUs and most RV's over 30' have two. In warm climes ours work continuously to keep the coach comfotable.
 

2006F350

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Rif, first congratulation on undertaking such a conversion project. Before you get to deeply engrossed (time and money), unless your plans include a lot of state and national parks, you will most likely get a cold shoulder when trying to get a site at a private RV parks .... many will not accept non-RVIAA certified RV's because of insurance and other issues.

Best of luck on you build.

Larry
 

Rif

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Thanks again for the replies.  Jeff, my vehicle is a mini-school bus, basically a Chevy G30 diesel van with a higher ceiling than a normal van.  It has two stock batteries and a 105 amp alternator.  So, now I am aware that that 105amp alternator may need some boosting to run an AC, water pump (on board WVO filtering) lights, DVD etc...  As far as the cold shoulder issue at RV parks I appreciate the heads up, but we will most likely not be seeking that kind of travel, more like overnight trips and some camping.
I believe a 5000 BTU will be more than adequate to cool the bus, and I'd preferable actually like to find something even smaller.  My idea is to use a house hold small room AC.
Now, if y'all have any experience with a bottoms up design as in what are the particular to build out an electrical system from the alternator t the end devices I'd love to here some input, or if you know of any "HowTo's" on the web that I could look at.

Thanks Again,

Rif
 

Lowell

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RIF, Please keep us up to date with your project.  My son would like to do something like that for a hunting camper that he can take into the back roads of the national forest.  I'm reluctant to pull our nice TT down those washboard roads so we usually ending up camping some distance away from the area we hunt and then driving over in the early AM.  That means we have to get up earlier and sometimes other hunters beat us to the area we prefer to hunt.  A reworked "rough" school bus would be just the ticket!
 

Rif

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Lowell,

Your son can search Ebay and the like for a used mini school bus.  For three to four grand he could get one in great shape for less you'll end up putting some work into it to get it up to snuff mechanically, but that's ok too.

Rif
 

John From Detroit

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Well if your alternator is over 100 amps it should take that kind of load,  I've seen a few very small AC's that were in that power range and used to run one on a kilo-watt generator (so it was under 1,000 watts starting)

Ii will add this,  When it comes to inverters there are two families, cheap Modified sine wave (I have a few) and much more expensive True Sine Wave... Trust me, the TSW  units are worth the money If you are runing radios or televisions in the rig they make a BIG difference in performance of the electroincs.  Other stuff (IE lights, and even motors) are more forgiving
 

Rif

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John,

Thanks for that tip about inverters, I did not know that there were two different kinds.  I will keep that in mind as I proceed...

Thanks,

Rif
 

Karl

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Rif,
Something doesn't sound right. A 5000btu a/c works out to about 1465 watts (watts = btu's/hr x .2931) If it really says 575watts cooling, that is only 575/.2931, or 1859btu's/hr - pretty small even for a window air conditioner. Can you take a picture of the spec. plate on the a/c and post it here? That might help. Numbers aside, running even a small (5000btu) a/c off batteries without replenishing them as you go, will drain them in short order. What type/size batteries do you have now? That too makes a difference.
 

Rif

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Karl,

These are not the specs of an AC that I have rather just for research's sake I took the first model off a google search http://www.airconditioner.com/#  see the specs of the first 5000btu unit it should b ethe first unit on the page.
Right now there are two Interstate 700 cold crank batteries in the bus.  I have already established that running appliances off of these batteries would be most undesireable so that puts us in the realm of deep cycle batteries.  So far I have determined that I'd probably do well with two deep cycle batteries in parrellel on the 12v side of the inverter, it has also been suggested that an isolator be installed between the deep cycle batts and the alternator, my question there is does the isolator connect directly to the alternator or to the start up battery?
Another question I have is most True Sine Inverter come with 2 or 3 a/c outlets, what does one do about the need for more outlets? 

Thanks,

Rif
 

Karl

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Rif,
Sounds like someone made a typo. You're right about the deep cycle batteries - much more rv-friendly than motor starting batteries. You could go with either flooded-cell (golf cart type) batteries or maintenance-free AGM's. If you're going to be charging from your vehicle alternator, the isolation switch can be located at the batteries, a single pole/double throw knife switch of suitable amp rating (at least 100 amps) or a cut-over solenoid. You CANNOT charge both deep cycle and vehicle starting batteries at the same time, as they have different charging characteristics, and you should not parallel them either for powering loads. I would also suggest getting a  three-stage charger/converter for charging off shore power and maintaining them at the proper float level.

The number of inverter outlets is somewhat dependent on the size of the inverter. I have a 600 watt pure sinewave inverter that is used for sensitive electronics that has 2-120V outlets, and a 2kW, modified sinewave inverter hard wired into the main distribution panel for everything else. Units smaller than 600W may have only one outlet, but you can use a "Y" or "W" adapter for plugging in more than one device.
 

Rif

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Karl,

Thank you for that wealth of info.  When I mentioned parralleling the batts I meant two deep cycle batts to the inverter, independant of the starter batts, is parralleling the deep cycle batts ok?
How does the inverter/isolator connect to the alternator, through the start up battery?
You mentioned you have a modified inverter for non-sensitive stuff, what appliances would you recommend running on a modified and which would work best on a true sine inverter. 
Do people usually use electric stoves or gas?  If I use a single or double burner electic stove, 5000 BTU AC, 1/2 HP water pump, DVD/Stereo system, outlet for computer, and some lights could I get away with one True SINE Inverter or do you think I could benefit from having  modified and True SINE inverters?
I know I am all over the place but I am trying to gather info and get educated before I start on the hardware.

Thank you,

Rif
 

Karl

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Irv,
Yes, you can parallel 2, 4, or however many pairs of 6V deep cycle batteries as you want. You just don't want to parallel, for example one 12V starter battery with two 6V deep cycle batteries(in parallel), or one 12 starting battery with one 12V AGM, even though they're in effect two individual 12V supplies. Simply put, don't mix battery types.

The inverter is connected directly (hard wired or with alligator clips if it's a small unit) to the house battery(ies) if you have separate house batteries, or to the starting battery if you don't, but that's not a good idea because you could drain it and not be able to start your vehicle!.

Some devices, like electric blanket and coffee maker electronic controls simply will not work with a modified sinewave. Their power supplies don't like it one bit. Other things like tv sets, microwave, (most) computers, toasters, etc. have no problem. If I could start out new, I'd get a large, pure sinewave inverter and power everything from it. Keep in mind that pure sinewave units are about twice as expensive as modified but, in the long run you may be better off.

Almost everyone uses propane to cook and heat. An electric stove is, for the most part, impractical, unless you have 50A service and are hooked up to it on a fairly permanent basis. If you travel and move around a lot it's next to useless and propane is the only sensible way to go.
 

Gary RV_Wizard

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I can't find a definition of "Cooling watts" or "cooling amps" anywhere, but you can rest assured no motor/compressor producing 5000 btus of cooling is running on any much less than 5.5 amps (continuous).  Since the inverter takes 10-11 VDC amps to produce 1 amp @ 120VAC, we can safely say that at least 60 amps of continuous DC must be supplied and that number momentarily doubles or triples under the compressor starting load. Does an alternator have instantaneous response when demand suddenly triples? I leave that to the alternator experts, but if not, the batteries will have to make up the shortfall for a few seconds.  The inverter also must be able to handle that peak surge in demand, which means the inverter has to have a surge rating of at least 11 AC amps (1400 watts), even though its continuous rating can be around 700 watts (5.5 amps x 120 VAC).

So it sounds like a large engine alternator, say 140 amps or more, could supply enough amps to feed a mid-size inverter and  the load of a small a/c unit.

I think you are optimistic about cooling a bus with that small of a unit, though. A 5000 btu a/c is intended to cool a small room, e.g. 10x12 or about 120 sq feet. Your bus is probably twice that (or more) and likely not insulated all that well either.
 

John From Detroit

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Roamer I don't think you will find a definition of "Cooling Watts" since I think it's a mis-translation of another language to English

very common

Think "Watts in cooling mode"  that is running watts in A/C mode of course standard comments about STARTING watts applies
(What 4x running)
 

Rif

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Wow, thanks again for all the input, y'all are a great resource and I hope to able to give back in the future.  Gary, my bus is a mini you mentioned ac unit of 5000 BTU would only cool a room of about 10x12, and that just about describes the bus!  It is basically a Chevy G30 with a higher top.
From what I can gather I have two choices, 1. try building the electrical system using my current alternator 105 amp.  The bus in turn has two stock batteries, I would attach the starter battery to an isolator, the isolator would in turn attach to perhaps two inline deep cycle batteries that feed the True SINE inverter maybe a 1500 watts continuous / 3000 watts surge.  Now, someone also mentioned a charger???  Where does that fit in.  And do you think I am starting to grasp this subject ;)?

Thanks,

Rif
 

Karl

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Rif,
And do you think I am starting to grasp this subject?
Well, yes and no :) We're just starting to get the details of your current setup and, without specifics, it's difficult to make recommendations. You mention two stock batteries. What exactly are they? Volts/Amps/ regular auto batteries/what?

A 105A alternator is pretty standard nowadays and will provide the necessary battery charging power as long as your engine is running. The separate battery charger/converter is for use when you have shore power. That way you don't have to run your engine to charge the batteries.

The isolator (switch or solenoid) lets you charge either the motor starting battery (what we call the chassis battery) OR the coach battery(ies) (what we call house battery(ies) separately, because each may have different charging requirements.

The inverter doesn't care if it gets its' power from the chassis batteries or the coach batteries, but remember that if you drain the chassis batteries, you may not be able to start your engine. Most coaches have an "emergency" switch that will connect the two sets of batteries together for starting the engine, but this is a momentary connection for starting only; not something you want to do on a permanent basis. Example: Let's say you pull into a campground late at night with your headlights on, get everything set up for the evening, and forget you have your headlights on. Hey, we've all done it. Next morning you try to start the engine and....NOTHING! That emergency switch will connect your house batteries to the chassis batteries and let you start the vehicle.

Give us more specifics on your current setup and we'll be able to answer your questions more accurately.
 
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