Alternator, Batteries, Inverter Question

[Highway Yacht]

Ok....here are my plans as of now. My coach will be built mainly to use with shore power. I plan on installing (2) 15000btu rooftop a/c units and want to be able to use these units as my OTR air since the stock OTR unit is history. I want to install and power an extra alternator off the original pto pully where the stock OTR compressor was once located and have all that current going to a small as possible house battery bank and then to an invertor so I can power the (2) rooftop units and the fridge while traveling down the highway.
So now I need a little help with figuring out and sizing what I actually need to be able to do this. Starting with the 12v alternator, what kind of amp output will I need? How big of battery bank will I need? How many watts does my 12v Inverter need to be? Not sure of what the current draw on (2) 15000btu a/c units would be and I haven't bought the fridge yet so I don't have that current draw. Maybe someone knows what the current draw would be on a small apartment style fridge. I know there are a lot of variables here like if I was stuck in traffic at idle for a long time, startup of the units, etc. I'm just looking for a system that will keep the coach cool while going to and from shore power. Thanks...

[Sean]

Sorry to be the bearer of bad news, but you can not do this with a single inverter on 12 volts.  Better if you had a 24-volt coach to begin with (you don't mention what coach you have), but, if not, you could still go with 24 volts for the house bank if you are planning to add an extra alternator anyway -- in which case, I would go right to the belt-drive version of the DN50 oil cooled brushless generator.

Here's the math:

A typical 15,000 BTU/hr roof air requires 13-16 amps while running, and 20-35 amps to start.  The spec likely calls for a 20-amp, HACR circuit breaker.

With both units running, that will draw 30 amps, give or take, which is 3,600 watts before factoring in inverter and other losses.  These can add up to 20%, so you need at minimum 4kW of inverters and probably more like 4.5kW, and that's before the fridge.

In order to start the units (one at a time), you'll need another 1,200-1,500 watts of "surge" capability.

4 and even 5kW units are available in 24 volts.  The largest 12 volt models are 3kW, and 2.5kW is more common.  You could get by with a pair of 2.5kW units if they have a "stacking" capability.  This would give you 5kW continuous and probably 10kW "surge."

As for the fridge, they are all over the map.  Can't give you any figures without a make and model.  Lots depends on frost-free vs. manual defrost (hard to find these days, except in the smallest "dorm room" type units).  However, I have never seen a 120-volt fridge that used less than about 200 watts, and anything taller than counter height is going to be more than that, possibly a lot more.  If you are serious about running a household fridge alongside two AC units from inverters, then you will need to shop specifically for a low-current manual-defrost model (or plan on disabling the automatic defrost when on inverter).

You alternator will need to be sized for the total continuous load (the batteries will take care of the starting surge).  That's at least 4,500 watts and probably more like 5kW, which would be 416 amps at 12 volts (no such animal -- you'd need at least two alternators), or 208 amps at 24 volts.  If you only ever do this while the engine is running, the size of the battery bank is almost irrelevant, and pretty much anything will work.

Lastly, you will need "true sine wave" inverters to run induction motors, and that's what you'll find in ACs and fridges.  Figure close to $0.30 per watt when all is said and done.

HTH,

-Sean
http://OurOdyssey.BlogSpot.com

[Zeroclearance]

OT,  Sean did you get your new batteries?

[JohnEd]

jlink,

I think Sean has xeroxed copies of his posted answer.  Yours is a popular dream that can't be realized.  Others have installed a 120 VAC Mini-Split and put the evap over the drivers area.  The home units are much much more efficient and accurately sized/labeled.  Thanks to those dirty rotten scalleywag green conservatives interfering in the ethical practice of corp Meurica.  They haven't been able to force their underhanded ways on the RV industry as yet so we have to take their (mfr) unbiased word for everything That guy was really pleased with his heat pump split.  Blow it directly on ya and have a good fan directed at the driver.

Consider rethinking your decision to NOT us a engine driven compressor.  OEM OTR is far to large to fix or operate.  Read thru the other posts of late about OTR arrangements/designs.

John 

[Len Silva]

After you do all the calculations based on Sean's advice, a decent diesel generator in the 8-10 KW range will probably cost less to purchase, about the same to operate and can be used when parked.

If you have a 24 volt coach, trying to use an inverter to run ONE air conditioner for the driver's area may make sense (though I don't think so), trying to cool the entire coach does not.

Even if you are extremely well insulated, two roof units will not be adequate on a hot day going down the road.

Retaining the OTR air conditioning or a generator and three roof units is probably the most practical and least expensive method. If you are planning any off the pole camping, you will need the generator anyway, might as well use it.

[Sean]

FWIW, we do run two air conditioners while driving just from the alternator and inverter.  But we have a 24-volt coach that came with a 270-amp 50DN alternator, capable of producing over 6.5kW of power.  All we needed to add was the SW4024 inverter, which we wanted anyway for other reasons.

If we had to buy this amount of alternator capacity, the economics would not have made any sense, and we'd have considered using a generator while driving.

BTW, on a coach that spends more time parked than rolling, I personally believe that OTR air, whether that's whole-coach or just a driver unit, does not make sense.  It's a whole other system that needs to be maintained, takes up space, and requires that much more refrigerant, all for something that can't be used while parked.  Conversely, all the systems to run electric A/C while driving are normally already on board and being maintained anyway.

JMO.

-Sean
http://OurOdyssey.BlogSpot.com

[Len Silva]

FWIW, if you have a 24 volt coach and your name is Sean, it may be a good idea!  For someone with his knowledge and understanding of what is involved, it can work to your advantage. Under the same conditions, I might try it myself.  On the other hand, for the average busnut, who just wants to start the coach and put the A/C switch in the "ON" position, and drive down the road, I think you are much better off with a generator system.

[James77MCI8]

Having maintained my OTR A/c for the last seen years , I personally am going to the inverter option on my bus. I have spent more money on maintaining the OTR A/C than I have on any other mechanical systems associated with the coach. Since I do not boondock I don't see the need for a generator. My coach is 24V. I am with Sean on this one.

[happycamperbrat]

Oh I am sooooo interested in this! I have an RTS which has the 12v and the 24v. I have the big mutha factory AC which works great BUT when parked it really sucks to be without AC. I live in HOT dry desert near Death Valley. I really believe in solar power, but the technology isnt quite there yet for me to use it on my bus...... close, but no cigar. I hate generators! I really dont want a genny. So my choice is to wait it out until the solar technology catches up with what I want to do and instead spend my cash on batteries and inverters. I was studying a guy's website about his bus and his name was Dave (if requested I will find the link and post it) He claims to have used 10 batteries, a 4200ish Trace Pure Sine Inverter in the Arizona desert for about 2 1/2 hours and then stopped the test to find out what his results were. He says he could have gone a lot longer. He was running a refer, 2 Carrier Roof ACs at 15,000btu, computer, tv, etc. If this is true, this would solve my problem of getting from one hook up to another.

[NewbeeMC9]

Google DC mini split heat pump,    Also search the board here.


There was one with the rotary compressor and DC motor that claimed a high seer rating.


you'll spend more on the unit but you won't need as much inverter.

[Highway Yacht]

FWIW, we do run two air conditioners while driving just from the alternator and inverter.  But we have a 24-volt coach that came with a 270-amp 50DN alternator, capable of producing over 6.5kW of power.  All we needed to add was the SW4024 inverter, which we wanted anyway for other reasons.

If we had to buy this amount of alternator capacity, the economics would not have made any sense, and we'd have considered using a generator while driving.

BTW, on a coach that spends more time parked than rolling, I personally believe that OTR air, whether that's whole-coach or just a driver unit, does not make sense.  It's a whole other system that needs to be maintained, takes up space, and requires that much more refrigerant, all for something that can't be used while parked.  Conversely, all the systems to run electric A/C while driving are normally already on board and being maintained anyway.

JMO.

-Sean
http://OurOdyssey.BlogSpot.com

I do have a 24v DN50 gear driven alternator on the coach already...but I was under the impression from some past posts that it isn't a good idea to power the a/c's from the coach's engine alternator because it MIGHT use all the current coming from the alternator to power the inverter and a/c and cause dim headlights and possibly not enough cranking amps to start the 6V92. If that isn't true then that would be my best route..right? Do you even use a house battery bank or did you just go from the alternator straight to the inverter without house batteries?

[TomC]

By the time you get either one large inverter, or two small inverters, one large or two alternators, extra batteries, etc, you're getting quite close to the cost of a Diesel generator.  I only use my inverter for small items (like microwave, toaster oven, water heater, bath heater, various plugs, TV's) and just run the Diesel generator going down the road.  You can get a decent 8kw Diesel generator for around $5,000-of which is not that far off the cost of setting up for an inverter system.  Then you'll have a generator, and will be much simpler in the end.  Good Luck, TomC

[Sean]

I do have a 24v DN50 gear driven alternator on the coach already...but I was under the impression from some past posts that it isn't a good idea to power the a/c's from the coach's engine alternator because it MIGHT use all the current coming from the alternator to power the inverter and a/c and cause dim headlights and possibly not enough cranking amps to start the 6V92. If that isn't true then that would be my best route..right? Do you even use a house battery bank or did you just go from the alternator straight to the inverter without house batteries?

Well, if you already have a 24v DN50, and you've removed the OTR air (which was what needed all that juice in the first place), then, yes, the most straightforward solution is to get a 4kW, 24v sine wave inverter.  You should be able to pick one up for $1,200-$1,500.

I do recommend you use separate batteries.  The inverter will need to be powered up all the time, not just when it is running the loads while driving.  You don't want any possibility that the inverter will accidentally deplete the start batteries.  Use a solenoid powered from the alternator's Relay terminal (or old blower circuit) to bridge the banks together only when the alternator is charging.  You can also use a relay from this same circuit to force the A/Cs off-line in the event that the engine is not running.

You will not use up all the power from the DN50.  Your 4kW inverter under normal circumstances will draw at most 185 amps.  Your DN50 produces 270 amps, leaving a margin of 85 amps, or over 2kW, to charge both battery banks.  Most heavy-duty vehicles have entire alternators smaller than this.

If you are going to do this, I would recommend further that you use 24 vdc for your entire house system.  I've written about this extensively in the archives, and even did an article on it in the magazine last month or so.

My guess is that, at some point, you will still want a generator.  But if you get large enough house batteries, you will at least be able to run your fridge for a night or two of dry camping with this system.  By contrast, with no inverter at all and a household fridge, you'd be committed to run the genny every minute you're away from the pedestal.

-Sean
http://OurOdyssey.BlogSpot.com

[NeoplanAN440]

I went with a extra house battery bank connected via HD relays to the 50dn.
they connect to the alternator while running,or i can override to use them for starting also.
i run 2 15k Carrier units on them with no problems.i also have a normal house size fridge on it and all the other small things you find handy.

i started my system small on a budget.i was running two tripp lite 2400 aps inverters.split for front and rear ac and circuit.these are often to get new for less then $300 a piece and can carry for a short time double the load,to start the ac.

the best is for sure the xantrex sw4024.
it runs both ac with power to spare.also its nice that you can program your incomming power max,so you can even hook up to 15A to charge and in case you need more to start ac,the unit will share and then recharge.lots of nice features.i like the sw4024 and it never let me down.
i use it on my house  also,and stacked it runs my welder,lathe.milling machine........


i even removed my 10k isuzu gen and build my own apu from a 6hp kubota mated to 24v alternator.
most gens are oversized and run without enough load to clear out(wet stacking)

my 2 cents

[Highway Yacht]

I do recommend you use separate batteries.  The inverter will need to be powered up all the time, not just when it is running the loads while driving.  You don't want any possibility that the inverter will accidentally deplete the start batteries.  Use a solenoid powered from the alternator's Relay terminal (or old blower circuit) to bridge the banks together only when the alternator is charging.  You can also use a relay from this same circuit to force the A/Cs off-line in the event that the engine is not running.

You will not use up all the power from the DN50.  Your 4kW inverter under normal circumstances will draw at most 185 amps.  Your DN50 produces 270 amps, leaving a margin of 85 amps, or over 2kW, to charge both battery banks.  Most heavy-duty vehicles have entire alternators smaller than this.

-Sean
http://OurOdyssey.BlogSpot.com

So right now this does looks like the best solution to meet my needs to get current to the rooftop air. Approximately how big or how small of a house battery bank would I need to do this setup to just run the a/c and fridge WHILE the coach is running? I can't think of any situations of where I would need to run the air or fridge for any length of time while the coach isn't running from home to a shore power pole at my destination.