Here is my situation: I want to add a small (5,000btu) A/C unit unto my sleeper. The sleeper sits on my 2000 Ford F-350SD. I have looked at the option of powering the a/c with either a generator or 12volt power inverter. I would LOVE to run the a/c with a power inverter. Here is my question: Can I run the inverter directly from the 2 - 12 volt batteries already on the truck or am I better to run it from a separate deep cycle battery? If I run it from a deep cycle can I use the vehicles alternator to also trickle charge the deep cycle or do I need a separate charging system? Any advice is appreciated. Note: I will not be using the inverter with the truck turned off, only when it is running. Thanks.

 

If you are only going to use the inverter with the truck running, you could get by with hooking it to the vehicle batteries. They, along with the alternator will contribute to the power demand of the inverter/AC combo. If you were going to use it with the engine off, that's when you would need a deep cycle battery(several probably).

Look at the A/C unit you are thinking about buying, and get the amperage it uses(or wattage) Size your inverter to this, and get one that can stand the surge of the A/C compressor starting. You are probably going ot end up with a decent sized one, and it will have instructions on what size wire you need to run to the battery(probably fairly large).

Of course I guess you know you can buy 12v A/C units, but they are sort of expensive. You probably could buy a cheap window unit from Walmart for $90, and a inverter from Harbor freight or somewhere, and still be ahead. I think the 12v units start at around $500-up.

 

As Dave suggests, you really need to look at the sizing here.

I work with inverters all the time and the rule of thumb is not to use them with AC or high amp draw appliances. Its not that it can't be done, but you normally will need plenty of batteries and a heck of an alternator. I have blown the terminals right out of the batteries from the heat they generate when pulling a heavy amp load. Even campers running small 7,000 btu high efficiency ACs have 2,800 watt generators for this reason.

Steve

 

5,000 btu air conditioners at 120 V draws 5 to 6 amps when running steady state.

Frigidaire - 6,000 BTU Compact Room Air Conditioner - White - FAA065P7A


Problem is the details... that unit can use a starting current of 10 to 15 amp when starting --- depending on the quality of the capacitor start system, which are often awful at the low end units.


6 amp, with some overhead, is 700watt.

10amp is 1,200 watt.

With some safety margin, you really need a 2,000 or higher watt power converter.


With 50% efficiency, that mean a 4,000 watt load at 12 volt nominal.

That is over 30 amps.

Assuming that your wiring is short... and that is not a factor.

You still want to do this?

30 amps - mean this. Most truck batteries are not even 50amp hour rated.

That is, they can do 1 hour at 50 amp when brand new and fully charged before the voltage dips below usable.

To pull that kind of load, you are looking at golf cart sized deep cycle batteries, and an awful lot of it to run more than 30 minutes.

Your car alternator MIGHT be able to spare 20 amps at 12V on a continuous basis... 250 watt of energy.

Use your alternator at close to its top rated power, and it will probably die on you real fast.


A 2,000 watt genset works real good!

 

Remember to multiple the amp load at 120 VAC by10 to get DC amp draw from the batteries in any calculations. Systems that typically run AC on inverters have, as a minimum, 8 house batteries weighting 125 pounds each. I have worked on them on large houseboats and conversion buses with ten of those hernia producers. High-end pure sine wave inverters are very efficient, but even then weigh, cost, and practicality, all argue against inverters in this application.

Measuring at the batteries, an alternator rated at 150 amps will usually be putting out about 120 amps, again having measured this type of system at the batteries in RV applications. How long it is capable of doing this without going to Heaven is open to conjecture. High-end inverters will put out their rating, but the heat at the battery terminals is unreal. Folks who have these systems normally have them custom designed. Inverter efficiency on varies greatly with cost, as does wave form.

I have also installed large inverters in Peterbuilts, but never to power air conditioners.

Personally I think a genset is a much more practical idea. I would second the suggestion above and would suggest a Honda or Yamaha generator intended for RV use as they are very quiet and, in my experience, reliable. I run a Yamaha 2400 on my service truck and it will even pop off RV air conditioners putting out 13,500 BTU and pulling about 15 amps to start.

Steve

 

Well, I would hope this would work out for him, since I am planning to do sort-of the same thing with a small dorm type refrigerator in my camper.

If he gets the smaller A/C unit, and uses it night for sleeping(best senario), the unit cooling the sleeper probably would not run continuosly(just the circulation fan), and with the engine running, if the alternator will not be able to quite keep up when the compressor is on, the batteries will contribute, and then when the compressor kicks off, the alternator can put some charge in the batteries till it cycles on again.

I have a regular dorm refrigerator(again 120v is cheap), and when I am at the campground, I will plug into the campground power. When I am traveling, I want the frig to stay cold till we get to the next destination. I am thinking if the door is kept shut, it may not cycle too much, and I will use a deep cycle placed in the trailer to run the inverter, and have a charge wire from the truck to help it out as we are traveling. The only time the battery would be on it's own, would be during a stop at a resturant while traveling.

 

Fridges are darn small draws as I recall. Even some fairly good sized ones have compressors drawing 3 amps or less.I bet dorm sizes are less than 2 amps. These also come in AC/DC models.

Air conditioners are power pigs by comparison. I have run RV fridges on inverters while traveling without a problem and regard that as a very reasonable use of inverters. Might need to add an additional battery, but if you keep the door closed and make your selections fast, I would expect little problem.

Steve

 

> Can I run the inverter directly from the 2 - 12 volt batteries already on the truck or am I better to run it from a separate deep cycle battery?

It will not run long and in the long run you will end up nuking the batteries and the alternator(s).

> draws 5 to 6 amps when running steady state.

(6 x 120 = 720)/12= 60 amps 12v

Most small A/C appliances (ac - frig - compressors) surge at 12-15 amps.

(12 x 120 = 1440)/12=120 amps at 12 volts

Better off just getting a diesel generator and muffler it well within a compartment. 6 amps is basically an idle, as it cycles it will draw 120x - 12 amps.

The house battery on many boats that want even a chance of running A/C is (4) 600 cca batteries and they get charged by a 120 amp alternator on each engine.

Plus, the inverter has to be heat sinked and have it's own fan and the real big ones have placement requirements about where it can be secured because it needs the air flow.

Swamp A/C works out better because the fan draw is very low and can be run on a single battery without conversion, which saves on the energy loss and resultant heat.

 

Swamp coolers work out well in dry areas, but work less well in many parts of the country m given high humidity.

Best to check actual amp draw on small fridges as the compressor kick is fairly small and good inverters do have surge capacity about double their continuous rating.

Steve

 

Problem with dorm fridges:

Unless you are getting the type that is using thermo couples (no moving parts), many of the motors in the compressor based units are not designed for travel.

So it basically rattles about in the case --- not good.

 

I did some back of the envelope work to see if a 900 watt microwave would work in my rig.

Turned out that the power draw is well into 12A on 120VAC, or 1,500 watt.

Add in the 50% efficiency, and you are into 3kv --- about the same draw as the starter (3.6kv).

Barely feasible --- if used for very short (10 min) to heat food.

Otherwise, it wipes out the batteries (dual batteries here).

 

Couple of thoughts. Be careful here not to throw the baby out with the bath water. While it does require some planning and design to utilize inverter power correctly, there are literally hundreds of household refrigerators of various size bouncing around the country with no ill effect. While it may not be what they were designed to do, they seem to handle the stress of travel admirably.

A typical microwave in an RV draws just under 11 amps, again measured with a meter at the box. Draw, of course, varies with wattage.

I have run microwaves with inverters in over half of the RVs we have owned ( twelve and counting) with nary a problem. Yes, inverter power will make the microwave give off an unusual humm, but a quick check with manufacturers will convince you it is not a problem.

I think where some calculations may be falling short is figuring efficiency at 50%. High-end inverters, while much more expensive, have ratings in the 90s.

I am not trying to argue with anyone here. There are definite problems with using inverter power for high draw applications and I think it is a very bad idea to attempt to use it for air conditioners without a total system redesign. For other less arduous applications, I think it can be reasonably done without harming alternators or other components, but I would model those applications after the myriad of successful designs already in use in RVs. The downfall with a stand-alone truck design, in my opinion, would be reliance on the starting batteries. I would add pair of house batteries with an isolator. Even in Peterbuilts I have added a pair of house batteries under the steps on the passenger side of the cab.

Just my thoughts on what I find to be an interesting discussion,

Steve

 

Yes, the key is high end inverters.

That gets you into $$$... don't know if you can approach 90% without going esoteric but certainly a lot better than 50%.

Key to a microwave is the relatively short duty cycle.