Running air conditioner off solar and batteries

[DavisBurns]

We have a 2004 Interstate. We've added an extra house battery to power astrophotography equipment and a solar panel to charge both batteries (house and extra). We try to camp where it's cool at night but occasionally we need the AC. It isn't too noisy when we have shore power but it is too loud to sleep if we have to run the generator. Has anyone else tried using batteries/solar to run the AC? We took the microwave out which is the only other big energy hog.

[Protagonist]

Originally Posted by DavisBurns

We have a 2004 Interstate. We've added an extra house battery to power astrophotography equipment and a solar panel to charge both batteries (house and extra). We try to camp where it's cool at night but occasionally we need the AC. It isn't too noisy when we have shore power but it is too loud to sleep if we have to run the generator. Has anyone else tried using batteries/solar to run the AC? We took the microwave out which is the only other big energy hog.

Even on my 2012 that came from Jackson Center with two Group 24 AGM house batteries, I wouldn't even try to run the rooftop A/C off the batteries through the inverter.

First, I don't think the inverter even powers the A/C circuit. Even if it did, I think the A/C would run the batteries down stony dead in just an hour at most.

One suggestion, if your onboard generator under the floor of your Interstate is too loud to sleep while it's running, do what the trailer owners do— get a portable generator and hook up your shore power cable to it. Having 25 or 30 feet of separation between you and the generator will do a lot to make your bed quieter.

[Minno]

According to the discussion Lewster had at the Vintage Trailer Academy, it is virtually impossible to run the AC with solar, even with a large number of solar panels. He said that one of his customers experimented with it and the AC ran for about 20 minutes only using 3 or 4 large panels. Maybe as the technology improves over time....

Kay

[idroba]

look at it this way. The average good battery system might have a 220 amp hour capacity at 12 volts. That is about 2640 watt hours of capacity. Now you don't want to run the batteries down much below 40% so you might have a usable range of 1600 watt hours of capacity.

Normal 13,500 btuh rooftop AC units take about 1600 watts per hour. So the batteries would have a running time of about an hour. Then of course you have to recharge them.

Sorry to say, it is just not an option.

[73shark]

Since the A/C draws 15+ amps at 120V, then it would pull 150 amps plus the inverter loss at 12VDC. Don't think the batteries could sustain that draw for an hour. But maybe I'm missing something.

[DavisBurns]

Our house battery is 100 amp hours and the supplemental is 245 amp hours. Both are AGM. Not sure about the class. We haven't bought the inverter yet but will check the load for the Dometic AC. I understand it draws a lot more power when it starts up. Good to know two out of two think it can't be done resulting in us exercising great caution.

[David M]

Roadtrek E-trek is able to run the AC on batteries, but they use a 5000 watt inverter, 8 AGM 6v batteries, a 245 w solar panel, and 2 alternators. Some even had the Efoy fuel cell generator, though I don't think there were many who opted for this option due to the cost. I'm waiting for Airstream or someone to put a Tesla battery under the floor 😃👍

[73shark]

The Dometic draws 15+ amps depending on the ambient temp.

[lewster]

Here's a little blurb that I wrote some time ago with reference to a Yeti inverter/battery system. When discussing battery capacity for very large loads like an air conditioner running thru an inverter, this little bit of information should go a long way to realizing the extreme limitations of large DC loads, even if you have a very large battery bank and inverter to match.

PM me if you have any specific questions.

PEUKERT:

Sorry to shoot down your aspirations, but your Yeti 1250 is simply a small AGM battery bank coupled to a small (1250 watt) pure sine wave inverter in one very expensivepackage. This is coupled to 60 watts of solar for charging? If it will take 24 hours to charge from a 60% depth of discharge, you are probably looking at batteries with a capacity of about 200 amp/hours (which, BTW, they conveniently omit from the specifications).

I calculate this by using their 60 watt solar array (5 amps max.) during 4 peak charging hours (10-2:00) and the residual for the rest of daylight hours yielding 40 amps to the batteries, 24 hours (3 days) gives you 120 amps of charging.....probably a 200 amp/hour battery pack.

That won't even come close to touching a roof A/C's 120VAC requirements, especially with only a 1250 watt inverter. Then there is a little thing called the Peukert effect:

Peukert's law, presented by the German scientist W. Peukert in 1897, expresses the capacity of a lead–acid battery in terms of the rate at which it is discharged. As the rate increases, the battery's available capacity decreases.
Manufacturers rate the capacity of a battery with reference to a discharge time. For example, a battery might be rated at 100 A·h when discharged at a rate that will fully discharge the battery in 20 hours. In this example, the discharge current would be 5 amperes. If the battery is discharged in a shorter time, with a higher current, the delivered capacity is less. Peukert's law describes an exponential relationship between the discharge current (normalized to some base rated current) and delivered capacity (nomalized to the rated capacity), over some specified range of discharge currents. If the exponent constant was one, the delivered capacity would be independent of the current. For a lead–acid battery however, the value of k is typically between 1.1 and 1.3. It generally ranges from 1.05 - 1.15 for VRSLAB AGM batteries, 1.1-1.25 for gel, and 1.2-1.6 for flooded batteries.[1] The Peukert constant varies according to the age of the battery, generally increasing with age. Application at low discharge rates must take into account the battery self-discharge current. At very high currents, practical batteries will give even less capacity than predicted from a fixed exponent. The equation does not allow for the effect of temperature on battery capacity.

Simply put, the capacity of any battery bank decreases exponentially with the increase in the draw from that battery bank. You would need a minimum of an 800-900 amp/hour battery bank coupled with at least a 2800 watt sine wave inverter. To keep this charged properly with solar, you would also need 800-1000 watts in your solar array. I have placed 800 watts on a newer 27' Airstream in the past, but that was about the limit. A 30-34' trailer could get 1000+ watts using the proper panels (AM Solar GS-100 to be precise).

Still, from all of this, you will still only be able to run a roof A/C for less than 2 hours after doing the Peukert calculations. PM me for the actual calcs. if you like, but basically you reduce a 900 amp/hour battery bank (6 Lifeline GPL-6CT AMG golf cart batteries) to 50% depth of discharge maximum draw. This give you a usable battery capacity of 450 amp/hours.

At a 16 amp draw (for the compressor and fan of a 15K BTU roof A/C), you would expect that you could run that A/C (not even counting inverter loss) for 450 amp/hours divided by 16 amps =28 hours. But because Peukert shows an exponential drop in battery capacity based on the higher amp draw, you actually will reduce the capacity of this 900 amp/hour battery bank to a mere 30 amp/hours, giving you less than a 2 hour run of your A/C.

Class dismissed!

PS: I've tried it and it works out pretty closely to the formula!!!

[overlander63]

Originally Posted by DavisBurns

We have a 2004 Interstate. We've added an extra house battery to power astrophotography equipment and a solar panel to charge both batteries (house and extra). We try to camp where it's cool at night but occasionally we need the AC. It isn't too noisy when we have shore power but it is too loud to sleep if we have to run the generator. Has anyone else tried using batteries/solar to run the AC? We took the microwave out which is the only other big energy hog.

Simple, effective solution: Buy some earplugs and use the generator.

[DaveFL]

Besides load draw on batteries you also have to determine the load needed, it will depend on insulation and sun. Using 4 hrs of peak solar radiation for collection also puts you in the rays for heat, put it in the shade and watch the ampmeter reduce output at collectors. Instead of earplugs take some of the money saved and super insulate for heat and sound and try to create some shade, install fans to move air for comfort.

[Boxster1971]

Originally Posted by overlander63

Simple, effective solution: Buy some earplugs and use the generator.

That's what I did in Key West last spring when I was dry camping and needed AC to cool down the van before sunset.


- - Mike
2013 Lounge EXT on 2012 Sprinter

[vinstream]

Fact: I can run my air off my inverter! 3000 watt inverter / 255 watt battery / 200 watt solar
HOWEVER: Its like watching the fuel gauge in a 70 muscle car head to empty!
I have researched this and have concluded that even the most efficient air still draws more than what a battery with solar can produce on both limited space on the roof and weight and size of battery.
I can produce 200 watt of solar and have 255 watt hour battery and in direct sunlight can run my air for only 45 min before my preset cut off of 11.5 volts cuts off the power to the air. That is why I also have installed a 3000 watt LP Gen to run the air....
Fact: based on real world situation you would need 1000 watt of solar and 1200 watt hour of battery(s) storage to run your air continuously for 7 hours... You don't have the roof space for 1000 watt of solar???? nor do you have the space or weight (800 lbs) for batteries in your coach?

[wahoonc]

While I love using solar and battery banks the best solution is generator. If the noise of the onboard generator bothers you buy one of the quiet run gas or lp versions. I prefer the current generation of Yamaha. It costs less in the long run than an equivalent solar system.

While you can run air off of a solar system the sizing and recovery time is just not feasible in most cases.

Aaron

[73shark]

I couldn't find it but there was a thread discussing changing out or adding another muffler to the generator to quiet it down.

[AFUser]

You can't do and if you try, unless your preset cutoff is working properly (and sometimes they can and do fail) you could ruin your batteries.

[lewster]

Originally Posted by AFUser

You can't do and if you try, unless your preset cutoff is working properly (and sometimes they can and do fail) you could ruin your batteries.

If you're talking about deeply drawing a battery bank to well below 50% depth of discharge, this alone will not generally damage batteries, as long as they are re-charged to full immediately after the deep discharge. It is leaving batteries in a discharged state for extended periods that cause sulfation and battery failure.

[DavisBurns]

Thanks to all who pointed out the futility of this option. We don't like the heat so we park in the shade and travel when and where we don't need AC often. When we stay in one place, we have aluminet fabric we use to reflect the heat away. It works pretty good.We use fans when possible and keep flexible 'blue ice' in the freezer. Since we cannot sleep with the generator running and the AC on, we mostly run it an hour or so to cool it off so we can sleep. We live in the west and humidity isn't a problem. A swamp cooler might work but that's another project. A portable generator would work but that's another piece of equipment to store in our tiny van. Wouldn't it be nice if there was a door and storage for the portable unit where the builtin unit is?

[Protagonist]

Originally Posted by DavisBurns

A portable generator would work but that's another piece of equipment to store in our tiny van.

A Sprinter van is only tiny by way of comparison to a larger RV, but the lack of storage space is a problem. For a portable generator, there's only one viable choice of storage location, a hitch-mounted cargo tray or cargo box. You don't want to carry any gasoline-fueled device inside your living spaces. You also want to be able to carry enough fuel to run the generator for your entire stay so you don't have to break camp just to make a fuel run. A cargo tray or (vented) cargo box allows you to not only carry a portable generator but also a gasoline can to go with it.

If you go the portable generator route, you might consider removing the propane-fueled Onan under the floor. You wouldn't gain any extra storage space under there, but you would save a couple hundred pounds off your curb weight to offset the added weight of the portable generator. Or a decent metalworker might find a safe and secure way to mount extra AGM house batteries where the Onan was…

[Tomzstream]

I opted for not having additional batteries or solar for my RT Sprinter van. I am not crazy about using the ONAN generator, but in a pinch, its there. I just didn't want the weight and I don't consider it a good return on investment.

Surprised no one has suggested adding the 3500 Watt MB engine mounted generator. An easy upgrade that utilizes the existing engine power and diesel fuel. I'd favor that over a separate externally carried generator. You still need a good inverter to use the AC, but certainly do-able. Any MB dealer can add it as an aftermarket item, and yes, its OK to fast idle the engine while using... its designed for contractors work vans. RT offers it as an option on certain E-Trek combos.