RVelectricity: Running an air conditioner from battery power – Part 2

By Mike Sokol

This is the big one I never thought would happen, but here it is. Yes, it’s possible to run an air conditioner from battery power alone, at least for a few hours. But is it practical and what does it take to make this happen? I’m beginning an experiment next week to see just what technologies need to work together to make this happen.

But why do this?

Well, many state campgrounds without electric power have strict generator running hours. So while you may be able to run your portable generator to power a rooftop RV air conditioner during the day, what do you do when you climb into bed after generator curfew, and it’s still 90 degrees outside? Wouldn’t it be great if your RV batteries could power the air conditioner for a few hours (at least) until you’re fast asleep dreaming of your next RV adventure?

In my Saturday RVelectricity article in RV Travel, HERE, I detailed all the basic power requirements to make this work. In this article I’m going to show you how I plan to hook everything together.

Let’s start at the beginning of the power chain with the batteries. While it’s certainly possible to use flooded cell or AGM batteries for this upgrade, they’re not really practical since you would need twice as many batteries that can only be discharged to 50% of capacity compared to using Lithium batteries that can be safely discharged down to 0% of capacity. I’m using a Briter Products 100 amp-hr battery for this demonstration, but there are a number of quality Lithium batteries you could also use from a variety of manufacturers.

To convert the 12-volts DC to the 120-volts AC that your air conditioner needs, you’ll need a hefty inverter of at least 2,000 watts, and 3,000 watts would be better. I’m using a CarGenerator™ SL-2000 Hybrid Inverter with a 125-amp charger for this experiment.

If you’re not interested in combining multiple power sources, then a standard 2,000-watt inverter could suffice. But it needs to be a pure sine inverter, not modified sine, since the harmonics of a modified sine inverter will turn into extra heat inside of your compressor motor windings, possibly damaging them. So a pure sine wave inverter of at least 2,000 watts is needed with 3,000 watts being better.

Of course, there’s going to be some serious current draw from the batteries (150 amps or so while the compressor is running), which means the inverter needs to be mounted as close to the batteries as practical, with large gauge wiring. Don’t throw away those precious watts due to a voltage drop in a too-small conductor.

I’m also including a DC to DC solar charger from REDARC for this experiment, even though it’s not totally necessary for nighttime air conditioner operation from batteries. However, it will allow your batteries to begin recharging as soon as the sun comes up. How fast that occurs depends on the number and size of your solar panels, but I’m not going to cover that at length in this first article. However, that is part of my experiment and I’ll make a few WAGs (predictions) at the end of this article.

I’m also installing a SoftStartRV™ controller on a Dometic Penguin II 15 kBTU air conditioner. As you can see from my HRDL diagram, this lowers the peak starting current of the compressor from 52 amperes down to 24 amperes of current, reducing the strain on your inverter. It also quiets the big BANG of the factory start capacitor cranking up in the air conditioner compressor while you’re trying to sleep.

Even though I have a box of hard-start capacitors from Supco, I’m not going to include them in the first part of this demonstration. My preliminary experiments and HRDL current data show that hard-start capacitors won’t do anything to reduce the startup noise of the compressor since they don’t reduce starting currents at all. If anything, they increase the length of time that start winding on the compressor is engaged from 150 milliseconds to 1.5 seconds (the green graph at the top), while still pulling 52 amps of peak current. However, because many of you have asked for my analysis of how hard-start capacitors work, I’ll run a dedicated Supco hard-start capacitor experiment later.

Nope, I firmly believe that a SoftStartRV controller that can ramp up the starting current to around 24 amps (gray graph) is the best technology for this experiment, especially if you want to sleep peacefully at night.

As you can see from the diagram, this is a pretty standard electrical hookup with a few specialized power distribution paths. Since I’ll be using a CarGenerator SL-2000 Hybrid Inverter for power it will allow me to segue into Part 2 of this experiment the following week using limited AC amperage from a 1,000-watt CarGenerator power source or a 1,000 watt Honda EX-1000 generator. The SoftStartRV is a must-have for any air conditioner you want to run while sleeping, and it certainly reduces the stress on the rest of the electrical system by lowering the compressor peak starting currents by more than 50%.

So what are my predictions? Well, I think that a 100 amp-hr Briter Products Lithium battery on a 2,000-watt CarGenerator Hybrid Inverter will run the air conditioner with a SoftStartRV controller for around 40 minutes at 100% duty cycle, and perhaps 75 minutes with a 50% compressor duty cycle and the fan running full time.

My 200 watts of solar panels should be able to recharge the batteries to about 50% of battery capacity on a sunny day since most solar users say a 100-watt panel can provide around 300 watt-hrs of energy per day. That suggests 600 watt-hrs per day charging a 1,200 watt-hr capacity battery. (See how easy this gets when you change everything into watts?)

See you next week with some very interesting empirical data. In the meantime, let’s stay safe out there…

 ##RVe34