By Mike Sokol
All the pieces of my solar/battery-powered air conditioner experiment are coming together nicely. Here’s the first lab data I gathered last week for proof of concept.
To be clear about how I create demonstrations, many times I’m integrating multiple pieces of gear together in strange ways that have never been tried before, which means I’m the first guy to try that combination.
So I first create a hypothesis on how I think it might work, design a mathematical model predicting the results, then run a test to gather empirical (real world) data. Then I compare the results of this data with my predictive model to see how close the reality is to my prediction.
Is it real, or just my ‘magination?
Sometimes reality agrees with my prediction perfectly, while other times it’s off by an order of magnitude or more. If I do nail the number, that suggests I probably understand what’s happening in the circuit. But if that number is way off, the implication is that I missed something in the calculations and need to rethink my mathematical model. To get in the mood, feel free to listen to “Just My Imagination” from the Temptations by clicking on the picture.
Math (slide) rules!
In this case, the data I’m gathering is matching nicely with my hypothetical model, which means there’s probably no easy (read cheap) way to run a rooftop air conditioner from storage batteries in your RV. But it’s not impossible, and it’s getting closer to affordable.
What’s the experiment?
For Phase I of this experiment I’ve connected a 100 amp-hr Lithium-Iron battery (this one from Briter Products) into a CarGenerator™ SL-2000 Hybrid Inverter, and used it to power my Dometic Penguin II 15kBTU air conditioner. I finally moved it off of my bench to outside on the back deck in order to reduce the noise and wind blast of an air conditioner running for hours next to my ear.
However, I quickly found that the SL-2000 Hybrid Inverter was having trouble starting a 15kBTU air conditioner compressor due to the high inrush starting current of 50+ amperes. But luckily I still have my SoftStartRV™ controller at the ready, and quickly connected it back in the circuit using WAGO lever connectors (I love those things). The inrush starting current reduced from 52 amps down to 24 amps, and the 2,000-watt inverter was able to start the 15kBTU air conditioner dozens of times.
I contacted CarGenerator’s tech support about this product, and they noted that the SL-3000 Hybrid Inverter would be a better choice for those wishing to run an air conditioner, but the SoftStartRV controller still might be a good idea simply because it reduces the starting inrush current to less than 50% of a stock starting capacitor.
I used a Southwire True RMS clamp meter to monitor both the current on the 120-volt AC side feeding the air conditioner, as well as the 12-volt DC side feeding the Hybrid Inverter. The Briter Products Ion-Ready battery also has an on-board display showing the voltage, outgoing current and percentage of charge available, so I used that for measurements as well. Since this is a Lithium Iron Phosphate battery it’s safe to discharge it down to 0%, unlike a standard flooded-cell battery that you want to discharge no lower than 50% of capacity.
You can see from the Southwire clamp meter that this air conditioner requires around 13.5 amperes of current at 120 volts AC while the compressor is running, which forces the battery to supply up to 170 amps of current at 11.8 volts DC. That’s a shot of the battery monitor itself.
My prediction was this single 100 amp-hr Lithium battery could power the air conditioner at 100% duty cycle (the compressor running all the time) for around 35 to 40 minutes, and that’s exactly what my initial field data confirmed.
And with a 50% duty cycle of the compressor running 1/2 of the time, and the fan running all of the time, I predicted 60 to 70 minutes of operation from a single battery. And indeed I was able to get 65 minutes of run time while manually cycling the compressor on and off 50% of the time every 5 minutes. Yes, now I have to build a 50% duty cycle thermostat controller so I don’t have to do this by hand. My work is never done…
What does this mean?
My first impressions of this technology are that it holds enormous promise for those of you wishing to boondock but still be able to run an air conditioner at night for sleeping comfort. But the numbers imply that you’ll need one 100 amp-hr lithium battery per hour of air conditioner run time. So two lithium batteries gets you 2 hours, 4 lithium batteries gets you 4 hours, and 8 lithium batteries gets you 8 hours of air conditioning. That’s 800 amp-hrs of current at 12 volts, which works out to 9,600 kWH of stored energy. This number agrees with the amount of battery capacity in the Winnebago Travato’s Volta storage system, which requires the equivalent of eight 100 amp-hr batteries for 8 hours of air conditioning. However, this is still pretty expensive for running an air conditioner overnight.
But wait, there’s more…
What I didn’t already write about is how this hybrid inverter and SoftStartRV combination can allow you to mooch-dock in a driveway powered by a 15-amp outlet by combining (blending) battery power with available shore power. So you can set the maximum current it’s allowed to pull from the shore power outlet to 13 or 14 amperes or thereabouts. at that point the limited shore power should be able to fully power the 14 amp requirements of the air conditioner while it’s running without adding in battery power.
However, if you have the SL-3000 unit you could not only run the air conditioner, but other 120-volt appliances as well. So you can also run your microwave, or hair dryer, and/or entertainment system for a limited time, all from a single 15-amp driveway outlet. Now, when the inverter needs to supply 25 amperes of 120-volt current, it will use 14 amps from the shore power cord, and make up the other 11 amps of 120-volt AC current by pulling 130 amperes from the battery, inverting it to 120-volts AC, and adding them together. A single 100 amp-hr Lithium battery could supply this extra backup power for at least 30 minutes or more. Talk about cool stuff…
Then, once the RV power requirements drop below your selected maximum current shore power settings, the Hybrid Inverter will seamlessly recharge the battery back to full capacity, all without exceeding your top current setting of 14 amperes from shore power.
“Just one more thing…” (yes, from “Columbo”)
My next experiment will demonstrate how I can use my vintage 1,000-watt Honda EX-1000 generator to power a 15kBTU air conditioner, or even utilize a 1,000-watt CarGenerator Inverter to power this all from my Nissan Frontier pickup truck for solar power backup. And this would be great for emergency power when hurricanes or fires leave you without power for weeks. I’ll post the results of these additional experiments in the next few weeks.
Yes, I think that the CarGenerator SL-2000 or SL-3000 Hybrid Inverter and SoftStartRV make a great symbiotic team for mooch-docking, even with 1 or 2 lithium batteries. And if you want to go in deep (and have deep pockets as well), then they would work well with 4 or 8 lithium batteries.
Find out more about CarGenerator products HERE.
I’m having too much fun with these experiments, so stay tuned for more in-depth analysis of how you can boondock with limited power.
Let’s play safe out there….
Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.
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