RVelectricity: Readers’ top 5 questions on running air conditioner from batteries

By Mike Sokol

Dear Readers,

Today I’m going to reach into my mailbag to answer the top 5 questions I’ve received in the last month about running an air conditioner from an inverter in your RV. You can watch me on video by clicking the screenshot, or read a transcription of my Q&A’s below.

1. Do you have any updates on whether it’s possible to run a rooftop air conditioner from a 2,000-watt inverter? Will a SoftStartRV controller help?

Yes I do know more, but my experiments have shown that a stock 15,000 BTU Dometic Penguin II air conditioner draws too much surge current at compressor startup to be able to operate reliably from a 2,000-watt inverter.  If you’re going to be running a stock RV air conditioner you’ll probably require a 3,000-watt inverter just to handle the 50+ ampere starting surge. However, if you install a SoftStartRV controller, you should be able to run your air conditioner from a 2,000-watt inverter, just as I have.

2. I have a 3,000-watt inverter with a Lithium battery, but it’s still shutting down when trying to start my rooftop air conditioner. It acts like the battery is shutting off. What’s up?

Many Lithium batteries have a peak current threshold set somewhere between 200 and 250 amperes of current. That sounds like a lot of current, but if you remember from my HRDL study on air conditioner surge current, a typical air conditioner compressor draws over 50 amperes of current during startup. That equates to 500 amperes of current from the 12-volt battery for 100 milliseconds or so, which is enough to kick the battery into self-protect mode. So you either need to add more Lithium batteries in parallel, or add a SoftStartRV controller to reduce the air conditioner surge current to around 24 amperes.

3. Do I need a pure sine inverter to run my air conditioner, or is a modified sine inverter good enough?

Every motor manufacturer I’ve spoken to has warned me that running an induction motor (like the one in the compressor of your air conditioner or residential refrigerator) on a modified sine inverter can result in premature short circuiting of the windings due to the harmonics. So they only recommend a pure sine inverter for these applications, as do I.

4. Will a SoftStartRV controller on my rooftop air conditioner give me more run time from my batteries when running from an inverter?

Perhaps a little bit, but maybe not measurable. The SoftStartRV controller still uses the same amount of energy during compressor startup; it just spreads out the surge current to twice as long as a stock capacitor, which is why it can reduce the peak surge current to less than half. I predict that due to the internal impedance of these battery systems, they’re going to waste more energy during the 50+ ampere, 150-millisecond startup surge of an air conditioner with a stock starting capacitor, compared to the 24-ampere 300-millisecond startup surge of the same air conditioner equipped with SoftStartRV technology. But it won’t be much of a gain, maybe only 1 percent or so. You’ll still need a lot more batteries and solar panels if you want to boondock without a generator and still run your air conditioner.

5. So how many solar panels will it take me to boondock without a generator and still run my air conditioner a few hours each night?

That’s a big question that needs a lot more information before it can be fully answered. Just know that a 100-watt solar panel is good for around 300 watt-hrs per day of energy, so it will likely take 400 watts of solar panels to recharge a single 100 amp-hr battery. A little quick math shows that each 100 amp-hr Lithium battery will run an air conditioner for around 90 minutes at a 50% duty-cycle, so 4 Lithium batteries would run it for maybe 6 hours, if it wasn’t too hot outside. That’s 1,600 watts of solar panels just to run one air conditioner for 6 hours at night. Still a little impractical with current technology, but certainly possible.

For more information on where you can purchase a SoftStartRV controller at a discount, please click HERE.

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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