How Long Can You Run Your RV AC Off Batteries?


Of all the questions I’ve been asked about RVing, this one is probably the most frequent.  Ever since we installed large lithium batteries from Lithionics in Lance 5 years ago, that seems to be mostly what people want to know.

Unfortunately, there’s been no way to answer that question without a million qualifiers.  Until now.  In this video, I give you a little sneek peek at our 20k upgrade, and introduce something I’m calling a

Normalized Runtime Estimate.

 

The whole idea behind the NRE (Normalized Runtime Estimate) is to create a metric that does two things:

  1. Allows us, the RVing public, to compare one rig to another using facts – instead of guesses and anecdotes.
  2. Create a “worst case” runtime estimate for any of these systems.  Something that most people should be able to beat in actual use.  It’s your safety number.

I think I’ve done that, and it only requires three pieces of information to get there:

  • The first input is the USABLE battery capacity, expressed in WATT-HOURS. (Bu)
  • The second input is the inverter efficiency (E)
  • And the third is the running watts of the air conditioner in question (L)

 

That’s it!  One multiplication, one division, and we have a standard way to describe these systems.

You can watch the video to get the complete description of how and why, and all the reasoning behind things, but that’s the formula.

I REALLY HOPE

we can start using NRE instead of relying on oftentimes not very relevant experiments or isolated reports that may or may not have anything to do with your particular situation.  The NRE formula eliminates the ‘yeah but’ variables, and brings things down to just plain data.

The next time you’re talking to an RV sales person, or to a manufacturer’s rep… ask them for the NRE of their battery/inverter/air conditioner system.  If we all start asking for it, they’ll have to start providing it.

Remember – someone had to be the first person to ask for “miles per gallon”… and the NRE formula has far less variability than that!

Let’s hear it down in the comments.  What do you think?

 

 



James is a former rocket scientist, a USA Cycling coach, and lifelong fitness buff. When he's not driving the RV, or modifying the RV (or - that one time - doing both at once), you can find him racing bicycles, or building furniture, or making music. In his spare time, he works for a large IT company.


    60 thoughts on “How Long Can You Run Your RV AC Off Batteries?

    1. ArtDeco

      Ok. Thanks for the reply James. I am now going down the rabbit hole on these 12 volt mini splits. The only other things in the RV that takes 110v is the microwave that I’d already decided is going out since we only used it once as a test, and the water heater booster that I don’t need b/c the propane water heater works well enough..
      I Haven’t fought extreme heat yet, but we have lots of miserable 90 plus degrees and high humidity here in the East..
      I expect to tear out some cabnets and put in some sokar anyway.
      Wish me luck and I will report back.
      BTW, James you probably know the reason the answer to the big question is 42. But for other readers, is that is the ASCII decimal code for an asterisk. Like the DOS command *.* , it matches anything.

      Reply
    2. TJL

      In summary, the “worst case” run time is the battery capacity divided by the air conditioner load corrected for:
      – usable battery capacity
      – inverter efficiency

      Correct?

      Reply
    3. Will

      Total geek out discussion!! I love it.

      We have a 12V mini-split Arctic Breeze Truck AC in our Fuso based camper. All the “guts” live under the rig. We’ve found that having a thermal curtain that separates the cab from the house is a game changer. All that glass up front is nothing more than a giant green house in the summer and a giant freezer in the winter.

      And BTW, we rarely use our AC because we have wheels and wheels allow us to pick our climate. Too hot? Head up a mountain, or north, or to the coast. Works like a charm!

      Reply
      1. James - Post author

        I really want to bring the Winnebago-supplied thermal curtain along… but…
        Stef really likes turning the passenger seat around, and that just won’t work with that curtain in place.
        If I can break her of that habit, I’d leave that curtain up the entire time!

        Reply
    4. Don Kane

      BTW, the reason that Batteries are specified in Ah is that their most crucial use is turning over a starter motor that uses 400 A. So a 40Ah battery will only last 40Ah/400A=6minutes of cranking.

      Reply
    5. Don Kane

      Interesting video.

      One thing you are leaving out of the equation is the desired interior temp, which is the whole reason you are running the AC in the first place. Running the AC for 10 h in 40C conditions but having the inside temp at 35C, probably doesn’t cut it.

      On the other hand, Mr Marketing Man could “cheat” the system (and get a high NRE) by just installing a small AC unit, which would run allot longer but not cool the van in desert conditions.

      I don’t think you can put this into your equation, I wouldn’t even try, but the point is, there are boundary conditions that also need consideration. Like what is the highest temp your AC will cool the vehicle.

      And, wanting to get into the acronym sweepstakes, I am thinking: HOT = Hottest Outside Temperature.

      (BTW, run an ultra marathon in 110F conditions? Who would do that?)

      Reply
      1. James - Post author

        You’d be surprised at who’d show up in the desert to run an ultra… (not me though).
        See my answer further down on a similar line of inquiry. Short answer – NRE is just one number, like MPG.
        MPG doesn’t say how fast you can go, or how much cargo you can carry, but we still use it.

        Reply
        1. Don Kane

          Still, I am curious. When your AC is running constantly, what is the temperature difference outside to inside? (In full sun but pointing the van Northward or at least away from the sun to give the poor van a chance….)

          Can you get the interior down to 80F when it’s 110F out?

        2. James - Post author

          When it’s 110, then yeah, 80 is about as far down as the current air conditioner can take it.
          It’s definitely more comfortable inside the RV than out. But you wouldn’t walk in and say “Man, this is cold!”

    6. Randy C Curry

      Thank you James, and your talented camera operator! NRE is the way to go!
      We hope to wave, acknowledge, say Hello, to you both at Hershey if you are planning on being there?

      Reply
      1. James - Post author

        As of now, we don’t have plans to be at Hershey. We are planning on Tampa though!

        (And start asking people for NRE at Hershey!!!)

        Reply
    7. Patrick Broos

      Thank you for bringing a bit of sanity (and proper electrical terminology) to this topic.

      I have tried three times to get Airstream to use proper electrical units in their marketing materials—with no progress whatsoever.

      Reply
      1. James - Post author

        The table base is from Lippert! It’s called “Planet”.
        And yes. It slides in four directions, AND rotates!

        Reply
    8. AlexG

      Excellent video! I know it would take some effort, but I think it would be nice to know what your energy cost is per kWh. Maybe a comparison of the life time cost of the lithum batteries vs. a fuel Generator.

      Reply
    9. Darren Cornell

      James some of your videos, just blow me away. And this one was next level.
      Thank you very much for the education. <3 from Spain

      Reply
    10. Joe McG

      Seems like the hardest part of a 20kWh pack is the recharge. Sure you could run the AC for 14 hours, but it might take a full day of driving to top it back up. I’ve often wished that I could adapt a J1772 EV plug to charge my Boldt. Sometimes in a city during in the warmer months I just want an easy way to get power without having to drive extensively or stay at a campground. Level 2 chargers only run 240VAC, and don’t have a neutral on the J1772 plug to grab just 120VAC. The Volta/Dimensions charger says it only supports 120VAC. I wish there was an easy way to convert split phase 240VAC to single phase 120VAC at 3kW capacity without a giant transformer. Not sure if you’ve ever investigated this or not.

      Reply
      1. James - Post author

        Yep. I’m always on the lookout for faster ways to charge. A larger alternator will help, for certain. But even that would still take a while.

        Using EV chargers would be killer! I’m not aware of a solution for doing that yet. But if one shows up, I’d be one of the first to investigate it.

        Reply
    11. William

      Excellent video James. However when it comes to energy efficiency, the RV industry is in the DARK AGES. As Scott stated, standard RV ACs are very inefficient compared to similar HVAC available for homes, even at 120V. But more important than mechanicals is insulation, double pane windows and air sealing to reduce BTU or heat transfer load on the HVAC. Walls, floors should be at least R18 and ceilings R36+ using closed cell foam and windows a U factor of 0.25. Using a blower door at -50 pascals the RV should have less than 1.0 ACH (air change per hour). Who’s doing this testing? Nobody! My point is put your money into better insulation and air sealing before wasting money on HVAC and expensive batteries. With these changes and an efficient HVAC, using only a few hundred watts per hour, Jame’s 20k watt hour batteries would last for many days, not just hours.

      Reply
      1. James - Post author

        Well, I’m certainly in favor of better insulated RVs, and the EKKO is definitely a step in that direction.
        A better insulated coach wouldn’t affect the NRE, as that’s specifically excluded, but it would make things more comfortable, and could lead to longer runtimes in practice.

        Reply
    12. Robert H

      The formula indicates battery life running an A/C, which is fine as far as it goes I guess, but says nothing about cooling efficiency. Insulation DOES matter, so does the size of the interior of the rig. Class B rigs are smaller than a Class C which is smaller than a Class A,. The more interior you need to cool, the harder it is for an A/C to cool that interior.
      Why would I care how long the battery lasts if I don’t also know the temperature the A/C keeps that rig at. An A/C running full blast lasting 20 hours has no value if the interior stays at 90 degrees. The question isn’t only “how long an A/C runs”… but also “how cool is it so my dog doesn’t die of heat stroke”.

      Reply
      1. James - Post author

        IF you’re looking for a single number that’s the answer to life, the universe, and everything – as far as RV comfort goes… get used to disappointment.

        The NRE will estimate runtiime, and that’s all it’s supposed to do. If we were to try to factor in interior volume, R values, air flow and distribution, and heat gain from radiant, convective, and even conductive sources, etc. etc. etc. the calculations would be virtually impossible. That would preclude almost everyone from actually doing them, and then it’s really not very useful at all.

        Think of this like miles per gallon. You can calculate miles per gallon for a moped. And you can calculate miles per gallon for a moving truck. They obviously have much different cargo capacities, occupancy limits, etc. It’s up to the user to engage their brain to determine which one is fit for their purpose. But they both can be rated on MPG. That doesn’t make MPG useless because you can’t load a couch on a moped. It means MPG is just one parameter for rating vehicles.

        Similarly, NRE is one measure you can use to help gauge the available capabilities of a battery/inverter/air conditioner equipped RV. Since the RV industry has given us literally zero pieces of information so far, I think one impartial number is a step in the right direction.

        Reply
        1. Robert H

          You are absolutely right. It does give an impartial number for runtime at extreme conditions, which at least lets you know a minimal amount of time you can be away from the RV and still have things running. And you are so right about trying to factor in all the variables… even if possible, it would be a waste of time… you could always just test it in the field when you own the RV… just like you have to learn how your car works in city or highway driving or in conditions like rain/sun/snow and anything else.
          But I would disagree with finding a “single number that’s the answer to life, the universe, and everything.” We already know that… it’s 42. 🙂

    13. Michele

      Okay, I am VERY interested in how you are going to configure the seating area on top of these compartments. An L-shaped couch??? One of the reasons we didn’t want the Ekko was those forward-facing automotive seats. Can’t wait to see the final result!

      Reply
      1. Other Steph

        I definitely want to see this as well, Michele! I love everything about the Ekko but those seats. If there’s a way to make a couch there, that’s a mod video I will DEFINITELY watch. (Not that I didn’t watch this one, because I totally did.)

        Reply
    14. Daniel Cogan

      Great job, James. It gives us what we need, in a very basic format. For instance, can I leave my dog in the unit on a hot day for 3 hours? This methodology will provide a safe answer. Thank you!

      Reply
    15. Jared aka Catmandeux

      Thanks for posting this approach as a normalized standard.

      Here’s my data point with a small 12DC minisplit.

      For battery watt-hours I referred to Will Prowse’s test results, where he depleted a fully charged SOK battery to where the BMS kicked in (at 90% depletion). As it turned out, AH times advertised nominal voltage, tracked the test results quite well.

      As I’m using a direct to battery connection without inverter, I don’t have the inverter efficiency hit.

      And then there’s the wattage of the small minisplit system, which the manufacturer published at 300-795W.

      So with my modest budget system of:
      Four 100AH 12.8v SOK batteries
      Fleet welcome / VeloSolar “ACDC” minisplit
      I’m calculating an NRE of 6.44 hour at max load, 17 hour at min load. Which does track well with my testing to date.

      I’m running a much smaller van / RV than you, even smaller than Lance. Promaster 136” WB low roof. We intentionally put the aircon in the bed loft, figuring the heat soak from the cab was a lost cause. But our experience with this system has exceeded expectations.

      Cheers from Charlotte!

      Reply
      1. James - Post author

        Cool!
        I should have run calculations for a couple “off the shelf” RVs in the video.
        We need some kind of repository for those.

        (And yeah – keeping the heat out of the cab is next to impossible.)

        Reply
    16. Graham Smith

      I applaud you for your work on this. For the past 2.5 years, I have been using roughly the same formula to answer the question about A/C runtime in the Travato L’s (Volta system) and found consistently that people did not seem interested in a “worst case” number. People want a “realistic” number – which as you point out doesn’t really exist because there are too many variables. Based on testing, I can tell others what MY estimated usage is in MY Travato, but someone else will likely have a different usage. And telling people what MY usage is usually results in someone telling me I’m full of it because they don’t get anywhere near that. Good luck.

      Reply
      1. James - Post author

        I hear you, Graham!
        For my part, I don’t plan to offer any other runtime information besides NRE.
        We’ll even start using NRE in our reviews of equipped coaches.

        People need to realize, it’s just like MPG.
        Nobody gets exactly what the MPG of their car is in actual use.
        But we all accept it as a yardstick for comparing different vehicles, and we can use it as a data point for gauging what our own “realistic” number is.
        I hope people hang with it until it gets to that point.

        Reply
    17. Kevin Scarbel

      I plan to order an Ekko as soon as the initial run of demo units is completed for the dealer body.

      Did Winnebago and/or Lithionics perform this battery upgrade for you?

      I ask because I want my Ekko to be so equipped. The national park camp grounds I frequent have no hook-ups, and generators are resented..

      If Winnebago was smart………, the company would offer this electrical system upgrade as a factory-built option in 3, 4 and 5 battery fitments.

      I also want a Winnebago option to upgrade from the Xantrex 2000W/80A spec Freedom XC to the 3000W/150A Freedom XC Pro.

      Reply
      1. James - Post author

        I actually completed all the work myself. (And it was kind of a lot of work at that!)
        I don’t think Winnebago has any plans to offer a similar upgrade, but it’s an intriguing idea.
        I had to change the floor plan in order to make this fit.

        Reply
        1. Kevin Scarbel

          I not only applaud your workmanship, but your drive and passion to do the project. Motivation often isn’t spoken on enough. Passion and motivation therein lies a great deal of fun and personal satisfaction.

          In the year 2021, we’re in a new era that requires Winnebago to revise their thought process so as to keep the company relevant in the industry, and boost sales in a time when volumes matter more than ever due to thinner margins.

          A meaningful number of customers today, tech savvy individuals, demand meaningful electrical options. If Winnebago won’t offer them, their competitors will, targeting that sales marketing mistake and profiting from it.

          The business model of yesterday is behind us. Today’s RVers are increasingly not yesterdays. You, Stef and myself for example.

          Today, and tomorrow, requires a different kind of Winnebago that is more attentive, and responds more quickly to customer demand. The Minnie Winnie era, is fading. A very different new customer base is far different, more exacting and demanding.

      1. James - Post author

        Right on!

        I had actually been thinking to make a “Watt-hours vs. amp-hours” video for a few years now. Finally got that incorporated into this one.

        Reply
      2. Art Deco

        Good afternoon James.
        My 2010 Sprinter (ERA) is very similar to your original “Das Bus” only it hasn’t been updated ( or used) much.
        So far I only added a tiny 400w inverter that should do to charge the laptop and (maybe) the eBikes, and replaced the house and coach batteries…still not Lithium, just ordinary deep cycle. I have room for another easily.
        Anyway, I don’t understand why there doesn’t seem to be any interest in 12 volt A/C systems. Between the noisy overhead A/C and the under the bed generator, sleeping without earplugs is impossible.

        Reply
        1. James - Post author

          I think the large cables required to run to the roof kill it for a lot of manufacturers. That, and they’re not THAT much more efficient or quiet, when you get right down to it.

    18. Scott

      You have to move on from these awful inefficient AC units that come standard on RVs and get an inverter unit with variable compressor speed. I did a mini-split conversion and now my AC sips power. I’m talking idling at 3-500 watts once it’s cooled down and that thing is whisper quiet. I can actually put some charge into the battery from my solar with the AC running if I have full sun.

      Now it looks like you can get a DC powered rooftop inverter unit intended for an RV that is probably even more efficient than my setup since you don’t need to worry about inverter losses. I hope you’ll consider looking into an efficient AC upgrade mod given how much coin you’re already dropping on this project. I’d love to see the results.

      Reply
      1. Kevin Scarbel

        Good point. Many heavy trucks around the world are using mini-splits in the bunk sleeping area for exactly the energy efficiency reason you point out.

        Reply
      2. James - Post author

        I would consider a DC powered unit, if I could figure out how to run the large-gauge cabling up to the roof without it looking ugly. Although, the DC units available here today seem to top out at about 11k BTU. Our current unit is 13.5k, and I kind of wish it was a little bigger. (110 degrees…)

        If something comes available in the future, the AC swap is super easy for me now.

        Reply
        1. Jared aka CatManDeux

          Regarding cabling, we opted to mount our minisplit compressor under the van. The supplied 12V electrical cables weren’t as thick as expected, plus they were limited to a 10’ run. Routing the Freon hoses took more space and planning, but we had several options. Ultimately we did put a hole in the floor, figuring it was more manageable than a hole in the roof.

        2. Scott

          ” if I could figure out how to run the large-gauge cabling up to the roof ”

          It’s unfortunate that you are stuck on 12V. It becomes pretty impractical when you get to the size system you’re running. Is it too difficult to swap that alternator to a 48V version like the Volta system uses? Then you could run a 48V air conditioner. Might even be able to use the factory romex that’s already run for the A/C. Adding the 12V step down converter for the other stuff is pretty trivial.

          Alternatively, instead of running a crazy thick gauge wire up to the roof for a 12V A/C, could you do multiple runs of 8 ga wire that would be easier to conceal?

        3. James - Post author

          Considering I just got this done, I’m not too motivated to swap the alternator, inverter, batteries (I have 5, no way to get them to 48v without giving one up.), and add the 12v step down, disable the chassis jump start, blah blah blah…
          For me to even consider swapping the AC at this point, there would have to be some real demonstrated benefit. Like 40% more runtime at greater BTUs or something.
          Without Beverly Hillbillying half of a mini-split to my rear bumper.

    19. Don Wilson

      Thanks for the NRE. I tried figuring the runtime out, but was lacking the proper/needed numbers to calculate it. As we live in the northeast, and have not traveled farther than Minn. west. This is a great number to start with. And as we are getting an Ekko with 2 batteries/no Gen., I feel we will be fine to get used to how GoGo does before we do make trips out west or South in hotter weather real world use. Keep the info coming, as I follow soaking up as much of your (both you and Stef’s) information as I can.

      Reply
    20. Liz

      Ok so I don’t own an RV but I live via YouTube RV people. This energy lesson was so thorough and taught so nicely. Now I want an RV really badly. Good job James! Say “hi” to Stephanie. I wish I could comment about this on YouTube .

      Reply
    21. Jim

      Maybe consider forwarding this to NRVTA that teaches owners, owners, technicians and rv inspectors. Could be a way to standardize the industry. Totally geeking out over the install. Amazed you got it in such a small spot. So jealous! You guys need a partnership savings code for lithionics and victron.

      https://nrvta.com/

      Reply
      1. James - Post author

        Interesting idea on the NRVTA. Even if they just joined the campaign against amp-hours, I’d be pretty stoked!

        Reply
    22. Ian F

      Some serious nerdiness here. Good stuff! I look forward to the rest of the battery upgrade videos so I can get more ideas to apply to my future van build.

      Reply

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