AC battery requirements, and 120v vs. 12v systems

[ReluctantTraveler]

I have what I suspect is an extremely dumb question for people who understand electrical systems better than I do, so sorry in advance!

12v AC units (like the Dometic RTX 2000 or Mabru RVSC) seem to use a minimum of 19 amps while running (at their lowest setting), probably around 30-40 when doing more heavy-duty cooling. Both recommend at least 500 amps of battery for use, with cooling capacity of 12,000 BTUs.

The Coleman Mach 3 has a 13,500 BTU capacity, and their own website claims 18 amps of draw while running in desert conditions. They also now offer a soft-start adapter to lesson startup load.

Given that, is there any compelling reason to choose a 12v AC over the Coleman Mach 3, particularly if you're planning to have an inverter in your electrical system already? And how would you calculate the minimum amps required for that kind of setup? Is it the same as with a 12v system, or is there some "formula" to deal with the energy loss from using an inverter?

I'm also considering retrofitting our cheap travel trailer with some LioN batteries and inverter to have short-term off-grid AC until I can build our dream rig.

 

[clydeps]

For a given cooling output (BTUs) the power draw will be roughly similar for 12V vs 120V systems, but the amp draw will be different (amps at 12V will be 10x amps at 120V for the same power.)

12V A/C is much more expensive to buy, assuming you are installing an inverter either way, but the power usage will be a little less than a 120V unit with inverter since with 12V (assuming sufficiently massive cables to the battery) you will save the conversion loss of the inverter.

I personally don't think that 12V A/C is worth the small saving in power for the big difference in price.

Either way don't expect to run A/C for more than short periods from a battery, unless you have a really big one. Then the challenge is recharging it (assuming no shore power.) I would suggest that to run A/C for a few hours at night you will want at least 400Ah of lithium batteries, and at least 1200W of solar panels. Add that up with a decent inverter and MPPT controller and it's not a cheap setup.

 

[ReluctantTraveler]

For a given cooling output (BTUs) the power draw will be roughly similar for 12V vs 120V systems, but the amp draw will be different (amps at 12V will be 10x amps at 120V for the same power.)

12V A/C is much more expensive to buy, assuming you are installing an inverter either way, but the power usage will be a little less than a 120V unit with inverter since with 12V (assuming sufficiently massive cables to the battery) you will save the conversion loss of the inverter.

I personally don't think that 12V A/C is worth the small saving in power for the big difference in price.

Either way don't expect to run A/C for more than short periods from a battery, unless you have a really big one. Then the challenge is recharging it (assuming no shore power.) I would suggest that to run A/C for a few hours at night you will want at least 400Ah of lithium batteries, and at least 1200W of solar panels. Add that up with a decent inverter and MPPT controller and it's not a cheap setup.

Thanks!

For my dream rig, the plan is to have a big LioN battery array (600-800 ah) plus a bunch of solar, as well as DC-to-DC charging from the truck (with an auto on-off setup to use the trucks a generator if the batteries get too low).

For now in our travel trailer, I wanted to add some LioN batteries and an inverter (to be reused in the dream rig) just so that we can run the AC when we stop to eat lunch in 90F weather or whatever.

Either way, I expect we're looking at $10k+ US, and that's fine if I can reuse it later in a future rig. I'm trying to maximize use of "what we have now" while I save up for my future thing.

Thanks for sharing some numbers. I appreciate it!

 

[clydeps]

Yes, running A/C during the day is feasible from solar/battery. The good thing is that if it's hot you probably have sun. I have a Truma A/C and 1200W of solar and that will run fine from the inverter with the solar keeping up so the battery stays close to 100%.

Having said that I've only every run the A/C a few times - good insulation, airflow (fans) and as little clothing as you can get away with deals with most hot weather.

BTW I don't rate the Truma very highly. The Dometic Ibis 4 looks better - it uses inverter technology so should be quieter and have lower startup current. I have not used it though (and not sure if it's available in the US.)

 

[1000arms]

Do you want vehicle height increases and hole(s) from a roof-mount air conditioner?

12 volt air conditioners probably aren't as common as many 120 volt air conditioners. If you find yourself needing to fix/replace one in a hurry, easily sourced units/parts become very important.

 

[Peter_n_Margaret]

12V A/C is low volume niche market stuff. Expensive and inefficient.
The most efficient, high volume A/Cs are all 120V (or in our case 240V AC). They have inverter motors.
Look at the Mitsubishi Heavy Industry small AC split units. They are the air conditioners of choice for motorhomes in Oz, ESPECIALLY if you want to run them from inverters and batteries.
Cheers,
Peter
OKA196 motorhome

 

[andy_b]

I have bought but not installed this 12 volt mini split:
I have no connection to this product other than to offer another option. It does not need to be mounted on the roof and it also offers heating functionality since it is technically a heat pump rather than traditional AC only.

Initially, our camper was not going to have an inverter. That and the ability to keep the AC off the roof and have heat made this unit seem compelling. Still won't be able to install it for a bit so who knows if I chose correctly.

 

[KiwiMtnClmbr]

I have bought but not installed this 12 volt mini split:
I have no connection to this product other than to offer another option. It does not need to be mounted on the roof and it also offers heating functionality since it is technically a heat pump rather than traditional AC only.

Initially, our camper was not going to have an inverter. That and the ability to keep the AC off the roof and have heat made this unit seem compelling. Still won't be able to install it for a bit so who knows if I chose correctly.

Any progress or update on the mini split Andy?

 

[andy_b]

Any progress or update on the mini split Andy?

I wish! It is still patiently waiting while the rest of the install progresses. I'll definitely update when I have something useful to report.

 

[KiwiMtnClmbr]

Don't feel bad. I have a Mabru rooftop unit waiting still in the box for me to finish ordering materials, to begin making panels, to begin assembling panels... I'm stuck on the design challenge right now however of a very small roof that desperately needs as much solar as possible and a 6" tall AC unit which is likewise desperately needed in my current environment (West Africa). A wall/vertical mounted unit would be soooo much easier to accommodate and not involve holes in the darn roof that I'm spending so much time and brainpower trying to make insulated, strong and light somehow all at the same time.

 

[Mfitz]

I don't want to hijack this thread, but it got me thinking. If I had a truck camper with a typical Coleman Mach _____ ac unit and sufficiently sized inverter, and I was running a DC-DC charger from my truck, could I run the AC unit while I drove so that the camper wasn't 1,000 degrees when I stopped? It wouldn't run while the truck was off, but while it was on?

 

[ReluctantTraveler]

Don't feel bad. I have a Mabru rooftop unit waiting still in the box for me to finish ordering materials, to begin making panels, to begin assembling panels... I'm stuck on the design challenge right now however of a very small roof that desperately needs as much solar as possible and a 6" tall AC unit which is likewise desperately needed in my current environment (West Africa). A wall/vertical mounted unit would be soooo much easier to accommodate and not involve holes in the darn roof that I'm spending so much time and brainpower trying to make insulated, strong and light somehow all at the same time.

I'll be REALLY curious to hear how the Mabru install goes. Looking at that same unit for our build.

 

[ReluctantTraveler]

I don't want to hijack this thread, but it got me thinking. If I had a truck camper with a typical Coleman Mach _____ ac unit and sufficiently sized inverter, and I was running a DC-DC charger from my truck, could I run the AC unit while I drove so that the camper wasn't 1,000 degrees when I stopped? It wouldn't run while the truck was off, but while it was on?

I did the math on this, and even the lowest consumption Coleman Mach AC runs for about 1/4 of the time per amp hour that a good 12v AC does.

 

[rruff]

I did the math on this, and even the lowest consumption Coleman Mach AC runs for about 1/4 of the time per amp hour that a good 12v AC does.

I don't know what that means... but power is in watts and energy in watt-hrs. 12v and 120v should be very similar if they are doing the same job. If they draw ~18amps at 12v then that's easy for an alternator to supply.

 

[ReluctantTraveler]

I don't know what that means... but power is in watts and energy in watt-hrs. 12v and 120v should be very similar if they are doing the same job. If they draw ~18amps at 12v then that's easy for an alternator to supply.

I understand energy is in watts. Most batteries are in amp hours, so you need to do some math. Let me break it down for you...

The entry-level lower-usage Coleman Mach, the Mach 3 Plus, draws about 15 amps at 120 volts (at max power). That's 1800 watts. The Mabru draws 55 amps at 12 volts. That's 660 watts.

That means the Coleman Mach draws 2.7x more watts per hour as a dedicated 12v unit. My guestimate above, 1/4, was wrong. It's closer to 1/3.

But that still means that a 12v AC system is going to be far, FAR more efficient if running off battery. It means you can run longer, and without an alternator.