AC battery requirements, and 120v vs. 12v systems | Expedition Portal

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 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. 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 W of solar panels. Add that up with a decent inverter and MPPT controller and it's not a cheap setup.

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 W 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! 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 W 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.) 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.

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?

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.

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.

Here's my take

This will b a little techie. If you're not up for techie, feel free to skip it.
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Dave -

The RTX specs say " watts/ BTU @ 12Volts". What does that mean?

At 12 VDC, watts is 167 amps. That is a whopper of a current draw! But watts can't mean 167 amps of current, since the unit's fuse is only 80 amps. In fact, the spec says that the unit 's max draw is 58 amps at 12 VDC. This means that the max electrical power used is 58 X 12 = 696 watts from the power supply, not watts. And running 58 amps through an 80-amp fuse is a reasonable number, so I tend to believe it.

At the same time, it is worth noting that Dometic also offers several 13,500 BTU RV rooftop air conditioners, operating from AC power, for about $-. These units provides twice as much cooling as the RTX (13,500 BTU vs BTU), for laboutn half the cost - about $ lower cost.

So the "big difference", the "new technology", is that the RTX unit operates directly from 12 volts DC, while an ordinary Dometic RV air conditioner operates from 120 volts AC. This might be useful if you always operate solely from solar and batteries. On the other hand, you could buy a -watt Renogy pure-sine power inverter for about $400, run the ordinary RV air conditioner from 12 volts, and save several hundred dollars. You could run the A/C on LOW COOL and get similar cooling capacity at a bit lower power consumption, or on HIGH COOL to get twice as much cooling at higher power consumption. Your choice.

The bottom line is that electrical watts are not the same as thermal (cooling) watts. Dometic's description is poorly written and confusing at best, and I am not sure where they are trying to go with this product. It is definitely an interesting device - thanks for flagging it to us, Dave. For TM use, though, I see no advantage in the RTX, and substantial added cost. According to the write-up, Dometic's target audience for the RTX is long-haul trucks, where it might be worthwhile. I find it interesting, though, that most long haul trucks have 24-volt or even 48-volt electrical systems, not 12 volts.

Bill Hi Bill,

Yea, I'm not sure what the advantage is of running a 12VDC A/C unit directly through the battery vs a 120VAC A/C unit through an inverter. Clearly, you'd avoid the 10-20% inverter loss, which is important if you're running it alot, but other than that I'm not sure what the cost premium gets you.

Interesting too about your power calculations....I'm not sure where the 2kw number comes from.

But, broadly speaking, I think this is a sign of the way things boondocker units are headed. I saw a van conversion recently that was 100% electric / battery. Zero propane. Induction cooktop, electric water heater, electric furnace (of some sort), DC fridge, etc. I think they had something like an 800Ah battery bank, and 600W of solar, and found that to provide essentially unlimited power for their needs. Dropping two 20-lb tanks of propane frees up about 100 lb, which is enough for over 300Ah in batteries alone, so the additional weight really isn't even that much.

Dave