Running AC with a Small Generator

AnOldUR

Running an air conditioner from an inverter is not practical, because of battery life. Also, there's no way to power the Air8 from a little generator like the Baja 700/900W. What about plugging the Air8 into a large enough inverter connected to the batteries, then plug the T@B's shore power cord into the little generator. Would the inverter handle the AC startup and running requirements while the generator did its best to maintain a decent level of charge.

There's probably something seriously wrong with this, but someone once said that there are no dumb questions. We very, very rarely use our AC and have no plans to take this approach. I'm mostly just curious if or how well it would work?

I put this question out there at the end of another thread, but it either got lost there or is not worthy of discussion. Trying one more time here on its own before letting it go.

MuttonChops

So why make it complicated?
If OK using a propane/gasoline/diesel generator for part of the power (if even possible) why not just purchase a higher output propane/gasoline/diesel generator in the first place.  The single correctly sized generator will also have a far lower initial purchase cost.

Grumpy_G

 There's a lot of efficiency loss between the converter turning 120V from the generator into 12V and then the inverter turning 12V into 120V again. You might still end up draining the battery albeit slower. 

Curious why the change for the AC ? The older T@b 320s have a simple 500W window air conditioning unit which runs just fine off a small 800W inverter generator (at least for me).

AnOldUR

@MuttonChops The Baja total cost was $260. It weighs 25lbs and is small and easy to carry on the roof rack of our 2D Wrangler. I'm happy for T@B owner with larger, more capable tow vehicles, but that's not me. I bought the Baja as a small footprint way to charge our batteries in the case of consecutive days of poor solar. An emergency backup. Just curious if it is capable of more?

AnOldUR

Grumpy_G said:

Curious why the change for the AC ? The older T@b 320s have a simple 500W window air conditioning unit which runs just fine off a small 800W inverter generator (at least for me).

Our 2020 320S has the Air8 that has a total input current of 10.5 amps. I'm not sure of when the change was made.

edit to add:
Just took a look at our Progressive Industries EMS readout with the AC turned on. The Air8 draws 7 amps with the fan and compressor running.

MuttonChops

Okay I played with this idea a little and IMHO it is just not practical as a huge battery bank would be required.  As @Grumpy_G noted there are several energy loss points but even under ideal power conversion the trailer battery bank would just be killed.

Baja would be powering the WFCO Power Center Convertor.
Baja would be outputting a steady ~ 600W.  It would not increase output during the Air8 compressor start-up {32 A @ 115VAC} as the WFCO is limited to a maximum output of 475W.

A 2000W continuous, 4000W peak Invertor might/should handle the compressor startup demand of 3680 W. Then while the Air8 is running compressor+fan at ~10.5A it needs ~1210 W.  For the Invertor to output 1210W its 12VDC input side will have to be ~1344 W.  1344 W minus 475W (best case) from WFCO means the Battery must supply ~ 870 W.  That is around 65 Ahr . . . . a huge hourly demand.

All my math is back of napkin and different VDC values {12.5, 13.5} may have been used for different calculations.  But general magnitude of the battery load is correct.

MuttonChops

As for a larger output generator.
There are several in the 50# range that cost under $400.
My i2000 (1800 W continuous) is carried on the TaB boondock platform.
Can run the Window A/C and WFCO at the same time.

AnOldUR

@MuttonChops thanks for taking the time to help with this. It has my brain spinning but that's a good thing. I'm following most of it, but the one thing that has me confused is your, "For the Inverter to output 1210W its 12VDC input side will have to be ~1344 W." Why the difference? 10% efficiency loss?

There are a couple of things that would work in favor of this. First is that my EMS shows that the Air8 is drawing around 7 amps with the fan and compressor running. Second is that solar would offset the deficit a little more.

For the most part this is just a numbers game that caught my interest. My biggest mistake was not knowing that the WFCO is limited to 475W of output. I've had adding an inverter in the back of my head for a while. Maybe I was just looking for additional justification beyond the obvious.

Thanks again.

Denny16

The Air8 AC was introduced on the 2020 model year TaBs.
 Cheers 

MuttonChops

AnOldUR said:
. . . "For the Inverter to output 1210W its 12VDC input side will have to be ~1344 W."
. . .  Why the difference? 10% efficiency loss?

A high quality (price) Inverter could have a 90% efficiency rating; so 100W DCV in = 90W ACV out.
Many inverters on the market have efficiency rating in the 80-85% range; making our little back of the napkin mind game nearer ideal for available power.

The WFCO also has an efficiency of about 80% as it needs 600W to generate 475W.

AnOldUR said:
. . . EMS shows that the Air8 is drawing around 7 amps with the fan and compressor running.
. . . Second is that solar would offset the deficit a little more

If the Air8 is actually using 7 Amps and all other conditions remain the same in the original calculations then the battery bank drain will change from 65 Ahr to around 31 Ahr; reducing the battery drain around 50%.

Having one or two solar panels will help under ideal conditions but cannot be counted on.
Say you have one 100W Ideal Rated panel, it's ideal output will be 4.7-5.3 Amps depending on the brand.  The panel will never operate under the ideal conditions to set its marketing Wattage output.  Even in direct sunlight lets pick an output level of 3 Amps ( 3 Ahr) . . . your best case battery drain is still in the 28 Ahr range.

          No matter how fancy the battery/inverter/solar panels & controller within a very short
          time period the battery bank will be dead (at an unsafe level to continue using).  Now how
          many hours of prefect sunlight will be needed to recharge that battery bank?

          Anyone that needs/wants A/C while boondocking should just bite the bullet
          and purchase a correctly sized generator . . . and in this threads mixed power
          source situation there is already a noise source generator running . . . a slightly
          larger one won't be noticeably louder.

Maxcamp8

FYI if you're thinking of just running direct from generator and bypassing the converter.
On our 2021 320 the Air8 had a 120v outlet in the Alde compartment, however the Air8 interface is wired from 12v circuit #2 from the (former Wefco) converter.  
Our little Ryobi 1200w max genset runs the Air8 (via trailer input) just fine the few times we've needed it to.

AnOldUR

Damn @Maxcamp8, you had me excited. With the high startup draw, I couldn't see this working, but I tried anyway. With the Air8 plugged directly into the Baja, the fan mode would work, but compressor wouldn't fire up. It didn't shut down the generator the way I had expected. I'm kind of curious of what type of protection the Air8 has? When I press the fan only button it appears like everything is working, but when switched over to cooling the fan stops. The panel stays lit up with no error messages, but nothing else. Do you have a soft start?

Maxcamp8

The 1200w genset ran the AC, fridge and lights just fine before installing the Soft Start in anticipation of future battery/inverter upgrade.  

See prior thread:
https://tab-rv.vanillacommunity.com/discussion/15060/air8-and-microair-easystart-read-this-first-so-that-you-can-avoid-our-drama

Elwell Air8 data:
6000BTU 2.3SEER
900W
starting current 15A operating current 9A.  With the soft start the starting current draw is less.
weight is unstated.  There is no technical data on Elwell website.

Veering off topic...

The Air8 cools quickly and well and can be set to cycle within setpoints.  We have several issues with it.

1. Compressor does not kick in on initial fan start, you have to wait up to a few minutes- very inconsistent.  The Soft Start data refers to "training" it with 5-7 starts however this behavior remains since inception.

2. On our highly defect ridden Jan.2021 TAB 320, the Air8 thermostat thinks the cabin is 12-15 deg F cooler than actual, and the separate Alde thermostat thinks the cabin is 15-18F warmer.  Grrrr.  A lot of time spent on forensics and calls without resolution.  Thermostat programming 'offsets' do not begin to correct the setpoint error.  I've seen 2020 posts here about relocating the thermosensors.

3. The compressor and fan are incredibly noisy. 

Having installed several minisplits, I am starting to look options to replace the Air8.  Ironically many residential heat pumps actually run at about 19v DC internally (as I understand it) so there's a lot of inefficiency Inverting 12V battery rack to 120V for the unit connection, and then internally back down to 19 or so.  There are 24v and 12v trucker units so starting research.

AnOldUR

Like you @Maxcamp8, I couldn't find technical data from Elwell, but I've attached the Air8 label. The actual load that I measured was between 7-8 amps with the compressor and fan running.

Unfortunately, my Baja is enough smaller than yours that it won't run the Air8. In my post above I said that the generator didn't shut down, but that was my mistake. I wasn't aware of the delay to start that you mention in your post. When I finally waited long enough, the AC actually fired up and blew cold air for maybe a minute, but then the generator shut down with an overload error.

Thanks.