RV Electrical question: paralleling a generator with a battery powered inverter

Ok- I'm almost done restoring my 72 Airstream. It's been great but I want to run our air conditioner off my Yamaha 2000is inverter generator. It kicks off when I try to power the 13,500 BTU air conditioner. I expected this. I think it will run the ac once it overcomes the surge to get the AC started. 

The IDEA: Get a pure sine inverter to supplement the boost required to get the AC started, and let the generator run the AC.

The problem: Can I parallel my "parallel ready" inverter generator with a battery powered inverter to make this happen?  More importantly- how?

I love the "simplicity" of this idea- rather not get another generator, a bigger one etc if I don't have to.  I've considered that, considered running the AC off of a 2000w/4k surge inverter powered by the battery and charged from my generator, but that destroys my battery in an hour.

I'm a software guy, not a EE, but I don't think that works.

You can get "soft start" boxes for ACs that will significantly reduce the initial inrush current, supposedly this allows most of them to be run off a 2KW generator.  They're typically a few hundred bucks and you wire them into the power input on the AC compressor motor.

It's theoretically possible, but I don't know if any off-the-shelf parts exist to make it happen. Unlike DC current, the AC current phases need to be in sync in order to do something like this.

The soft-start capacitor is probably what you should do. 

This is a problem for Data and La Forge. I suspect the solution will involve reversing the polarity on something but you may tear a hole in the space/time continuum.(and let the smoke out of some of your equipment)

I'm no expert but I've never seen an install like that, nor seen a transfer switch that'd allow it. My sneaking suspicion is that whichever source was putting out more juice would attempt to power the other source... through its outputs. Some bigass Zener diodes might keep that from happening, if it was gonna, but I dunno...

Thanks all. I'd looked at the micro air soft start capacitors- not sure if my Yamaha will start the ac even with that. Looks like the equivalent honda has a better track record. I know my idea is possible- on my ship we had to manually align phases when putting the generators in parallel... just this might be more pain than it's worth.

To operate two sources in parallel you want the droops set up so the two machines share the load in proportion to their ratings. Adjusting the no load freq of one machine will change real load between the two machines and changing no load voltage will change apparent power loading.
 

You also need a way to parallel the two machines to make sure they are in phase when bringing the 2nd source online.

It might work to start your AC or it might not.  If there is not a product out there to do all of this automatically, I would get a bigger generator or a smaller AC. 

Is there an Inverter that can also operate in reverse (ie- as a rectifier/battery charger) and transition between the two seemlessly? If so, that may take care of your synchronizing issue. Bring on the inverter as an AC load/DC source, then have it change to AC source/DC load. 

One big inverter, and then charge the batteries with the generator using a charge controller.  There are some good "all in one" set ups that have solar and generator inputs that feed batteries and then balance all of that to AC power.

I have a Micro Air Easy Start on the 15k BTU air conditioner on my Jayco 5th wheel (also mentioned above).  It is more complex than a soft-start capacitor.  It allows my Honda 2000i generator to start and run the AC, and I can also do it from a 15A house plug.  I think that is the solution to your problem.  

If you want to get more complex, Google "RV hybrid inverter."  These things can mix and match battery power, generator power, and shore power to do what you want.  I was looking pretty seriously at the Magnum Energy MagnaSine hybrid, but I have not pulled the trigger yet.  I have a small solar array, but no inverter yet.  

Good luck!  

Brian

Paralleling is possible. We do it with generators at work all the time, but it seems to require expensive equipment for our stuff that I don't think you'd want to buy for your setup. We had a facility that had 3 gas turbine generators, and the added a 1.5 Megawatt diesel generator to the building, and a fancy paralleling board. It took almost a year, and loads of experts to get it to work, and then they decided it wasn't necessary, and ditched it. Probably cost around a million dollars to get working, and then they didn't even use it. Crazy!! Capacitors are probably your friend here.

Something that may help- https://www.amazon.com/AMPINVT-Inverter-Converter-Adjustable-Frequency/dp/B07QYF8THG

And then look for RV automatic transfer switch.  You can have various inputs to that.

But also you can use a hybrid system that will draw both on a generator and batteries at the same time.