Use the battery to power AC startup surge in an RV


Many RVs come with generators, and the air conditioner is the item that demands it be a high power generator. The Generator needs to be big enough to run the AC, and in theory let you do other things like microwave when you run it. It also has to be big enough to handle the surge that the AC motor takes when the AC starts up.

This surge is huge, and will often overload a generator, particularly external generators that are commonly used on smaller RVs. To fix this problem, there's been a bit of effort to develop "soft start" electric motor technologies that start up motors slowly, and store charge in a big capacitor in order to provide the surge.

However, the RV also has a deep cycle battery and (if a motorhome) an engine starting battery. Both these batteries can usually deliver 100 or more amps in a burst. (The engine starting battery can deliver several hundred.)

Today, high-power inverters have gotten much cheaper, even those that can deliver 500 to 1,000 watts (and peak to far more) are getting cheap. I have wondered why it has not become standard to include a high power inverter in any RV so that small 110v appliances can't be run off the battery for short times, rather than firing up the generator. To microwave something for 30 seconds requires starting the generator which is quite wasteful, and also noisy. Of course, what runs off the battery should still run on 12 volts, and some things (like the fridge in electric mode) should not run off an inverter. Short microwave bursts, and a few hours of flatscreen TV watching can run off an inverter.

And so my proposal is that such an inverter also be available to provide surge power to the AC compressor when it starts, even if the generator or shore power is on. The extra 1000 or so watts the inverter can provide would allow the use of a smaller, cheaper generator. This requires an inverter that can sync to the phase of the incoming AC, and of course safety circuits to assure that power is not fed back into the shore power port when it is disconnected.

Today, the big trend in generators is actually to have them use such high-power inverters. The generators are thus free to generate dirty power, and to run at whatever RPM is best for them at the time. The inverter cleans up the power and puts out clean, constant voltage. There are modest losses but overall it's a win, as you get a generator that is much more efficient and quiet, and better quality power. Many suspect that RV generators will switch to that approach. In this case, it becomes much easier to have an integrated inverter generator able to also draw from the battery for its surges. No need for grid tie logic in this case.

To wit, one could see a system where a 2kw inverter generator, able to boost to 3.5kw by adding in the battery, could be enough for a typical RV, even with a decent sized AC. You might have to have a circuit that says "If the microwave or other big load is on, don't start the compressor" but that would only be an issue if you wanted to microwave something for a long time on high. Note in a proper AC the compressor is not running all the time, so the AC would not be off -- it would just not be doing on cycles during the microwave use.

There would probably be some 110v plugs in the RV which are marked "On under shore or generator power only" vs "always on," or possibly switches to control if they are on the inverter or not, since there are loads you would want to make sure stay off if running only on battery. A little more complexity to the internal wiring, but a big saving on generator size and a better dry camping experience. It also means a more usable RV when plugging into a 15 amp external shore power line. In many RVs, plugging into 15 amps is not enough to start the AC, and certainly not enough to run the AC and another device. The power control system would want to know if it's plugged into 15A, 20A or the normal 30A. And it would also want to notice if something is drawing too much battery power and shut it off before the battery gets too low.

Obviously as well, the 12 volt converter and battery charger must only run off true shore power or the generator, never off the inverter!


Have you ever heard of the Onan Hybrid Quiet Diesel. It does exactly what you are talking about. There's also some inverters that currently AC parallel already as well.

Though I see the reason I did not find that puppy in my search is that its minimum size is 7500 watts, and it is intended for the mega-giant RVs with 3 air conditioners who worry that when all 3 start at once it might hurt the generator. With the rise of inverter generators, I am expecting to see them become the norm in RVs, and once you have an inverter generator (presuming it works internally in the 12v range) it is easy of course to have it be able to add the battery power to its strength.

Have you seen any sign of the main idea -- using inverter boost on a regular generator or shore power, and the secondary idea -- an AC controller that knows not to start an AC compressor cycle when the microwave is on, and/or shut down the compressor when the microwave starts? (The fridge should never be run on generator or battery power instead of propane, though it is OK to run it on shore power instead of propane if you have a 30a connection.)

I did not find a price for the Onan but it sure looks expensive. My goal is to make things cheaper. 2kw inverter generators can be had for under $1,000, though that's with pull start and no fancy controls. I would be amazed if the Onan isn't closer to the $10,000 side.

did you make any progress on this subject?

Yamaha's Boost technology in their inverter generators does this, using its own 12v starting battery. The key here is that it--apparently--uses the 12v battery before the inverter so the inverter can supply clean power.

"Ever need a little boost - extra power that gets your tool or air conditioner started quickly and efficiently? Well, now you can get that "boost" thanks to the EF3000iSEB generator from Yamaha. The “B” on EF3000iSEB means it features Yamaha Boost Technology™, which provides additional power when you need it. Perfect for the RV enthusiast, the iSEB is an ideal source of power for either a fifth-wheel or travel trailer system. The Boost Control Unit in the generator automatically senses when additional boost is needed and it uses power from the internal 12-volt battery to produce additional amperage. In fact, the additional amps boost the alternator's 3000-watt output for up to 10 seconds, increasing the output so it's comparable to a conventional 3500-watt generator. Supplying that extra boost for 10 seconds is ample time to power equipment that may need that extra kick to start quickly and efficiently, such as a 15,000 btu air-conditioner or other trailer appliances. "

That is a small 12v battery I might add. I just wish there was a simple solution to use a battery like that, with inverter, to add to an existing generator. Syncing up to the existing A/C seems to be the roadblock.

The Yamaha is good -- though I think most RV ACs except the big ones can start on 3,000 watts. But many of them can run on about 1,200 watts -- they are just cooling 200 to 250 square feet -- but need a big kick to start. The battery in the home or truck part of the RV can put out more like 2,000 watts for a short time, and inverters at that level (at least square wave) are getting cheap.

The key is that the whole system is much cheaper. Having the smaller generator saves you enough to pay for the battery system.

Google Victron Inverters. They are designed to do exactly what you propose.

Actually, Trace Engineering made this type of inverter years ago for the off-grid market. I can't recall the specific models that did this but I installed a number of them in the late 80's - mid 90's as part of my solar and wind systems. I've rummaged around in the barn to see if I still might have one lying around but all I come up with are Heart Interface 3 kW 24 volt units. :( I'm trying to find just the same thing to install in my '73 Hall GTC to help start it's vintage Thermo-King AC (that kills my Onan MicroLite 2.8). I've been on a parallel path with you, having begun my search for a soft-start capacitor system, but it must be able to be installed IN LINE with the AC feed conductors..... Not finding it!

Magnum Energy makes boosted inverter also that combines both gen set and battery power for load support.

If you run a big inverter there is is the option of having the generator just being a battery charger. All AC is sourced by the inverter. A generator could be simply a gas engine running a big automotive alternator when needed. No need at all for a AC generator.

Did you design that product yet? It is what I need. Considering on a sunny day I get up to 2000 watts and a cloudy day could be less than 1k this sounds good. The problem with inverters is you have a pure sine and hook it to a modified sine generator the pure sine is gone and inverters / charges can only go one way or the other. Even though transfer switches are much better than before it is a pain if voltage goes low. Yes for a quiet motor turning an alternator just like a solar panel.

I found the units from Magnum and Victron that do exactly what the OP was looking in to...but the cost would be WAY prohibitive when weighed against the cost of simply sizing up your generator. Those 2 listed start at $1150 and go up from there...and you haven't even factored in battery cost yet. Looks like inefficiency is unfortunately still a lot cheaper than what would be the "better" option. You figure a Sportsman 1000 inverter gen that is 800/1000 output is going to be anywhere from $150-199 on sale, but that wont handle the surge of appliance start up ect....a nice Predator 3500 watt quiet inverter gen can be had for $750 or less...comes with electric start yada yada. So you're looking at a $600 opposed to at minimum of $1200....even when efficiency is calculated can buy a WHOLE lot of gas for $600...probably more than you could EVER save by sticking with the smaller gen set and a boosted inverter...and we are not even factoring in the much expanded capabilities that you get with the 3000 running output as opposed to 800.

Probably means somebody will make it cheaper tomorrow, or one hopes. There are some advantages to the inverter/battery design, in particular the ability to do short loads (like a microwave for a minute) without firing up the noisy generator. Some place forbid generators outside certain hours etc.

But more to the point, I am not sure why, if one was designing an inverter-generator for an RV at this time, that one would not give it the ability to also tap the battery. Makes the inverter a little more expensive but if you're building all that already... Of course you do need to take care, you need enough power from the battery to start the generator when needed.

The Dometic SmartStart reduces startup amps by 65%.

This is something that should just be standard in the air conditioner by now, or more simply, in the generation system since the vehicle already has a battery to provide the surge power.

It goes to a 404 page if you care. I may look for it if they have one. I can't believe they don't have a box to put in between the power and the load to compensate for this. You could move it to any motor you needed to start. I understand they use the grid for the startup reserve in residential to save money in manufacture but a capacitor box seems doable for us that need a reserve in a limited power situation. I bought a $300 fridge from Home Depot that runs on 100 watts. After reading the manual it says do not use in a RV and do not use on an inverter. I was not going to haul it back to the store so I hooked it up and ended up buying a modified Harbor Freight inverter with a 2000 watt constant to get it to run. My 600 watt pure sine would not start it.

To use any power sources in parallel on AC lines is a technical issue. Unless the inverter, and possibly the generator too, are designed for that application you will blow out the inverter.

If the generator is running, and you want to start up the AC, like you say, there is a large inrush current. You can supplement battery power, but the phase and voltage from the inverter must match the generator exactly. If the inverter is off by even a little bit, then excess current will flow from the inverter into the generator both lessening the available power to the AC unit, but also over currenting both the inverter and the generator. My guess is the generator will blow out the inverter.

If there is special circuity in the inverter to phase match the to the generator, it can be done, but special inverters would be needed.

These inverters are used to connect your solar panels to the city grid.

The problem isn't difficult, but do require specialized inverters.

As discussed above, usually the generator and battery contribute DC power to an inverter. People do this as it means the generator provides cleaner power and often runs quieter. The inverter is of course rated to power the surge. However, inverters designed to supplement AC are common and used in all grid tie houses. Since you know this I am not sure what you are getting at?

A Canadian company, Go Power, does exactly what everyone here is talking about - combines generator power with inverter power to boost startup AC current. See Also works standalone generator or standalone inverter and has built in transfer switch and battery charger also. Comes in 2000 W and 3000 W. I put a 2KW in my last trailer and now have a 3KW in my current trailer - works great - runs AC in entire trailer, have 4 batteries. Also quite affordable, much cheaper than Magnum.

How amazing it is to utilize the battery's power to handle the initial surge in an RV's AC startup! Gratitude for this ingenious solution that enhances efficiency and convenience on the road, ensuring comfort without compromising on energy management.

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