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Old 03-19-2013, 12:35 AM   #15
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Run current is easy for the inverter, it is the start current that overwhelms the inverter.

The inverter has limits to what it can output as well as protection circuits to prevent damage.

The generator can provide better surge to support the starting current.

If you proceed seek guidance from the manufacturer of the inverter and ac to see if the inverter can supply the starting current and if the compresor will operate properly with the inverter.

Country Coach has units configured this way so there are units that will work.

The generator consumption of 2 gallons per hour would be spread out over the distance traveled in the hour, still very little operating cost per mile, return on investment still long
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Old 03-19-2013, 09:11 AM   #16
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Regardless of the argument about whether it is more economical to run a separate generator to generate 3.6kW to run two aircons, or generate that 3.6kW by running the engine alternator continuously at rated capacity, there is the additional consideration that at the moment you have two inverters in perfect working condition that may continue working for another 20 years so even if you lose one, you will be able to continue your trip with only limited inconvenience.
If you replace those with a single large unit, you have no inverter if it fails.

Also - all those extra batteries incur an opportunity cost plus a cost of replacement and that needs to be weighed against the marginal extra cost of running the generator during the limited times you are on the road when aircon is required.

Other considerations that need to be addressed - the larger the number of parallel strings of batteries hooked together, and the larger the maximum single load or charging source, the more difficult it is to ensure that all batteries share the load equally. Even apparently minor variations in lead and connection resistances at 500 amps can mean dramatic variations in the share of the load that each battery is supplying.
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Old 03-19-2013, 10:31 AM   #17
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battery strings

Parallel strings will not be a concern if done properly.

The fuel cost to operate this will be different, the needs of the ac are in BTU or Watts, these can be converted to amps amp/hours and gallons of fuel.

One could contact Country Coach to determine the inverter and AC units used in the Magna that had the front AC off inverter to determine what equipment to use as well as the battery design to save a lot of time.

Then the AC unit can be checked to see if generic or special, manufacturer could advise.

We wanted to do same thing last year due to factory air needing coolant, you can find our thread to see what we put up with and learned the hard way.

Yes, this has an can be done.

To do it properly will be very expensive.

To do it all at once is noting more than an ego trip which is fine, been there done that, it is a great idea.

Now is one set that as a "future goal" then it is not so crazy, we assume the inverter may fail as well as the AC dropping dead at some time.

If one has currently 2 inverters, then one could upgrade one to the unit that would be suitable for this application and either sell or keep the removed one for a spare.

Then one could either upgrade the one AC to the type required if the existing one is not suitable, and keep the harvested one for spare for the remaining unit.

With this plan one could justify the investment and ROI is not as critical as the "investment" includes the spares that could be used later, or sold to recover some $$$

ROI based on fuel savings is moot, unless one gets substantial savings in MPG it will take many many miles to get there.

To ne exact, one can only compare apples miles to apples miles, those where they would be running the genny on those hot days.

If one only does that say 50% of the time then the ROI may never happen.

But we do not do things to our coaches with ROI in mind, we get wild ideas and convince ourselves they are good ones...

Do your homework and engineer this properly before investing any money.

Contact companies that already have done this to see how they did it, assume they had engineers working all the numbers to determine the most cost effective plan, then work from there.
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Old 03-19-2013, 06:29 PM   #18
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Quote:
Parallel strings will not be a concern if done properly.
Agreed, but 8 large batteries in parallel?
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Old 03-19-2013, 07:49 PM   #19
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Your 300 amp alternator is not enough.

A 4,000 watt load at 12 volts is 333 amps.

A 5,000 watt load at 12 volts is 415 amps.

8,000 watts at 12 volts is a whopping 665 amps!

Run the generator.
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Old 03-19-2013, 08:27 PM   #20
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Quote:
Originally Posted by Rich-n-Linda View Post
Your 300 amp alternator is not enough.

A 4,000 watt load at 12 volts is 333 amps.

A 5,000 watt load at 12 volts is 415 amps.

8,000 watts at 12 volts is a whopping 665 amps!

Run the generator.
In addition (or subtraction) there's the draw of the other systems that the alternator has to power, the 10% inefficiency of the inverter (average) the weight and maintenance of the batteries, etc. You can buy a lot of diesel for the cost of all the components this would take to make work.
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Old 03-19-2013, 09:51 PM   #21
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I am an engineer and it does not make technical or economic sense to me to double up on batteries, inverters and change the alternator to get a higher charge rate versus running the generator as required to run AC's.

To start.with, everything in an engine and its ancillary components is engineered for specific forces. If you double the generator output. You likely more than double the force required to produce this doubled output power. This additional force is placed on the drive belt and increases the engine load hence reduces MPG. There is no free lunch re power generation and consumption.

If you double the battery quantity, you have increased weight, taken up more space, reduced the MTBF, and increased initial costs plus increased maintenance and replacement costs.

By putting a second inverter in, the wiring and physical engineered space would need a retrofit and MTBF again would be reduced because the hardware is doubled.

Everytime one converts energy from one form to another, or to change to different voltages. Energy is expended in this process. Generally in the inverter will have a power factor of 0.9. Energy is also expended by turning belts and running an altenator as noted.

As others have said, running an 8kw gen just to run two 13500BTU AC's it would be operating at much less than 1/2 power so the fuel consumption would be about 1/2 gal per hr.

Of course one must assume the weather is not always so hot where the Air needs to be on so the gen is not always needed.

The great thing about an RV is one can move to more moderate weather so the AC is not required.

Would I engineer and configure my RV AC's to run off batteries?

Not a chance, I see no net gain in doing this and actually there a net loss in every respect from my perspective.

I run the gen as required, but to each his own, if it makes sense to you, then go for it.
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Old 03-19-2013, 11:24 PM   #22
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I have many years in telecommunications, we had Battery plants built from small 48 volt with 2 strings of 90 amp hour batteries to 5 strings of 2050 amp hours.

As long as batteries are same type and condition along with proper conductor and buss installation you would be fine.

Problem with most folks doing this in an RV is they do not install proper sized conductors or protection devices.

A single 8D Battery can produce 1500 cranking amps without breaking a sweat, that is while maintaining a voltage suitable to run the starter motor.

Connect more in parallel and consider short to chassis ground instead of starter motor and we are talking KA (KILO-AMPS), yes thousands of amps, this can vaporize tools and large conductors so extreme care must be taken when designing and installing them

Regarding power consumption, the 13500 BTU unit is not going to pull more than about 2000 watts when running, they usually pull about 12 to 15 amps or so.

This load will be easy for the stock charging system.

However a large Battery system can easily overwhelm chargers like alternators if allowed to discharge too much.

Usual charge rate is about 20 amps per string or Battery, and can be upto the capacity of the unit

So 4 8D batteries in parallel would need 100 amps minimum @25 amps each, but could be 800 to 1000 amps worth of load if discharged more than 50%.

The charging device needs to be current limited to avoid smoke from overloading caused by the large batteries.

If you are only looking at using inverter for AC on the road then not too much Battery is needed as long as it can support the high starting currents, the alternator will maintain the running load and recharge the Battery.

If you plan on doing this while parked then many batteries needed, handy in rest area but not wise as recharge limits advised above.

Yes, we did all of this planning before as we tried to convince ourselves that it was a good idea...

We discovered that " Hot Shot" was a direct replacement for R12 and we could get it for reasonable cost so we charged the dash AC and called it good.

Will need to top off but we have plenty of HS, and can run the genny if needed.
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Old 03-20-2013, 07:37 AM   #23
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Personally, I wouldn't use the inverter to run the A/C but it's worth mentioning that from what I've seen, 2 x 2k inverters is fairly common for all-electric coaches. It's not so much being able to use 4000W, it's probably more being able to use more than 2000W (e.g. microwave, tv and res fridge as the same time).
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