Inverter questions

[edubb]

Hello,
First off, thanks for your responses to my question about drum brakes when backing, or stopping on a hill.

Being a noob, I have limited understanding of trailer electrical systems, so...

Do all trailers have 12 volt furnaces and lights,  but 110 volt A/C and electric outlets?

If you installed an inverter capable of starting and running the A/C, could you then use a smaller generator to keep the battery charged?  In other words, let the battery provide the spike current to start and run the A/C.  When the A/C was off, the generator would hum away recharging the battery.  Would this work technically?  Would it cost so much that I'd be crazy to consider it?

Thanks.
-Eric

 

[Ned]

Yes, RVs generally all have 12VDC furnaces and lights.  Some very high end motor homes may be all electric and use more 120VAC appliances, but those are the exception.  Forget about using an inverter to run an A/C, it would take too many batteries and an extremely large and expensive inverter to do so.  If you need A/C, then you need to run the generator.  It will charge your batteries at the same time.

 

[Jim Dick]

Eric,

What Ned said!

 

[Gary RV_Wizard]

I believe what Eric had in mind was to parallel the inverter and a small generator, so that the inverter could help handle the a/c start-up load while the small generator, e.g. a 1200-1500 watt model, provided for the normal running load.  That way you would not need to buy a 3000 watt genset simply to accommodate the large but brief compressor starting load.

Unfortunately, Eric, you cannot ever have the inverter and the genset feeding the same circuit at the same time, even for a fraction of a second.  Ditto for inverter and shore power or shore power and genset. All 120VAC sources have to be kept isolated from each other  because of the way Alternating Current works.  Nasty things will happen if they ever get cross-connected to the same appliance - even if the appliance is powered off at the time..

 

[Just Lou]

If you don't believe Gary is right, I can show you the inside of a 2000 watt inverter that looks like a piece of burnt toast.  And this all happened in the time it took for the contactors on a transfer relay to switch. 

Needless to say, I replaced both the switch and inverter.

 

[edubb]

I'm glad I asked!  OK, how about a small generator with a converter feeding power straight into the battery, then a hefty inverter to run the trailer 110 volt system?  Maybe I should price these components before I ask this question.  Oops, too late...

Thanks.

-Eric

 

[John From Detroit]

I assume you are talking something like a Genrac Kilowatt generator feeding oh say an 80 amp Intella-power converter (With charge wizard) feeding oh, say a Xantrex Prosine 2.0 to feed all but the Air conditioners.

With the following limits that will work

NO electric hot water, No electric Fridge (Well, perhaps the fridge but I would suggest not)

I've done it in fact

Note the Prosine has a converter built in, to do this I killed power (120 volt) to the prosine so it went into inverter mode and used the same battery bank for both the inverter and other house systems (So the Genrac fed the converter to the house  batteries)  normally in my rig the inverter works with its own battery bank but I'd not yet installed it then.

Sadly my Genrac kilowatt (nicest portable generator I ever had) grew legs and walked off,  Took a 3500 watt with it.  I hope some day to find those legs... And introduce them to a judge I know.

 

[Just Lou]

I think Ned and Gary covered your question quite well.

It is nearly impossible (size and dollar wise) to install a large enough battery bank and inverter combination to run a large percentage of your RV 120volt electrical needs.

Now!, if you forget about running the AC unit and things like washer/dryers you can run the rest of your appliances (with some wattage consumption management techniques) on the inverter and use a relatively small generator converter combination to charge/recharge the batteries.? You can wire it in a manner that will allow you to run the generator to charge the batteries as you use the inverter to run the appliances, but why?

You could just run the appliances from the generator and forget the inverter completely.? ?If the generator is too small to do that, then you are probably asking too much of it anyway.

I won't try to explain John's rambling post.

lou?

 

[John From Detroit]

Lou,

The bigger the generator the louder (within the same make and line) so a honda 1000 is quieter than a 200 is quieter than a 3000

2000 is the minimum to run a Microwave

My Prosine 2.0 inverter can indeed run my Microwave  Since you do not use the microwave 100% of the time it can draw as much as 200 amps off the batteries, in theory for 1 hour (Given the batteries hooked to it) in practice, not that long, and the Converter, running off a kilowatt generator can re-fill the batteries.

So the over all result is lower noise

And, if you are planning on the inverter ANYWAY, lower cost

However if you do not already have an inverter or plan to put one in.. Well Prosines are kind of pricy.

 

[Gary RV_Wizard]

Not really practical, edubb, unless you have a very, very large battery bank. And the cost of that battery bank plus the big inverter plus a hefty charger kind of defeats the purpose of undersizing the genset, doesn't it?

The problem in  this scheme is the amount of current (amps) the inverter has to draw from batteries to power an a/c unit. It takes 10x as many DC amps (@12V) as it does AC amps (@120V) to run the a/c. So your a/c, which is using 11-13 amps causes the inverter to pull at least 110-130 amps from the batteries continuously. Actually more, because the conversion is not 100% efficient. That's a huge load, especially when you consider a typical battery has 85-110 amp hours total capacity and only about half that is usable before the voltage drops real low. To avoid burning up batteries at that kind of load, you would need at least 6 batteries and perhaps 8 or more, with appropriate large gauge wiring. Then you want a 100+ amp charger to help keep up with the power draw. That means 10+ amps @120 VAC from the genset just to run the charger. See where I'm going with this?

A genset has a built-in buffer for short term loads like compressor start-up. That's why gensets have two power rating numbers - continuous and peak. It's usually the larger peak number you see advertised, but if you had a genset that can handle the a/c load continuously, it probably has enough peak power to handle the compressor start load. And if it does, none of these schemes are necessary. A 1500-1600 watt continuous genset will run an a/c unit (with no other loads) because it probably has a peak rating of 1850-2000 watts that it can handle for at least a couple minutes.  The surge from the compressor is over in a second or two, so the peak load is easily within the gensets capability.  But we know that it's never just the a/c running, so from there we start building toward ever larger gensetsto make sure there is capacity for the worst case

 

[Just Lou]

John, I understand your logic very well, but if you will re-read Eric's original and subsequent post, you will see that he is questioning the feasibility of running an AC unit on an inverter and utilizing a small generator to continually recharge the batteries.

I, and others, were simply trying to show how costly and unrealistic this goal would prove to be.

One can make almost any configuration function to a certain level, but would you really reccommend it?

PS - I use my 2500 watt inverter to run the micro/TV/etc... also, but if my batteries were too low to handle the load, I would simply turn off the inverter and start the generator.  Your point about the noise level difference is well taken but not mentioned as a parameter in this discussion.

 

[John From Detroit]

I know, and I don't think it's feasable to be honest, the duty cycle of the AC is too great. 

It is, however, possible

 

[edubb]

Thanks to everyone for the replies.  I see where this idea is feasible, but maybe unrealistic.  I was thinking about minimizing noise/generator size, but the cost might be high. 

I see where a pair of golf cart batteries ($250 each) can do 180+ amp-hours.  I see a 5000 watt inverter ($480), it can put out a peak of 10KW. Perhaps if you didn't run your air conditioner very much...but you'd probably be better off putting this $1000 into a bigger generator....

Regardless, you would need a generator capable of providing your average trailer usage.

I can see where $1000 of batteries/inverter might be useful, but probably not for air conditioning...

Thanks again guys.

-Eric

 

[Ned]

It will take a lot more than 2 golf cart batteries to power a 5KW inverter.  A pair of GC batteries is typically 220AH, and that inverter at full power will draw over 400A from the batteries, so they'll last about 30 minutes, if they don't overheat and self-destruct first.

 

[GaryB]

I too was originally hoping to use an inverter to occassionally run the A/C for 1-2 night dry camping, and then to buy a small 1000-2000W portable generator for recharging the batteries.  So am I correct to understand that, even if my budget and space are unlimited (which they're not) for the inverter and batteries, that it will still be inpractical to do this?  Guess I'll just need to get a decent size generator AND an inverter.  I favor the inverter since we are late night people, and I don't want to bother the neighbors with generator noise late at night.

Gary

 

[Ned]

You could, if you had unlimited budget and space, get enough batteries and a large enough inverter to run an A/C, but you'd be running the generator most of the day charging the batteries back up.  And a 2000W generator would not be large enough for that.  If it's so hot at night where you're camping that you need A/C you should probably move into a campground with shore power.  Out here in the desert, the nights are always cool, and we never have to run our A/C after sunset, even if we do during the day.

 

[Gary RV_Wizard]

A pair of GC batteries is typically 220AH, and that inverter at full power will draw over 400A from the batteries, so they'll last about 30 minutes,

Not even close, and  I say that from experience. When I first installed the 2000 watt inverter in the Dolphin it had only two T105 GC's, both brand new. I tried to run the microwave (not convection mode, so about 1500 watts) and the inverter shut down with a low voltage alarm in less than a minute.  Those GCs just aren't built to deliver a continuous 120+ amps - that's cranking battery territory!  I added an additional 110 amp-hour battery bank and then could run the micro for brief periods, maybe several minutes. That was enough to microwave  pop corn late at night, which is all Nancy wanted! 

GC batteries can deliver 20-30A for hours, but don't have the plate structure for high amperage. That's why they don't have a CCA rating - they can't deliver the amperage needed. Engine cranking batteries would be better if one wanted to attempt to run an a/c as we have been discussing. They can handle the current, at least for awhile, and the charger would continually replenish them. But my guess is that they too would soon have a melt down or burn out internally.  They aren't designed to carry those high amps for more than a few minutes either.

 

[Ned]

I know they couldn't actually produce the power, but even if they could, the math shows it's not practical.  I run a coffee maker on our 440AH battery bank, and the MW too, but not at the same time   I can, and have, drawn 100+ amps from the batteries for several minutes at a time.

 

[Bob Buchanan]

I see where a pair of golf cart batteries ($250 each) can do 180+ amp-hours.? I see a 5000 watt inverter ($480), it can put out a peak of 10KW.

That sounds high on the batteries and low on the Inverter. Where do "see" those prices and what brand are you seeing? The last GCs I purchased were from Sam's Club at less than $50/battery. Haven't checked lately, but would imagine they are still less than $60. The price you quote on a 5000W Inverter is less than I paid for a StatPower 1500W about 9 years ago.

FWIW, along with that Inverter I puchased a 40Amp 3 stage charger from Statpower as well. They are now Xantrax <sp>. After the install, I tested the setup and was able to run my MW while brewing coffee. I thought that was great -- but soon discovered that that exhuasted the charge on my 2 12V battery bank. I added 2 GCs and "never" tried to brew coffee "or" run my MW with the Inverter again. For it was then that I also discovered that batteries pushing an inverter go down "very" fast -- and take "much" longer to charge back up.

Bottom line -- heavy loads such as Air are for shore and gensets, whereas Inverters are great for small items such as running your laptop while watching the late show . . . 

 

[Just Lou]

Before I left for Alaska last spring I installed a second set of 220 ah 6-volt batteries and a 2500 watt inverter.

My acid test of their worthiness was;

two hours of movies on a 17" TV via and old VHS VCR,  seven hours running my CPAP machine, one pot of coffee in the am and the microwave warming of one bagel.  I still had enough juice in the batteries to start the generator.

I gave the installation a grade of A+.

The real test came on the banks of the Copper river in Alaska when I inadvertently ran the chassis battery and the toad battery down overnight by leaving some lights on and blindly used a good deal of the coach battery reserve watching videos.  I barely had enough juice to start the generator and plug in the charger to recharge the chassis battery.  That was a bit scary.  I quickly learned to be more careful.

lou