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Old 05-10-2010, 09:35 AM   #1
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Inverter Use and Battery Level

I have replaced my univolt on my 66tradwind with a 45 amp converter. I now want to install an inverter so I can power tv, mw, coffee pot and hair dryer. I have done some investigation on inverters and am currently planning on installing a Xantrex 2000 watt PROwatt SW inverter. I am also planning on adding at least one more battery. My total battery reserve capacity will be about 420 minutes at 25 amps. I am not sure how to figure out how much operating my wifes 2000 watt hair dryer for 5 minutes or the 1000 watt coffee pot for 5 minutes will affect my battery state of charge. I have a 1000 watt Honda generator for recharging the batteries when necessary.
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Old 05-10-2010, 11:05 AM   #2
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I have replaced my univolt on my 66tradwind with a 45 amp converter. I now want to install an inverter so I can power tv, mw, coffee pot and hair dryer. I have done some investigation on inverters and am currently planning on installing a Xantrex 2000 watt PROwatt SW inverter. I am also planning on adding at least one more battery. My total battery reserve capacity will be about 420 minutes at 25 amps. I am not sure how to figure out how much operating my wifes 2000 watt hair dryer for 5 minutes or the 1000 watt coffee pot for 5 minutes will affect my battery state of charge. I have a 1000 watt Honda generator for recharging the batteries when necessary.
You can divide the AC wattage by 12.

That will give you the rate of battery usage.

However, you must also include a "efficiency" factor into that formula, of perhaps 60 to 70 percent efficiency.

As an example, using 2000 watts from an inverter, will take 183 amps at 12 volts DC input. Then add the efficiency factor which will add another 50 to 70 amps input at 12 volts DC.

That would boil down to using about 230 to 240 amps at 12 volts DC, to power the hair drier.

You will need a bunch of batteries, or find 12 volt DC replacements.

I do not know of anyone using a "inverter" for a microwave, coffee pot or hair drier. 12 volt TV's are available.

Coffee is easily made on the stove using LPG, as well as replacing the microwave.

Hair drying, maybe a towel or two, and a little "wait" time.

The use of an "inverter" in a self contained RV, has very limited uses, since they are a big time watts hog.

Please keep in mind, that the above numbers are "ball park" since the efficiency of the inverter you plan on using, is not known.

Another way of ball parking the currect draws, is that for every 100 watts output from the inverter, you wish to use, requires about 150 watts input, or 12 1/2 amps from the batteries.

Since W (watts) = E (volts DC) X I (amps) or W=EI or I= W over E, you can easily see the huge amounts of current input that would be required to power the items you mentioned.

Then, also keep in mind, that "IF" and that's a big "IF" you still wanted to do that, the size of the wire from the batteries to the inverter, would be at least a 1/4 inch in diameter, if not more, depending on the distance between the two.

Perhaps you can come up with an "alternate" game plan.

Andy
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Old 05-10-2010, 11:15 AM   #3
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Here are some questions and answers from an inverter manufacturer, that advertises on the Forums.

Andy

Q & A

What can power inverter do for you?


A power inverter can change DC power from a battery into conventional AC power so that you can operate a lot of electronic products and appliances , which including, but not limited to electric lights, kitchen appliances, TVs or radios. Basically, you need to connect the inverter to a battery, and then plug your AC devices into the inverter and you can have power wherever you go.



Which inverter is suitable for me?


We carry many different sizes of power inverters, from 400 watts to 5000 watts (including charger and remote control). The size you choose depends on the watts (or amps) of what you want to run. (the information regarding the power consumption should be on the appliances). It is always safe to get a larger model than you think you need.
First, you need to determine the continuous load and starting (peak) load, that is how much power your appliance (or combination of them) requires to start up (starting load), and also the continued running requirements (continuous load).


For example: The terms continuous-3000 watts and peak surge-6000 watts is that some appliances, such as ones with a motor, require an initial surge of power to start up (starting load). Once started, the tool or appliance requires less power to continue to operate (continuous load)


How to convert AMPS to WATTS: AMPS X 120 (AC voltage) = WATTS
The above is an approximation of the continuous load.


To find out the approximate Startup Load: WATTS X 2 = Starting Load
This formula is an approximation of the starting load of the appliance, though some may require an even greater starting load. Some appliances, such as air conditioners, refrigerators, pumps etc may have a start up surge of 3 or more times the continuous rating.


powerinverter
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Old 05-10-2010, 11:45 AM   #4
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A 2kW hair dryer for 5 minutes or about a tenth of an hour is 200 watt hours of energy. That represents about 20 pounds of battery. (a typical RV battery runs about 60 pounds). BUT

With large loads like this, the Peukert coefficient can be important. The rated capacity (20 hour rate) of a battery is at a load of about 60 watts. There is about a ten percent decrease in available battery capacity for every doubling of the load. There's about 5 doublings to go from 60 to 2000. That decreases a typical KwH battery from 1000 to 900, then second to 810, then 729, then 656, then 590. (double the load five times, decrease capacity by ten percent five times).

That means your 200 watt hours of hair dryer uses a third of a single battery's total energy capacity. Since you shouldn't use more than a half of a battery's capacity as a routine thing, that means the 5 or 6 minute 2 kW hair dryer use would use about 2/3 of the most you should take out of a typical RV battery.

Another factor here involves the 6v vs 12v discussion you so often see in battery discussions and why many diesel pickups use 12v batteries in parallel. Batteries in parallel will provide large currents easier (but the difference for small banks like this is probably not worth arguing).

A 2 kW load is almost 200 amps at 12v. That means that wiring and connections can make a big impact. Short leads with large wire (2 gauge or better) and solid connections are important. You should also keep negative and positive leads matched and properly dressed to reduce impedance effects.

Keep in mind that it takes 8 to 12 hours to fully and completely charge a battery bank if you have a good three stage battery charger. 45 amps is a good rate for a two battery bank but will push that 1 kW genset when going full tilt.

Where many amateur system designers make errors is in working with improper units. I suggest using watts for power and watt-hours for energy. Only convert to amps when considering wiring and avoid amp hours by always thinking of battery capacity as watt hours (amp hours times volts).
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Old 05-10-2010, 11:54 AM   #5
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I think a Honda Eu2000 is in your future, call it a $900 hair drier

A quick note - the PROwatt 2000 is rated at 1800 watts continuous and as a inexpensive sine-wave unit is not as robust as we could hope for. It is considerably cheaper than SW units used to be but I would be tempted to dedicate it to the electronics, microwave, flat-screens, audio, satellite, compact fluorescents, and other sensitive equipment only and leave the hair drier and other resistance loads to shore power or a heavier modified sine wave unit dedicated to one or two outlets...

2000 watts is a bit high for a consumer grade hair drier - the PROwatt efficiency is listed as 85-90%; wiring and cable losses come in around 3 to 5%, so a 12-amp 1440w hair drier run off batteries is commanding better than 1700 watts or 140 amp draw off the batteries. With that amount of drain it would be easy for the inverter to see the 10.5 Volt low voltage disconnect if the batteries are not nearly fully charged or have minimum or undersized cabling - if you have the Converter helping power the load that 45 amp can supply 500 watts so the batteries will be taxed by 1200 watt draw offs.

Since the batteries are less than 90% efficient on the charge-discharge cycles, with the honda running during the hair drier usage you need to continue running the AC generator powering the converter-charger another 4 minutes minimum for each minute the drier runs, more likely 6 or 8 minutes though to get batteries better than when you started.. .
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Old 05-11-2010, 11:51 AM   #6
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Guys

Thanks for all your help. You gave me a lot of information to think about. This is exactly what I was looking for. Here is where I am at right now. I guess the 2kw hair dryer is overkill. I guess she will need to use a much smaller capacity one although her hair is real thick- not like mine. I still want to be able to run the mw (700 watts), coffee pot (1050 watts) and maybe I can knock my wife down to a 1200 watt hair dryer. I don't need to operate the TV. I just found what maybe the perfect RV TV at Sams- A 22" LED 12v, 1080p, 3.3 amp by Vizio for $248. I think I have room for 4 6v batteries in the rear compartment of my TW. I can buy 4 GC batteries at Sams with 220 ah capacity for $75. each. I figure the energy stored in each battery is 1,320 watt-hrs or 5,280 watt-hrs for all 4 batteries. Now, running my wife's new 1,200 watt hair dryer for 6 minutes will require 120 watt hours of energy or less than 2.5% of the total stored battery energy. Double this for inefficiency and losses and it is still only 5%. If my calc are not to far off and as a guide line for operation, every minute I operate the hd, mw or cp will use up about 1% of my total battery capacity. I don't see wire size as a major problem as I will run one wire to the inverter from each bank of 6v batteries.
I have already replaced my coverter with a PD 45amp unit. What have I missed here or are my calculations screwy? What modified sine wave inverter would you guys recommend?
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Old 05-11-2010, 12:29 PM   #7
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Dan,
You got good advice so far. I use my Honda EU2000i to charge the batteries, and run the microwave and hair dryer and use my Xantrex Freedom 10 (1000 watt) Inverter to run everything else, i.e. flat screen TV, laptop, satellite receiver and cell phone battery chargers.

I tried using the wife's hair dryer on the low setting using my inverter and it wasn't worth it. It's much faster and doesn't wear the batteries down when I just use the genny.
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Old 05-11-2010, 01:39 PM   #8
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Guys

Thanks for all your help. You gave me a lot of information to think about. This is exactly what I was looking for. Here is where I am at right now. I guess the 2kw hair dryer is overkill. I guess she will need to use a much smaller capacity one although her hair is real thick- not like mine. I still want to be able to run the mw (700 watts), coffee pot (1050 watts) and maybe I can knock my wife down to a 1200 watt hair dryer. I don't need to operate the TV. I just found what maybe the perfect RV TV at Sams- A 22" LED 12v, 1080p, 3.3 amp by Vizio for $248. I think I have room for 4 6v batteries in the rear compartment of my TW. I can buy 4 GC batteries at Sams with 220 ah capacity for $75. each. I figure the energy stored in each battery is 1,320 watt-hrs or 5,280 watt-hrs for all 4 batteries. Now, running my wife's new 1,200 watt hair dryer for 6 minutes will require 120 watt hours of energy or less than 2.5% of the total stored battery energy. Double this for inefficiency and losses and it is still only 5%. If my calc are not to far off and as a guide line for operation, every minute I operate the hd, mw or cp will use up about 1% of my total battery capacity. I don't see wire size as a major problem as I will run one wire to the inverter from each bank of 6v batteries.
I have already replaced my coverter with a PD 45amp unit. What have I missed here or are my calculations screwy? What modified sine wave inverter would you guys recommend?
Two things to keep in mind.

1. Do not mount the 4 batteries rearward of the axles. To do so, will have a negative effect on the tongue weight, and more than likely will contribute to some rear end separation.

2. A 45 amp charger, will not keep the batteries up to any degree, when used in the manner you propose.

What size charger you will need, depends on how you will be discharging the batteries. Certainly, a small charger, as you propose, won't even offer a tickle for recharging in a reasonable period of time.

Andy
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Old 05-11-2010, 01:58 PM   #9
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Another option is to look for 12 volt DC appliances. They're not cheap themselves, but cheaper than an inverter,eaxtra battery, and all the heavy wire.

Ricky
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Old 05-11-2010, 03:36 PM   #10
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Good posts so far. A couple more Facts to Consider.

1) Modern, high-quality inverters generally run around 90% efficiency at a wide range of output levels. The 60-70% efficiencies we once saw aren't present on inverters currently in the marketplace.

2) At the higher wattages you would want to consider combined inverter-charger-converter-transfer switch devices like the Xantrex Freedom, since you will need a bigger converter anyway, and the combined units are lighter, cheaper, and work better than two separate units.

3) I have penciled out a couple of designs and it seems to me that the best way to handle really big battery+inverter lashups is to put the batteries and the inverter in the tow vehicle. This helps with weight distribution, provides the possibility of the tow vehicle doing some recharging while you're out and about, and keeps the batteries out of the living area.

4) For hair drying, you could use one of the better 12 volt hair dryers although it's going to work barely as well as a household one on "low." Still a battery hog and will require special wiring but at least you don't have the expense of an inverter.

5) For coffee, if you're used to a drip machine, use a cone filter, a thermos, and a tea kettle on the stove. That's what I do, and it works great. Good coffee and the grounds neatly separate out and go in the trash.

6) )Lots of 12 volt TVs or you can get a really small inverter to run most smaller TVs. They're cheap and plug into a cigarette lighter outlet.

7) For the microwave you'll have to use something else. More a matter of habit and planning than anything else. Frozen stuff you put out early, and use the oven and the stove for cooking.

8) People usually ask about toasters too. We have a thread on that somewhere but the short of it is that you can do it on the gas stove with a pan or a trivet or one of those special-purpose toast racks. In a pinch you can do it without any of these, just with a fork and constant supervision, but you have to watch all the time or you'll burn the toast.
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Old 05-11-2010, 08:20 PM   #11
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Thanks, Andy- I thought about all that extra weight behind the axle. I was not thinking about the affect on the tongue weight, but mainly that I did not want to beat the s out of my nice old trailer especially on my dream trip to Alaska some time in the future. I had also thought about the 45amp charger not being adequate for my 4 GC batteries.

Jammer- I think you nailed it. I like the idea of putting a 2000w converter/inverter and 4 6v GC batteries in the back of my pick-up truck. This has the following advantages:

1. I would mount the converter/inverter and batteries on a board and carry the weight at the front of the truck bed- right between the wheels.

2. Don't have to mount 4 heavy batteries in the rear of my trailer and deal with what ever problems this may cause structurally and handling wise.

3. If I am not going boondocking and will have shore power available just leave my power board at home in the garage.

4. I could even add 2 more batteries to my power board. However, one of my favorite sayings is that new solutions have new problems. How do I now secure, load and unload my power board in the truck. With 6 batteries it may weigh almost 500 lbs.

5. I could boon dock with either trailer- the 66 TW or the 84 Excella.

6. I don't have to make any electrical changes to my 66TW. The new 45 amp converter is fine along with my small group 24 Interstate (light weight) battery in the rear of the trailer.

Ya, I like this solution. Thanks, Jammer.
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Old 05-12-2010, 10:21 AM   #12
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re: " I figure the energy stored in each battery is 1,320 watt-hrs or 5,280 watt-hrs for all 4"

keep in mind that only half of this is available for routine use if you value the life of your batteries.
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Old 05-12-2010, 03:48 PM   #13
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You might want to consider L16 batteries to get the kind of capacity you want.

If you rig your power board with suitable lifting loops, then with proper rigging you could remove it with an engine hoist, if you have a garage with a concrete floor. There are cheap engine hoists from places like Harbor Freight, and Amazon has them.
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Old 05-12-2010, 11:22 PM   #14
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Jammer- What are L16 batteries?

Since I have a cap on my truck an engine hoist may not help all that much. I think that I may mount the converter/inverter to my power board, lift the board into the truck bed then lift each of the 4-6 batteries into their place and secure them in place with a steel bracket and some bolts. Lots of design details to work out. I will probably design it and then modify the design as I build it. Now I know that somebody on this forum has already designed and built a power board. I would like to see your design. I am not to proud to copy stuff I see that I like.

Bryanl- Ya I know that I don't want to use more than 1/2 of the battery bank capacity. I will install a battery guage to keep tabs on my battery bank capacity. If my calculations are correct, if I run the hair dryer, microwave or coffee pot for 6 minutes or so, I am only using about 10% of my usable battery capacity. I feel pretty good about that.

This whole thing is about convenience. I don't want to have to get the generator out, start it up, listen to it (or bother my neighbors) even though it is quiet and then put it away again every time I want to make coffee, my wife dries her hair or I want to heat something up in the mw. Also, some campgrounds restrict operation of generators to certain times of the day, or do not allow them at all at some sites. This way I can run the generator when I want to in order to charge the batteries.

I guess I could find a 12v coffee pot, but then I need to carry two coffee pots and install a 12v line and receptacle large enough to power the coffee pot. New solutions have new problems.

The more I think about this, I may go ahead and install 6 6v batteries, then I will run 4 gauge copper wire from each pair of batteries to the inverter. This will be good for 210 amps total. I would think that this would work fine for a 2,000 watter inverter.
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