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Old 03-18-2006, 06:47 AM   #11
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I'm all about using what you've got. (A look at my patchwork free wood panels will show that) Give the "Sears alternator" a try. If it doesn't work out.....do something else. What's the worst that could happen? maybe learning something in the process. One of my favorite thing about watching people's buses evolve on this site is the resourcefulness of the builders. This is fun!!! hee hee hee!!
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Old 03-20-2006, 11:12 PM   #12
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I just ran my old dorm fridge off my 750-watt inverter (the mislabeled one off ebay that everyone was talking about a month ago) all the way to Texas & back & it worked fine. I also ran a TV, Xbox, Stereo & some lights at times all with the fridge plugged in.... that inverter ran everything just fine.

My batteries were pritty much shot when I left for Texas & on the way home one of them started shorting out & leaked acid. I replaced both batteries with Deep cycle Marine/RV batteries from Wal-Mart (only $55.00 each) in Waco Texas at about midnight. We stopped a few hours later because the rain was so bad it was hard to see the road. I turned the bus off while we all went to sleep for about 3-4 hours & left the inverter running so the fridge kept cold. When we woke up the engine cranked over & started with no problem. I figured if the batteries got to low the inverter would shut off.

Jason has "house batteries" on his bus too but I just run my inverter off the main bus batteries... Now I have deep cycle marine Wal-Mart batteries with 1 year warranty so if this set up kills the batteries I’ll just exchange them for new ones. When I'm on shore power I turn the inverter off & run a trickle charger all the time. I have everything on one circuit in my bus; I figure that the bus is small enough that I can see if someone is trying to vacuum & run a hair dryer at the same time.

My fridge is one of those 3' high ones with a small freezer inside, the label says it runs 1.05amps & 115V, which isn’t much, I was a little worried about start up power but it ran fine the whole trip. The fridge never once interrupted the Xbox or TV or anything... Actually that reminds me, after we ran out of fuel & restarted the fuel system still had some air in it and stalled a few times, we were playing the Xbox off the inverter while cranking the engine and had no problems (that was after the new batteries of course)
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Old 03-21-2006, 04:14 AM   #13
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I'm not worried about the batteries getting hot. They will be in a compartment under the bus. In fact, just the opposite is normally the issue here in the north country as I'm sure you know.

The heat I'm worried about is in my inverter compartment under one of the seats of the dinette. I just figure with an insulated floor, compartment sides, and the seat above it.....that compartment might get mighty warm. Heat doesn't do inverters any favors so I figure I'll have some sort of venting method.
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Old 03-21-2006, 10:33 AM   #14
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Re: Dorm fridges

Quote:
Originally Posted by the_experience03
Obviously the 120 volts and 60 hertz seems reasonable enough to me, but the 1.2 amps....I figured it would have drawn MUCH more than that.

Remember the 1.2 amps @ 120 volts = 12 amps @ 12 volts. 12 amps is not much current draw however the fridge might run as much as 50 % of the time it's plugged in, depending on ambient temperatures. Plus you have to figure in the loss from the inverter So you're really drawing more like 15 amps. It could easily consume more 150 amp hours per day. Not a big deal if you're driving, or you have shore power connection. That's a good deal of juice to pull from batteries, especially if you're gonna be parked for multiple days. Steve has a big enough battery bank that he could survive 9 months after a nuclear bomb blast....but most of us don't have that kind of reserve power.

I think having a fridge in your skoolie is an excellent idea. I had one in my bus during my 10 days or so in the desert. Just trying to warn you about the power consumption. A honda EU geni is a great option if you don't have shore power.
For just occasional use, dry ice put into an insulated bag (like you'd transport pizza in) then placed inside a cooler can keep things frigid for most of a week. Many people use this method at burningman.
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Old 03-23-2006, 10:57 AM   #15
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Part of how I make a living is designing electrical systems for cruising boats. I've learned a lot over the years and lots of new components are availalbe.

I'll throw out a few tidbits...

For the best estimate of actual power draw always convert to watts first 'cause while amps and voltage change from system to system...watts is watts.

The voltage at which AC appliances are rated varies (some 115v, some 110v, some 120v, etc) and we're going to use a factor of 10 to convert that to our typical DC system so inacuracies get multiplied. If you figure watts on the AC side (amps of draw X the rated voltage) then you can divide by your DC system voltage to get actual amperage draw. It isn't typically 12-volts by the way; that only exists a one point in time. It's not unsual for the system voltage to be well over 13 volts when charging and quite a bit lower as power is consumed from the batteries.

Going back to our Watts = Amps times Voltage you need to know that Watts is the power the appliance uses and is a static number (that's not ture for things that have variable speeds or temperature settings but humor me and pretend for now; the max draw or various ratings are usually on the label) ; that makes it easy on us since that means the other two numbers (Amps and Volts) must always multiply together to equal Watts.

Lets do the math...if you have a refrigerator that draws 3 amps at 115 volts that's 345 watts. Now you know how much of your inverter's capabilites this thing will use. To figure the DC side there are two options (at least)...the first is to multiple the amps draw on the AC side by 10; in this case that would be 34 amps (which would be #6 wire if the inverter is 15' away from the batteries). We use 10 rather than 12 because it's an easy number to divide by and because it leaves a "fudge factor" for inverter efficiency and less than ideal wiring runs. The second (and more precise way if it makes a difference) is to divide the Watts you calculated by DC system voltage. A fully charged battery (the kind we're talking about) is 12.8 volts and a 50% discharged battery (the level you shouldn't go below and unless you have "traction" (deep-discharge) batteries) is 12.1 volt; so using 12-volts presents sort of a worst case scenario. The amperage in the DC side in our example will be 28.75 amps but then we have to figure in inverter efficiency (listed in most of the Owners Manuals). If it's 85% then that works out to 33.82 volts; awfully darn close to just using our "multiply by 10" method.

Unless you have a 3-stage charger (either an AC model you plug in at home or to a genset, or one on your bus alternator) it's difficult to charge deep cycle batteries up past around 85% of their capacity. On the flip side (unless we have special deep-discharge batteries) typical deep cycle batteries should not be dischaged below their 50% capacity level. For day in and day out use that means we typically have available about 35% of the batteries rated amp-hours.

Using the refrigerator above as an expample which is using 34 amps, that means if the refrigerator runs 50% of the time we'll consume 400 amp-hours over the course of a 24-hour day. We would need a battery bank of 1165 amp-hours to support it without damage to the batteries (that is, to not go below 12.1 volts). That's three 4D batteries or eleven group 31's!

A battery is (by definition) "dead" at 10.5 volts but plates in typical deep cycle batteries are physically damaged when they're discharged below 50% of their capacity (about 12.1 volts).

An inverter should have a battery bank to draw from that's at least 20% in amp hours of the inverter's rating in watts. That is, a 1000-watt inverter should be connected to a minimum 200 amp-hour battery bank.

A battery bank should not be larger than 3 times (ideally) to 4 times (ok) in amp-hours the alternator's rating in amps. That is, the 200 amp-hour bank above should be charged with a 50-amp alternator as a minimum and better yet with a 65-amp or higher alternator. [As an aside...the 1165 amp-hour battery bank we needed from above to run our refrigerator should be charged by 388-amp alternator!]

When calculating wire size remember the rating is always based on the *round trip* from the battery to the load; not just one way. They're also based on the wire being in "free air" and not bundled; if your wire travels through tight conduit and/or is bundled with a bunch of other wires step up a size.

Allow for voltage drop; we shoot for less than a 3% drop on a wiring run in boats for sensitive electronics, running lights (their output has to meet legal requirements), pumps and motors. Other circuits get along fine with a 10% maximum drop. There are wire charts available for both (West Marine has them on their web site http://www.westmarine.com/pdf/0660_ETRIC_MC04.pdf).

If you locate your batteries far from your alternator make sure you size them for the engine's starting loads even if they're house batteries unless they're isolated (manual battery switch, battery combiner, etc). Also remember to use the round trip distance to calculate the size necessary to carry all the amperage the alternator will put out. For example...you'll need #4 wire for an alternator of 130-amps if the batteries are 15 feet away.

Just remember that we're talking about a SYSTEM; every component is interelated. With the room available in the typical bus is wouldn't be much of a stretch to get 500 amp-hours (or way more) of batteries installed; but then you have to charge them with something and if that something is an itsy-bitsy alternator on the engine you'll have to drive from Detroit to Dallas to charge the batteries if they're discharged much.

Lots of folks have systems way outside of the parameters I've outlined here. That doesn't mean they won't work; they just happen to work in specific circumstances. For instance if you use the example from earlier (1165 amp-hours of batteries and a 130-amp alteratior) it would work fine for someone who goes away for a weekend of camping. Let's say they drive 4 hours to where they're going, spend the night, the next day, and another night then drive home where they back the bus in and hook up the shore power cable and the battery charger goes to work until the next outing. Chances are with that setup they never took the batteries down all that far in the course of a couple of days and they really didn't need them recharged on the road (although the alternator did what it could) since they were heading home where the charger was available. If you take that same rig and keep it out longer with no shore power available and short road trips the batteries will just stair step down until they're dead since there's no practical way without an onboard AC charger connected to a power pole or a genset to recharge that large a battery bank with that small an alternator (unless the bus is always on the move).

There is absolutely no implication here that you have to do your system any particular way. I'm only posting this as "knowledge base" so to speak for folks that are interested in such things. It's your bus, do it your way but if you're interested in getting the most out of your system (and it does take a bit of learning and monitoring) there's some information here to start you out and it outlines the basics (even if you're just tring to purchase an portable inverter).

Cheers!

Les
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Old 03-23-2006, 11:19 AM   #16
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Fantastic post! You're right on the money with what you posted. I don't think 4000 amp hours would be enough to completely satisfy me.
Basically it just comes down to limiting my useage to what the system can handle and adding batteries as I can afford them. Either way the fridge is going in. It will be my one "high draw" appliance in the whole place. Coffee comes from the truckstop or in the freeze dried form, cooking is done on the grill or propane stove top, and under no circumstances will a passenger of my bus need (or be allowed) to use a curling iron or hair dryer unless it is one of those butane fired jobs my sisters both had way back in high school. You can't imagine the terror in my elementary school eyes as I rode with them in their roving beauty salon. I digress....

Anyway...worst comes to worst the fridge runs hard when the engine is running on down the road and becomes an expensive cooler after that.

Thanks for your input, Les.
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Old 03-24-2006, 10:03 AM   #17
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Thanks for the kind words; glad it helped.

There is an alternative but it comes at an expense. A few years ago RV and Marine refirgerators got a big boost when the Danfoss compressor starting being used. It's now possible to have a nice refrigerator that only draws around 3 amps on the DC side...now that's efficient! The Norcold DE line is one that uses this technology and they run on 12 volts or 110 volts (that is, it's a 2-way refer). You can see them here:

http://www.norcold.com/builtin.cfm

They also make portable models...just follow the links.

They're not really inexpensive (as far as I'm concerned) but they are very efficient and last years and years.

Other companies make them too; I just used Norcold because I'm more familiar with the line but do shop around.

You may be able to score one of the smaller units on the used market; quite often to keep prices down on boats and rv's the small units are installed. After a while folks want one of the large models and the smaller ones are pulled out and sold.

I know you were probably thinking "thrifty" since you originally asked about a dorm fridge and maybe that's the best way to go for your use. Just thought I'd let you know what else was out there for ideas. I'm using the DE-0061 in our conversion.
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