Stealth Camper Build Thread

[Ran D. St. Clair]

RTFM – Read the F--ing Manual

The second of my assumptions to fall was the idea that I would use a more or less standard RV style inverter – charger – transfer switch.

I had been considering the Tripp-Lite RV750LHW, (Inverter - 750W, 1125W<1hr, 1500W<10sec, Multistage Charger - 45A, and Transfer Switch) or something like it. There are similar models made by Xantrex, Magnum Energy, and others. As far as I can tell, they all have the same killer problem for my application. When they see a valid AC input they want to turn on their transfer switch and pass that AC to their output, or in other words, to all the AC appliances.

The problem is, my wimpy little EU2000i is no where near strong enough to handle the air conditioner, battery charger, and all the AC appliances that could potentially be turned on all at once. If I was connected to shore power then turning on the transfer switch would be a fine plan, but they gave me no way to disable the transfer switch. It’s a shame, because technically speaking it would probably be an easy thing for them to do, but they didn’t.

On the other hand, my mighty little EU2000i is plenty strong enough to provide all of my daily energy needs, even in a worst case scenario. It just can’t meet the potential peak demand. I could get a 2nd EU2000i and put it in parallel for twice the current, but I don’t want to have 2 generators running if I can avoid it.

The solution is simple enough. I just need to buy a separate inverter and battery charger. A transfer switch might have been nice in some situations, like when on shore power, but I don’t really need one. The only down side is that the inverter is running all the time, but it is made for that, and the energy wasted is pretty trivial.

Another small problem is that inverters put a small load on the batteries even when no AC power is being drawn. I say a small load, but depending on the size of the inverter it could be a few amps. Some inverters sense the load, or lack thereof, and go into a sleep mode to minimize this current draw. I might want to use a relay with a switch inside the living quarters to turn off the DC power to the inverter at night when I don’t need AC for anything. On the other hand, I will be running the generator every day so the overnight loss of energy isn’t much compared to my daily needs.

To help with any back of the envelope calculations you might do, I will offer these conversion factors for AC to DC (Battery Chargers), and DC to AC (inverters). These are just approximate as battery voltage varies with load, temperature, and state of charge, and conversion efficiency varies with technology and by individual brand or model.

AC to DC (Battery Chargers) 120VAC/14VDC x.95 = 8.14, or 1A AC translates to roughly 8.14A DC. If you know the DC current you need then divide by 8.14 to get the AC current into the battery charger.

DC to AC (Inverters) 12VDC/120VAC x.9 = .09, or 1A DC translates to roughly .09A AC. If you know the AC current you need, then divide by .09 to get the DC current into the inverter.

To be continued…
?

[Ran D. St. Clair]

Two down and one to go…

The 3rd preconception to go was the idea of calculating battery capacity based on the energy needs for a day (or defined time period) without a generator.

In my case the generator is going to have to run every day. I could theoretically go a day or two with minimum energy use, but as a practical matter, if I am actually living in the truck, I need to run the generator for at least a couple of hours a day. Web’s have to be surfed and TV’s have to be watched you know…

The limiting factor is charge and discharge current, or the 25% rule. The maximum practical battery current with the generator and the charger working, and not running the microwave at the same time as the air conditioner, is about 87A. That means I need at least 4 x 87 Amp Hours, or 348Ah. The batteries I am looking at come in increments of about 220Ah, so 2 batteries in parallel is 440Ah, which more than meets the need.

There are still lots of details underneath these high level figures that I haven’t shared with you. In true engineering fashion, that involves a spreadsheet, or multiple copies of the same spreadsheet to play out different scenarios like a maximum energy usage hot day, or a minimum energy usage comfortable day.

I made a list of all the electrical devices that I could think of. Everything from 12V electric blankets, to a Play Station 3 with a flat screen LCD TV. Don’t forget the basics like overhead lights and a refrigerator, etc. Then I researched the current, or power required for each of these. A good source for some of this information is:

http://www.oksolar.com/technical/consumption.html or http://www.psnh.com/Residentia...l/Applianceusage.asp or
http://www.warehouseappliance.com/elecfridge.htm

All these devices break down into two general categories, AC and DC. The AC currents can then be translated into DC currents using the conversion factors I provided earlier. A time factor can be applied for each device, in other words, how many hours per day it will be on. That allows the calculation of Amp Hours, or Watt Hours if you prefer. With all that information in one place, plus battery charger current and a few others, it is possible to calculate a bunch of interesting metrics. At least these are the ones that I thought were interesting.

11.4 Battery Energy replacement time at full charge current (Hours)
7.48 Battery Energy replacement Time at Full Generator Capacity (Hours)
241 Maximum Battery Current if Generator and Charger are not working (A)
87.07 Maximum Practical Battery Current with Generator and Charger working and not running the microwave at the same time as the air conditioner (A)
348.2 Minimum battery Amp Hour rating based on 4X Maximum Practical Battery Current
66.1 Average Battery Current for 12 hours per day (A)
2397 Peak Inverter Wattage assuming everything possible is on simultaneously (W)
1597 Maximum Practical Inverter Wattage assuming the Microwave is not on at the same time as the Air Conditioner (W)
10.7 Generator Load Assuming only the Battery Charger (A)
81% Percentage of Generator steady state capacity at maximum Generator Load
0.6 Battery Hold Up Time (Hours) with everything on without the generator or the Battery Charger
2.2 Typical Expected Battery Hold Up Time (Hours) without the generator or the Battery Charger
4.2 Typical Expected Battery hold Up Time (Hours) without the generator or the Battery Charger or the Air Conditioner
83% Average Battery Current for 12 hours per day as a percentage of Battery Charger Capacity

The above figures are for a maximum energy usage hot day. By changing the number of hours that each individual appliance is on you can model any sort of day you like. As it turns out, if it’s a nice day, meaning I don’t need to run the air conditioner or the overhead fan much, and I don’t watch an excessive amount of TV or play on the computer all day, I actually can go the whole day just on batteries.

If anyone wants the actual spreadsheet, PM me with your E-mail address and I will send it to you.

To be continued…

[Ran D. St. Clair]

Stealth Camper Design Drawings have been posted along with the earlier pictures. They can be found by searching for key word "Stealth".

[Ran D. St. Clair]

Assault and Batteries…

This web site is an excellent source of information about deep cycle batteries.

http://www.windsun.com/Batteri...ery_FAQ.htm#Amp-Hour Capacity

Now I can begin to choose the actual batteries. There are many technologies to choose from including the cheaper flooded cells, or the higher quality AGM (Absorbent Glass Mat), or even gel cells, and those are just the lead acid batteries of one sort or another. Nowhere on this list are car batteries. They are just not suitable.

There are also many configurations, like 6V batteries, 12V batteries, 2V batteries (or individual cells), and various capacities in series and parallel combinations. I may be cheap, but I am willing to spend money for quality when it’s something I really need to rely on. That means AGM (Absorbent Glass Mat). I would also like to be able to pick up one of these batteries without killing myself. A 12V battery at 210 amp hours is 135 pounds. I could probably pick that up if I had to, but wrestling it into a battery compartment sounds ugly. Even then, I would need 2 of them to meet my needs. Better to use 6V batteries at 220 amp hours and 66 pounds each. I would need 4 such batteries, two parallel sets of two in series. That would give me 440 amp hours and a total weight of 264 pounds.

All of this brings me around to money. Each of those batteries is $280, so my 4 battery design is $1,120 for batteries alone. I could cut that almost in half by using good quality flooded cells, but my longevity would be reduced from 1,000 cycles to 750 cycles, or something like that because I don’t trust those figures to be more than approximate. 1,000 cycles is less than 3 years at a cycle per day. I would hope to do a bit better if they are mostly shallow cycles staying well above 50%, but even so, in 5 years or so I will be spending another $1,120 plus inflation, disposal fees, etc.

Before I leave the subject of batteries I should mention that putting batteries in parallel isn’t an ideal situation, though it is often done. If one of the parallel strings is weaker then it tends to drag the stronger string down to its level. This is one place where a battery combiner or electronic feedback relay can come in handy, not to keep the house batteries separate from the starter battery, but for keeping the parallel strings of house batteries separate. It will drain the house battery strings one after the other depending on how you have it programmed. It can also sense the battery voltage and prevent you from accidentally discharging the batteries more than some pre-set level, like 50%. Since I will be starting with brand new and well matched batteries a battery combiner won’t be an immediate necessity, though I may want to spend the money on one eventually.

If actual life experience dictates that I need a 3rd or even 4th string of batteries to increase my capacity, I can still do so, The physical size of these batteries is about 10.28” x 7.06” x 9.63”. I can put 4 batteries on my mounting tray with no difficulty, and 6 batteries with just a little effort. With 8 batteries I have to stack them 4 batteries each, two high. I really don’t want to do that though. The idea of batteries crashing down on top of other batteries, shorting them out, catching fire, exploding and spraying sulfuric acid all over the place doesn’t appeal to me.

Here is a listing of some of some possible batteries:

Lifeline AGM 6V Golf cart Battery, GPL-4CT http://www.powerstridebattery....y=214&idproduct=2016

Lifeline AGM 12V Deep Cycle Marine Battery, GPL-4DL
http://www.powerstridebattery....gory=0&idproduct=837

UPG UBGC2 Golf Cart Battery, UBG-C32
http://www.apexbattery.com/upg...cart-batteries-.html

US2200 XC 6-Volt Golf Cart Battery, US2200 XC 6V
http://www.powerstridebattery....y=214&idproduct=6878

Battery___________________________________________ Ah___________________Longevity
Part_______________Length___Width__Height__Weight_ (20Hr__Tech-___________Cycles
Number______Volts__(in)_____(in)_____(in)_____(lb) ___Rate)__nology_____Price__At 50%
GPL-4DL______12____20.76____8.7____8.63____135____210_ ___AGM______$609___1000
GPL-4CT_______6____10.28____7.06___9.63_____66____220_ ___AGM______$280___1000
UBG-GC2_______6____10.25____7.07___10.19____64____200_ ___AGM______$211____???
US2200 XC 6V__6____10.25____7.125__11.25____65____232____Flo oded____$154____750

To be continued...

[geofkaye]

...look into "fork lift batteries" and also add a 12vdc air to air heat exchanger to your plans....BAL makes them and they are about $200 if I remember correctly...if they had used them in the Katrina trailers there would be no problems with the build up of cooking odors/people odor/chemicals/VOC's/and moisture.....but the government had their head up someones exhaust pipe again....and there was sure to be some foolishness with the manufactures as they were aware of the problems with trailers as with any coach......I will not turn any unite back without one now.....they proved them selves to me!......geofkaye

[Bob86ZZ4]

Are you sure agm lasts longer than a standard flooded cell? I thought I had read they don't. Only advantage to them is they can be mounted upside down.

[Ran D. St. Clair]

Originally posted by Bob86ZZ4:
Are you sure agm lasts longer than a standard flooded cell? I thought I had read they don't. Only advantage to them is they can be mounted upside down.

Bob86zz4, I can't say that I am sure. The manufacturers published specifications that I posted listed 750 cycles for Flooded cells vs. 1000 for AGM. That's just one example, and I don't claim to have done exhaustive research. I suspect that there are tons of convoluting variables, just one being how dilligent you are with maintaining the batteries at the proper fluid level. I am also told that the AGM's are supposed to be a bit tougher mechanically because the glass mat separates the plates and keeps them from shorting together. All I know is what I read, and while it sounds plausable I can't personally confirm it.

[Ran D. St. Clair]

Originally posted by geofkaye:
...add a 12vdc air to air heat exchanger to your plans....BAL makes them and they are about $200 if I remember correctly...geofkaye

Goofkaye, can you provide any more information on these. I have done a web search and while I am finding some cross flow heat exchangers, or HRV (Heat Recovery Ventilation) as they are sometimes called, I am not finding anything for BAL, nor anything that is 12V, nor anything that is for a small space. Most seem to be for very large houses.

Also, from what I am seeing they only recover 60% to 70% of the heat/cold, so I wouldn't want to have much airflow through one for my application. I am even wondering if I should just go get about 12 ft of double wall stove pipe, wrap it with insulation, and mount it under the truck somewhere. I could use the inner and outer for opposite airflow and put a little tiny 12V fan on the inner where it comes up through the floor. I am also a little concerned that warm air coming into my air conditioned truck would condense inside the heat exchanger, so I would have to make provisions for the condensation to drain, which would be relatively easy if its just a long straight pipe with a downward slope. Likewise warm moist air leaving the truck going to the cold outside would condense in the exchanger but sloping down to the outside end solves that problem as well.

[Ran D. St. Clair]

Give me a sine..

First, I need to give credit where credit is due. The Magnum Energy web site offers free access to their owners manuals and even though I can’t use their product, I think their owners manuals are really well written with a lot of useful information. One such manual can be found here:

http://www.magnumenergy.com/MMseries.htm

They do a really good job of explaining in simple terms the difference between a true sine wave inverter and a modified sine wave inverter. The following is taken directly from that manual.

“Today’s inverters come in two basic output waveforms: modified sine (which is actually a modified square wave) and pure sine wave. Modified sine wave inverters approximate a pure sine waveform and will run most appliances and electronics without any problems. These inverters are less expensive and, therefore, offer a viable alternative to more expensive pure sine inverters. The output of a modified sine wave inverter will run most electronic and household items including but not limited to TV, VCR, Satellite dish receiver, computers and printers. Some devices such as rechargeable power supplies for phones, drills, and such devices may not run or be damaged by modified sine wave inverters.”

To this I would add that I have heard of problems with video distortion on TV’s or monitors, audio distortion or buzzing, some appliances with motors making more noise, some appliances like microwaves having less than full power, and general loss of efficiency in the appliances themselves possibly resulting in their early demise. Mind you, this is all hearsay, not fact, and should be taken with a big grain of salt.

Using the Magnum products as a rough guide, The MMS Pure sine wave inverter/charger is rated for 1,000W continuous at $1,199. The MM1212 modified sine wave inverter/charger is rated at 1,200W continuous for $899. That’s 20% more power for 75% of the price, or in terms of watts per dollar, the modified sine wave unit provides 60% more value.

So the decision is obvious right? Exactly, I am going to use a pure sine wave unit regardless of the make or model. Why you ask? I would guess that a modified sine wave has a 90% chance of meeting all my needs and saving me money. Given my luck, that means I will buy it and find that it doesn’t work, then have to resell it at a big loss, and buy the pure sine wave unit anyway. Mostly I don’t want to risk the time or the money. I will take the 90% probability that I am wasting money to make a choice that I know will work, at least for this issue.

You may note that I didn’t mention the efficiency of the two types of inverters. That’s because I don’t think I can. The modified sine wave inverter might be more efficient on paper but it really depends on the load. If the load operates inefficiently because of the modified sine wave then it’s not fair to measure just the efficiency of the inverter. The bottom line is that I just don’t know.

Here are some possible inverters that I am considering, but not endorsing:

AIMS 1500 Watt, 12 Vdc, Pure Sine Wave Inverter 12 Volt. FREE Remote, Model #:PWRI150012. $449
http://www.theinverterstore.co...=pwri150012s-top-rgb

AIMS 2000 Watt PURE Sine Wave 12 Volt . FREE Remote. Model #:PWRI200012S, $599
http://www.theinverterstore.co...=pwri200012s-top-rgb

2500/5000 Watt Pure Sine Wave Power Inverter with 12V DC Port PE-2500PSW-12-110, $495
http://www.topsalesdepot.com/bwapusiwapoi.html

Those web sites also carry lots of modified sine wave inverters if you are luckier than I am and want to save some money.

To be continued…

[geofkaye]

GIVE BERNIE A CALL AT 1-574-262-3400 HE HAS ACCESS TO THE BAL UNITES AND WILL GIVE YOU A NUMBER IN CALIFORNIA....which is the manufacturing plant. It is a neat little machine and very well built considering it is used in the RV industry....it is all steel and has the drains that you are asking about.....12dcv 2 wire...use a mechanical timer.....figure out your cubic feet and set the time to change out the air...I use it in the AM while I sleep to freshen up the morning air so to speak....the dam girlz use hair spray and other toxic chemicals to make the air un-breathable when they are dressing for a show....and the alcohol/cigarette stink on them makes me choke in the AM...cuz they just can not come back to the trailer and get ready for the next day without drying up every source of alcohol available and leaving a lot of old sales-fools hung over/broke/feeling like death/and easy marks to buy drinks/[SELL STUFF AT BELOW COST]-all nite.....and think they are going to have some "cheap tricks" to tell the other bozos about the next nite.....[little do they know...]....Show girlz run the show[RANT OFF]....geofkaye

[Ran D. St. Clair]

I know I left my electrons here somewhere…

Lets talk electrical monitoring systems. Ideally the electrical system should have tons of margin and you would never have to even think about how much current is being drawn or whether your batteries are sufficiently charged. Everything would be automatic and the generator would start itself.

Clearly that is not the reality of my world. I need to be able to keep track of things and make sure they don’t get out of hand. Perhaps in time things will fall into a pattern and I will develop a good feel for what I need to do to keep everything happy, but I still need some basic information to help develop that feel.

There are lots of super cool and expensive systems out there with remote readouts and programmable alarms etc. They are probably all better than what I have planned. I intend to install low cost digital panel meters at various locations to measure voltage or current or both. Measuring something like house battery voltage is easy. Just hook up the meter with an appropriate pot to calibrate it, and possibly a small regulator to provide power and you are done. It is always on, but it draws a trivial amount of current so who cares. The only issue is where to place it for convenient readout.

Measuring DC current is only a little harder. Even the heavy duty wires I plan to use will have some voltage drop, especially at 100 Amps or more. It’s just a matter of measuring the voltage drop across the wires and calibrating the meter accordingly. These units are commonly available with 0.2V full scale input. If I have to I can make a little op-amp circuit to boost the voltage, but I suspect that won’t be necessary.

http://www.futurlec.com/Panel_Meters.shtml

So what kinds of things do I think I need to know?

1. House Battery Voltage
2. DC Current from the Battery Charger
3. DC Current into the Inverter
4. DC Current to the internal 12V appliances.
5. Net Battery charge/discharge current (can be calculated from #2, #3, and #4 above)
6. AC Current from the Inverter (can be approximated based on #3)

At a minimum the house battery voltage should be viewable from inside the living quarters. The DC current from the battery charger, to the appliances, and into the inverter would be very useful as well.

You may note that there is no such thing as a battery gas gage on my list. Certainly such things are possible but they are notorious for being wrong. It is possible to integrate the net current into and out of the battery over time, and knowing the size of the battery and a fixed starting point like when the battery is fully charged, to estimate the current state of charge. The estimation errors add up over time though, so you have to keep resetting the fixed starting point. Some circuits will attempt to figure all this out automatically just by monitoring the battery voltage and current, but they are easily confused. I don’t plan anything so sophisticated. I will simply keep an eye on the battery voltage under nominal load, and also on the charge current when the battery charger is running.

I have purchased a clamp on ammeter that can measure both DC and AC current. It’s also a voltmeter and a few other things, but mostly I needed the ammeter. It’s an Extech 380941 for about $240.

http://www.extech.com/instrume.../310_399/380941.html

http://metersuperstore.com/Mer...ch_Type=AND&Offset=0

It might seem a bit pricy, and compared to a cheap multimeter it is, but there are many more expensive makes and models for measuring DC current, and having to insert a shunt resistor into a high current line is a big hassle. I have the unit now and it seems to work well enough. I set up a 20A current loop with a regulated bench power supply and it measured 20A so I guess it works as advertised. Speaking of the specifications: CAT III, 300V, AC Amp Max Resolution: 200A / 10mA, DC Amp Max Resolution: 200A/ 10mA, AC Volts: 400V, DC Volts: 400V, Jaws: 0.9", Resistance: 400 Ohms, Frequency: 10kHz, Display Range: 4000, Other: Min/Max, Hold; Auto Zero, Leads, Battery and Case Included.

To be Continued…

[geofkaye]

.....with all this techno stuff-are you going to have any time to do anything outside of the coach?.....I dearly love gizmos but at this time and with all there is to do outside of the vehicle....I wouldn't spend any time messing with or even thinking about volts-amps-storage capacity-current or anything else but enjoying my time doing other things........sure you don't want to just hit the start button and let the generators do their thing?.........maybe it is my age-but I like automation without any effort on my part....geofkaye

[Bob86ZZ4]

I'm kind of with Geof here on this. I think just a voltage meter would be best. Then you can check the voltage once in awhile and when they get low you start up the generator and top them up. I'm planning on putting in a voltage meter on mine with a momentary contact push button so I can just push the button for a reading. Then it won't draw any power when that button is not pushed. Very simple.

[Ran D. St. Clair]

All,

I agree, there is a priority to these things, and battery voltage is at the top of the list. I could probably live without the others, but the idea was to have always on displays in the living quarters so I won't have to think about it much, just look up and see it. Modern Liquid Crystal Displays draw so little current that you don't need a push button.

KIS (Keep It Simple) is a great idea, and I will wherever I can, but a stealth camper needs to have power available without running the generator all the time. Plus my Honda is a pull start so I can't just hit the button. (I could possibly run the pull cord up into the living quarters though - hmm?)

Yes, all this volts and amps stuff has been going on for a good long while, but hey, I am an electronics engineer, it's what I do. Don't worry, it's almost time to move on to other exciting things, like refrigerators and my favorite, toilets!

As for doing things outside the coach, for now its work - coach - work - coach - sleep a little...
In a little less than two years it's going to be sleep, goof-off, nap, drive around a little, nap some more, nosh, visit friends, nosh some more... The wait is killing me...

[Ran D. St. Clair]

Refrigerators leave me cold…

At this point I still had lots of choices like whether to use an AC, 12V DC, or a propane refrigerator.

My first assumption was that I would be using a conventional propane refrigerator of the sort that is most often used in RV’s. They are, after all, made for just that purpose, and there must be some good reasons why they are so common. My research suggested that they are quite efficient, using between 1.5 and 2 gallons of Propane per week. I am not sure if I trust those figures though, especially when my son is standing in front of the thing with the door open trying to figure out what he wants.

As I studied the literature more deeply I became concerned about their heat carrying capacity. They warned of having to wait a long time for things to get cold, and even suggested waiting until after it was cold to put the food in.

I read the owners and installation manuals and while nothing about it looked terribly difficult, I wasn’t entirely happy with it either. For one thing there is the whole issue of piping propane into the living quarters. Now I will say up front that I fully intend to use a propane range, a propane heater, and possibly even a propane toilet, so having propane for the refrigerator is not much of a stretch. There are, however, some differences. The range would only be used intermittently and when I am there to keep an eye on it. The propane heater might be on a thermostat and might be on intermittently all night long, or possibly even when I am not there, so it is more similar. Regardless, I fully intend to have a smoke alarm, propane gas detector, low oxygen detector, fire extinguisher, and whatever other safety gear I can think of.

Still, I want my refrigerator to be running 24/7, whether I am on the move or not. Some people seem to think that running a propane refrigerator in a moving vehicle is fine, while others think it is shear madness. I tend more towards the fine category, especially since the owners manuals seem to suggest that it is not a problem technically, while simultaneously taking no responsibility whatsoever for the consequences. It seems that bouncing around helps to keep the fluids flowing in the gravity fed system.

There is also the issue of combustion air inlet and exhaust ducting. Obviously it can be done, but I am not thrilled about punching more holes in my floor and roof. I would also have to be very careful about sealing things up around the fridge so combustion gasses didn’t enter the living quarters. I am going to a lot of trouble to seal things up nice and tight for energy efficiency, and there isn’t a huge amount of air in there to begin with. I would probably also want outside access for burner cleaning and maintenance, which means punching a hole in the FRP outer wall and mounting an access door, also entirely possible but not something I am crazy about.

From what I have read at least some models of propane refrigerators will not work well at temperatures above 90 degrees. I imagine all of them become more inefficient at higher temperatures. The back side of the refrigerator would be conceptually on the outside of the living quarters. It needs to draw in outside air for combustion and exhaust hot gasses. That means if it is 105 degrees outside, my refrigerator is trying to dump heat into that already hot outside air. It doesn’t matter that the refrigerator is physically inside the living quarters and mostly surrounded by 70 degree air conditioned air. This also implies that I would need to be very careful about insulating the outside air ducting so it doesn’t dump heat into my air conditioned living space.

To be fair, an electric refrigerator is just a heat pump, taking heat from the inside and dumping it into the room. I have to believe however that burning propane to make cold has to generate more net heat than an efficient electric powered compressor. Then again, if that heat is dumped cleanly overboard then who cares? At least with an electric fridge, it is dumping heat into a 70 degree room, presumably kept that way by the roof top air conditioner, even if it is 105 degrees outside. The refrigerators performance shouldn’t suffer, though the AC unit will have to work a bit harder.

The biggest thing that bothers me though is the requirement that a propane fridge be level. Now I know that some of you will say that having your rig level is a requirement for many reasons, but I don’t see it that way. Sure, level is better, but I want to be able to pull over to the side of the road and hit the sack without making a big production of it. If I am in one place for a while I might use leveling blocks or jacks, or whatever, but I don’t want to have to. Most roads have a crown so normal parking on the side of the road is going to have the rig leaning to the right. I designed my bunks with the head to the left for that very reason. I find that slightly feet down doesn’t bother me.

The manuals seem to suggest that if the fridge isn’t level then the fluids won’t flow correctly. The internal liquids will puddle in the wrong places blocking the flow. To me that implies a burner flaming away, trying to cool the fridge down, meanwhile my food is spoiling, and I am probably sleeping 10 feet away too stupid to recognize what’s going on. I don’t want to be a slave to a temperamental appliance.

To be continued…

[Ran D. St. Clair]

An electric ice box? Ridiculous!

All of this set me on the path of at least looking into some very efficient 12V refrigerators. Heck, I would even consider a very efficient AC powered refrigerator. The loss of efficiency through the inverter isn’t that bad.

In broad general terms anything that runs off of 12V DC is going to require less power than the same thing that runs off of AC because the inverter that converts DC to AC has a limited efficiency, around 90%. An exception to this rule would be a 12V DC appliance that has its own inverter built in, like some microwaves. In that case you are trading a large, hopefully efficient inverter, for a small inverter that may or may not be very efficient. You would have to look up the specifications on the individual appliance.

There are 12V coolers using Peltier effect, meaning solid state thermocouple type devices that act as a heat pump. They work fine for keeping cool stuff cool, but they don’t have a lot of heat transfer capacity, meaning they won’t cool things down very quickly, and they also use a fair amount of precious electrical power. I have a cooler of this sort that draws less than 5A at 12V, but it takes forever to cool anything down. I want a real refrigerator with a real freezer section that can reliably keep frozen things frozen and make ice cubes as well.

I found some very efficient 12V refrigerators made by Sun Frost. The Sun Frost RF12 is a 12 cubic foot model that draws somewhere between 1A and 2A on average, weighs 230lb, and will cost me almost $2,800 with shipping taxes and all the rest. These are made for stationary use, so it is not clear to me how well they would hold up in an RV. They also offer a 19 cubic foot model that would draw between 2.58A to 5A, weigh 320lb, and cost me about $800 more. By contrast a modern and relatively efficient Frigidare 20 cubic foot model is rated at .52A at 110VAC, or an equivalent average of 5.3A at 12VDC, assuming it was powered through an inverter.

I think I can live with 12 cubic feet. $2,800 is a lot of money for a small fridge but there’s nothing about this project that’s cheap. A small fridge like that would best be mounted in a cabinet of some sort, so I might as well design it with some shock absorbing materials to take some of the sting out of bouncing down the road, better insulate it, and keep it from sliding around. The radiator coils in the back would need good air flow but I think I can manage that. Yes, the heat it generates would dump into my living quarters but it’s tiny by comparison to what the roof top AC unit can pump. The fridge creates about 24W of heat energy, or about 82 BTU/hour, and the AC is rated at 9200 BTU/hour, so the fridge accounts for less than 1% of its capacity.

At 2A DC compared to 80A for my battery charger, I would have to run my generator a minimum of 36 minutes per day to support it. I think I can live with that.

The following are some useful web sites I found:

http://www.sunfrost.com/refrigerator_models.html

http://www.firemountainsolar.com/refrigerators.html

To be continued…

[Bob86ZZ4]

Wow, I dunno. I sure think my propane fridge is a great thing. Extremely efficient, runs forever on hardly any propane. I don't worry about the level thing. It works great running down the road and I've never noticed a problem with it on minor off level settings. The only problem I know of was at high altitude it didn't work well. I guess for your application maybe that one you've found will be better.

[Bob E]

On the other hand,...I went with a high efficiency 120VAC refrig. It doesn't need leveled, cools down (from being shut off)quickly, & only costs a little over $250 to replace if it crapps out (not $1k+ like propane refrig).
We're set to dry camp, so with 400 watts of solar, 8 deep-cycle 6V batts, & 3600 Trace inverter we can run almost indefinately (refrig & other intermittent appliances) without gen - so long as the sun shines. If AC is needed, obviously we would need the gen.

[geofkaye]

....I use a 120vac 5.7 cubic foot freezer made here in Ohio...it is very efficient and only requires 120vac for about a half hour a day[ I use a mechanical timer] to keep stuff cold enough-[some times frozen like a brick-OOPS!]...it is a top loader and foam insulated...$149.00 at hh gregg out the door lifted into the trailer-mounted on/glued to a 3/4" plywood square w/casters with a 2" foam pad under it to help with the shock absorption....I can roll it in and out as needed as it is used for an Ice chest/beer cooler/wine cooler during the party season....AM I CHEAP AZZ YOU ASK???.....the answer is HELL YES! it works for me and that is all that is required.....[BTW- if need be-one could drop in a body to freeze it solid in about 19 hours...should you need a storage for a fresh corpse-].... but since my cadaver dog is dead now and we no longer search I have room for many other things like steaks and chicken-and of course 50 bottled water and 50 iced tea cans.....geofkaye

[Ran D. St. Clair]

Hey guys, I'm glad to see you're still out there. I figure the real value of this thread is for the hundreds (thousands?) who will read it in the future and hopefully learn something. Most of them will never post, and even if they wanted to, a year or more from now we will all have moved on. So keep those ideas and alternative opinions coming...