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Old 06-03-2019, 09:30 PM   #1
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Default I connected bedside AC outlets to the Transfer Switch

Our FunFinder came with a 1000W Pure Sine Wave Inverter which was only connected to the LG Refrigerator which draws only 2.8 Amps. I rewired the bedside AC outlets to the Transfer Switch so that we now have AC to run our CPAP machines even when dry camping. Each CPAP is under 10W when the heaters are defeated.
Photo 1 shows the new junction box I added.
Photo 2 is with the cover attached.
Photo 3 shows the bedside AC outlet.
Photo 4 shows the original wiring.
Photo 5 shows the manual Inverter disconnect switch, the 1000W Inverter and the Transfer Switch mounted to the top of the storage area (unchanged from factory).
Photo 6 I reinstalled the paneling to hide and protect the wiring.

Sorry I don't know how to rotate photos for posting!
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Old 06-04-2019, 09:55 AM   #2
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One correction/clarification: The LG Refrigerator draws 2.3Amps (~250 Watts) while the compressor is running but essentially nothing the great majority of the time as the compressor is OFF. The Magnum CSW-1012 Inverter has a maximum no load current draw of 1.2 Amps from the battery line and has an ON/OFF control switch in the trailer.
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Old 06-04-2019, 12:34 PM   #3
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Jimbo, you must have a boat load of batteries and solar panels/generator too run that thru the night.
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Old 06-04-2019, 02:01 PM   #4
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Okay, let's think this through.
I'll be overly conservative and use an average 3 Amp discharge rate which would be 24 amp-hours in a night. Using an average 2 Amp discharge for the other 16 hours would be 32 amp hours. That means the total used is on the order of 56 amp-hours plus or minus each day. I have two 2016 12V deep discharge lead acid batteries which are each probably rated around 40 amp-hours. Would you agree (I am guessing here)? That means I probably need to replenish some of the charge each day or risk damaging the batteries.
If I were to purchase a 100Watt portable 'suitcase' solar panel I would only be able to recover perhaps 2 Amps (average) for maybe 5 hours per day. The 10 amp-hours of charging is only on the order of 20% of my daily consumption. By this logic I would need several panels (perhaps 4-6).
In contrast my 2000i Honda inverter generator or my TV could relatively easily and quickly recharge the batteries on a daily basis.
Our LG refrigerator and our CPAPs are two strikes against boondocking (with no hookups).
Comments? Am I missing something?
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Old 06-04-2019, 10:30 PM   #5
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Jim, by my calculations, a 100 watt panel at 12 volts produces 8 amps. I could be wrong, though!!
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Old 06-05-2019, 10:02 AM   #6
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Well, you know the definition of Watts equals Volts times Amps! The only issue is that a 100 Watt panel represents its ideal power output under test conditions. That means that the light intensity and the angle of incidence is ideal. I have attached a spec label of a '100W' Kyocera panel. You will see that the rated wattage is at a light intensity you are unlikely to achieve here on earth due to the atmosphere. They list a more realistic output of 73W at a lower light intensity and a higher cell temperature. That is approximately a 29% reduction and neglects the angle of incidence, clouds, haze, any shading, dust on the panel, actual ambient condition, etc, etc. I have read that the actual output of a panel over several hours is probably closer to 25-30% of the rating.
Also, another point of trivia is that a solar panel is better thought of as a constant current device rather than a constant voltage device. The reason is that the panel will tend to output a similar voltage regardless of the light conditions (but it will vary with temperature). However, if you measured current you would tend to find that the current output is almost directly proportional to the light intensity. The better MPPT Controllers for charging batteries from solar panels try to maximize the Voltage and Amp points to thereby maximize the Power output.
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Old 06-05-2019, 10:54 PM   #7
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Jim. I'm not an expert but I think you are going to need at least 200 watts of solar in order to charge those batteries daily. I would go with moncrystalline type cells since they are more efficient than polycrystalline. I would mount a 100 watt panel on the roof and a second panel that is portable. For the one on the roof, I would run the wiring up the frig vent stack and tie it in parallel to the SAE connector for the portable unit. That way combined the give around 20 volts and 6-8 amp/hr.
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Old 06-09-2019, 12:36 PM   #8
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Seems like only the AC wattage is being considered here. Inverters draw a lot of DC current from the batteries to make AC. I looked up the specs on your Magnum unit. It draws 94 amps DC to make rated output of 1000 watts AC. If your estimate of 3 amps AC draw is correct, that is 360 watts AC. Your inverter would draw over 35 amps DC per hour from the batteries to make that load. Magnum's estimated run time for 350 watts, using larger 120 amp hour batteries is 3 hours. Your 80 amp hour batteries would deplete in less than 2 hours making 360 watts AC. Think you will need more battery power to meet your needs.
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Old 06-09-2019, 02:55 PM   #9
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Default Relation of AC and DC Watts for an Inverter

Good comments! First let's talk about the relation of AC and DC Watts. The equation (don't let me scare anyone) is DC Watts equals AC Watts divided by the efficiency of the Inverter expressed as a decimal number. If the efficiency were 100% then DC Watts would equal AC Watts as the divisor would be 1.0. This is not the case as all Inverters are less than 100% efficient because they have a no-load current draw as well as a loss dependent upon the load. The 94 Amps of current draw for 1000 Watts of output is specified at a nominal voltage of 12.5 Volts DC. The DC Wattage would calculate to be (12.5)*(94) = 1175 Watts. The efficiency at full power would be (1000W)/(1174W) = 85% or 0.85 as a decimal. Also note that as the battery voltage drops that the current draw in Amperes will increase even though the Wattage remains essentially the same. In our above example of a 1000 Watt load on the Inverter the input DC current at 12 Volts would rise to (1174Watts) / (12Volts) = 97.83 Amps. I note that the FF factory installed a 110 Amp fuse in the circuit feeding the Inverter. in my trailer.

Now lets talk about the current from the battery. Some loads are continuous and nearly constant. In the case of our CPAPs they are nearly constant. A light bulb or TV would be another example of a constant load. Other loads are intermittent use like microwave ovens, induction burners or blenders. The compressor on the LG refrigerator is only drawing the AC current when it is actively running. While it is running the DC current draw would be high (as you stated) but when the inside temperature reaches the set point the compressor will turn off and only cycle on for brief periods of time unless the refrigerator door is opened. When I guessed at the DC current draw on the battery I tried to make an estimate based upon the ON time of the compressor being very short compared to the OFF time. It is certainly true that the heat loss of the refrigerator is certainly bound to be much higher on a warm day than during a cooler evening and that opening the refrigerator door during the day will cause the compressor to run longer. I am agreeing with you that perhaps I have not been conservative enough on the DC current draw during a 24 hour period. Keep in mind that FF provided two 12V deep cycle lead acid batteries which probably had ratings of close to 50 Amp-hours when new and I used a derated number of 40 Amp-hours each or 80 Amp-hours total. To limit possible damage to the batteries the current draw should not exceed ~50% which means that the total discharge before recharging is needed should not exceed ~40 Amp-hours. In a 24 hour period this would equate to only (40) / (24) = 1.7 Amps of average current draw. This points to the need for more batteries or higher capacity batteries like Lithium to support a longer time between charging and better tolerance of discharging.

I started this thread to illustrate the complexities of trying to predict battery life before recharge is needed and whether solar, the Tow Vehicle or my Honda generator might offer the best recharging method where cost is a consideration. I already own the TV and the generator but would need to purchase a solar charging system or upgrade the battery storage. It seemed to me that I would need 4 panels or more, plus an efficient solar controller. I am grappling with how all of this fits my wallet. Thanks for your contribution!
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