where's the power going? - Fiberglass RV


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Old 08-10-2019, 07:26 AM   #1
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where's the power going?

hi, i have a 2013 scamp with a 200w solar system i installed. works fine but i have one perplexing problem. with plenty of power available through the solar system (180W as measured by my in-line meter), when i put the frig on 12V, the batteries have a rapid drain (10 minutes or so) down to 12.4v or so. i have an in-line meter on power from batteries that shows about 115-120W going to the load (frig). no other circuits are active - before turning on the frig, the system is idling along with full batteries and only utilizing a few watts to keep the batteries at float.


i've verified with volt meter that the in-line meters are accurate so i am losing about 60w somewhere. i can't believe the meters would self consume that much (or they'd get very hot) and the charge controller is a sunsaver 15A mppt that should be well over 90% efficient.


my major suspect is the 12v converter but i don't know the details of how that works. i know the 12v power for the frig gets routed through the converter but i cannot tell if it's just a pass through of if the converter has some function when you're not plugged into shore.



i think the next step is to bypass the converter so the frig power comes straight off the battery and see if that changes anything.


any helpful ideas or insights greatly appreciated.
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Old 08-10-2019, 07:34 AM   #2
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Even with my full 320W in the sun giving maybe 50% maximum charge I would never run my fridge off of the 12V system, as I know it will bring the batteries down faster than it can charge them and run the fridge.

One day I travelled with 240W of solar power along with the power from the truck, and it slowly dropped the voltage of the batteries and allowed the fridge to warm up very slowly.

I now will only use 120V or propane to power my fridge. I have used the 12V when running out of propane while I go get some more.
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Old 08-10-2019, 07:53 AM   #3
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Even with my full 320W in the sun giving maybe 50% maximum charge I would never run my fridge off of the 12V system, as I know it will bring the batteries down faster than it can charge them and run the fridge.

One day I travelled with 240W of solar power along with the power from the truck, and it slowly dropped the voltage of the batteries and allowed the fridge to warm up very slowly.

I now will only use 120V or propane to power my fridge. I have used the 12V when running out of propane while I go get some more.

but why? the frig only pulls around 120W? why does that run the battery down when your solar power input is well above that?
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Old 08-10-2019, 08:10 AM   #4
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where's the power going?

I’ve heard nothing good about running absorption fridges on 12V. Even with plenty of available power, they are inefficient and often ineffective at maintaining safe temperatures.

I’ll leave the technical analysis of why yours is not getting enough power to those who know more, but I think you’re beating a dying horse. If I wanted to refrigerate using 12V as my primary power source, I’d switch to a 12V compressor fridge.

Given your robust solar array, and assuming it’s mated to adequate battery storage type and capacity- you didn’t mention what you have- you seem well-positioned to make the switch.
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Old 08-10-2019, 08:11 AM   #5
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Originally Posted by semievolved View Post
but why? the frig only pulls around 120W? why does that run the battery down when your solar power input is well above that?
Yes, but if you have 200W of solar, unless constantly aimed at the sun you will lose a lot of that. I am usually well less than 50% of my fixed panels full power potential, but my portable can be a lot better if I keep moving it.

Then there is all the other items I want the power for.

Solar 12V fridge does not work with my setup for full time use, that's all I know.
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Old 08-10-2019, 08:54 AM   #6
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jon and jim i agree with both of you that the dometic 12v option is very inefficient and not very effective either. it should however cool as well as the propane or 120v which are also not very effective! my point is simply that if i have an excess of solar power available during the day, i should be able to power the frig with that. yes, a few ounces of propane is a small price to pay but there's no reason not to save the propane and use the sun instead if i can.



jim, as i said in the OP, i have an in-line meter between the panels and the controller and it was measuring ~180W of power, which would have been more than enough to power the frig and keep the batteries charged.


i realized that another potential source of the problem could be a loose connection in the charge side such that the batteries are being allowed to drain faster than charge. i need to look into that.
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Old 08-10-2019, 08:58 AM   #7
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The batteries will always show a lower voltage while under load. This does not tell you their state of charge. The battery must be "rested" to measure the voltage for an accurate state of charge.

If you are measuring the collector output, before your controller, you are not getting the true picture. The controller has some inefficiency and the batteries have about a 25% loss during charging. It would be good to put a second meter on the system to measure the delivered wattage from the batteries, compared the collector wattage.

If it seems that your battery is not carrying the load it is rated for, it is probably sulfated.

An inverter runs at about 90% efficiency. So, if you have a load from an appliance of 100 watts, there will be a draw of about 110 watts from the battery. That would be about .83 amps at 120 volts. Or 9.2 amps load on the battery. I just take the amps at 120 volts X 11 to come up with the approximate amp draw at the battery.
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Old 08-10-2019, 09:04 AM   #8
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While I can't make measurements from here, I suspect that your 40 watt loss is spread across the system. Voltage drop over the wiring & connections, efficiency losses in the controller, etc. Trying to pull every watt out of a solar panel(s) is tough. Under ideal conditions you can get close (or even exceed) the rated wattage, but under average conditions, usually quite a bit less. Powering a large load such as the refrigerator on 12V can put a fair amount of stress on a system.

If you have a IR thermometer, you might compare the temperature of the wiring & connections without the load, and 15 - 20 minutes after adding the load.
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Old 08-10-2019, 09:33 AM   #9
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We are going through this right now. If you go to Go Powers website they just launched a calculator and the inverter and fridge pull a lot of amps and like the others have said above to put your fridge on gas.
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Old 08-10-2019, 11:29 AM   #10
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I measured 15 amps when running my fridge on 12vdc. So a solar system would need to be able to supply about 20 amps to keep the battery charged. So if you have excess solar and battery, you could run on DC for a while. But the fridge only sips propane.
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Old 08-10-2019, 11:36 AM   #11
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[QUOTE…………...
jim, as i said in the OP, i have an in-line meter between the panels and the controller and it was measuring ~180W of power, which would have been more than enough to power the frig and keep the batteries charged.
[/QUOTE]

180 watts at 12 volts is 15 amps. The 12v heating element in the fridge takes a lot of power. You may want to measure the voltage at the terminals on the back of the fridge..
Our 2000 Scamp/Dometic 3-way was showing only 10.5 volts while the battery was at 12v. The Scamp wiring runs all around the perimeter and through the converter. It was too light, had too much resistance.
Solution? run a heavy (12gage) wire direct from battery to fridge. Add a 20 amp fuse in that line.
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Old 08-10-2019, 11:50 AM   #12
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I suggest you all read the fridge manual.
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Old 08-10-2019, 12:03 PM   #13
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I suggest you all read the fridge manual.

What specifically, do you want us to see in the manual? Can you just tell us what you are referring to?
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Old 08-10-2019, 12:05 PM   #14
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More solar panels or run on propane. We took a trip to Alaska, ran the refrigerator on propane the entire trip. We did not even use a tank of propane. Even our 1977 Trillium has either 12 or 10 gauge 12 V wire running to it. So I would check wire size regardless.
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