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Old 06-12-2019, 05:37 AM   #1
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Solar panel information

Hey all, new to this form.
Looking to get some info on how to run my rv fridge off solar when i am not at the park. My current set up is a 100w (mc4 connector) panel running to a 30a charge controller, from there to my original rv battery battery. From the battery I have my in 1500w inverter hooked up to which i plugged my fridge into while i am gone; however the battery went dead the first day and my low voltage warning went off on my inverter and the fridge stopped.

Since this I am just running fridge off propane with the panel charging my 1 rv battery.

This week however i picked up another deep cycle battery and a 50w panel (coleman brand name - has different connectors). I may have a chance for a 3rd battery.

I was wondering if I hook the 3 batteries up in parallel, run my 100w and 50w to the charge controller, from there to the battery's then hook up my inverter from the battery's then plug my fridge into the inverter. Will this keep my fridge goin when not there???

Would this work or is there another way I can set this up???

Thanks in advance
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Old 06-12-2019, 07:17 AM   #2
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The fridge will draw about 300 watts on AC. So you will need over 300 watts of solar just to run the fridge in full sunlight. Lets assume the fridge runs 50% of the time . So it would use 3600 watt hrs of power. Since the panels will only see full sun for about 5 hrs a day , you would need 720 watts of solar . With the efficiency losses and if your fridge needs a longer run time , you will need more solar.
Your current setup will work with the fridge running on propane.
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Old 06-12-2019, 07:45 AM   #3
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So if I hook up 2 deep cycles battery's (my original and a another in parallel) this would be fine with the 100w panel to run my fridge off propane during the week??

And if I use my 3rd battery with my 50w panel this will be fine to us with me inverter to run led Xmas lights after quiet time???
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Old 06-12-2019, 07:47 AM   #4
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Need to know more about the fridge make/model to assess the power draw, but soppy's numbers are in the ball park for an RV absorption fridge. Wattage will be 240-350 and it will be running 50-80% of the time. A residential fridge draws more power but for shorter times, so that often works out about the same total power. The net is that you need a lot more solar, many hours of direct sunlight and around 400 AH of battery capacity to operate that way. Your original battery is probably about 80 AH capacity, so maybe 5 of those or 3-4 of a larger size.

If you have an RV fridge, I suggest you operate in in LP gas mode while away. Your existing battery and panel should handle that fine. The fridge draws only a few watts of 12vdc when in LP mode, just enough for the circuit board and gas valve.
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Old 06-12-2019, 07:58 AM   #5
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My residential averages 24watts per hour in a 24 hour period or around 576 watts.
The absorption refrigerators are inefficient on AC. Best to use LP when no shore power.
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Old 06-12-2019, 09:01 AM   #6
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Same size 120 volt residental fridges use about 1/3 the power of a absorbsion fridge on 120 volts.

My 7cf 120 volt fridge draws 9 amps@12 volts DC.
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Old 06-12-2019, 01:16 PM   #7
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It's supply and demand.
Your battery and solar system is not able replenish the charge. You are using up more than you are replacing.
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Old 06-12-2019, 03:35 PM   #8
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Hi ! Welcome to IRV2! We're sure glad you joined the gang!

Noticed you are kinda new on IRV2 and wanted to say hello! Have fun and keep her between the ditches!

Good luck, happy trails, and God bless!
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Old 06-12-2019, 04:35 PM   #9
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For the past 2 years I have run the following in my 2009 Sprinter ERA. I currently have 525 ah worth of house battery storage, 400 watts of solar on the roof and a 5000 watt inverter. I also have an onboard Onan 2500 generator that I seldom use. My setup provides power to my 120V ac refrigerator, microwave, water heater, coffee maker, blender (occasionally), wife’s blower drier and curling iron, computer, phone and camera chargers and 120v ac TV and I might have missed a few things. Regardless, over the past two years I have not had any issues with not having enough power. Most times we are disconnected and remote and like I said power is not an issue.

The house batteries will also charge from the alternator if we have no sun and when we are heading down the road at night. The secret is knowing what your power consumption is, what’s your biggest power consumer and how often you plan to use the biggest power consumer. For you, it’s your refrigerator. We can actually run our AC for anywhere between 1/2 hour to 45 minutes straight off the inverter.

In order to determine how many watts your set up can handle you need to do some math - watts = amps x volts. For example, my dorm refrigerator I have in my van require 1.9 amps (120v ac) when it runs (3.9 to start the compressor). 1.9 x 120 = 228 watts of power is required while running. Now assume the refrigerator ran a total amount of 12 hours combined which would be 2736 watt hours.

Solar panels are commonly used to charge a battery – not to charge a device directly. There are a couple of reasons for having batteries. Solar panels might not generate enough wattage to directly power an appliance, but they can build up a higher wattage via a battery. Another reason is that a battery can regulate the power going in to the appliance at a constant rate. When solar panels are charging a battery it is usually at a varying rate which could damage the appliance if not regulated.

Your battery capacity is measured in Amp Hours (100Ah). You need to convert this to Watt Hours by multiplying the Ah figure by the battery voltage (12V) 100Ah x 12V = 1200 Watt Hours

For Example:
If a battery is rated at 100 amp hours it should deliver 5 amps of power for 20 hours or 20 amps of power for 5 hours. Taking into account a camper with a small 45W fridge running for 6 hours the minimum consumption would be expected is 270W. Take this wattage and divide it by the voltage, 12V, gives 22.5Ah. With the aim of leaving 50% in the battery brings the requirement to 45 Ah per day.

What size solar panel do you need?

Solar Panels power generation is commonly given in Watts (100 Watts). To calculate the energy it can supply the battery with, divide the Watts by the Voltage of the Solar Panel.

100 Watts / 18v = 5.56 Amps (on a perfect day with the controller that can handle it)

Please note that Solar Panels are not 12v. Any one who works out the Amps of a solar panels using 12v as the voltage calculation does not understand solar or has been misinformed. All solar panel voltages, amps, watts etc. are normally (should be) are displayed on a sticker attached to the back of the panel.

I know all of this can be very confusing so here is a very simple way to calculate how much solar panel you need to charge a 100Ah battery. I will assume the battery is 12V nominal. I will also assume, you are getting 5 hours of full sunlight.

100Ah/5-hours = 20A which means charging your battery at 12 volts and 20 amps for 5 hours will full charge your 100Ah battery.

20A x 12V = 240 watts

So you need to do the calculation/math on how much (watts or amps) your refrigerator is using per day and then compare that to the time needed for your solar panel(s) to charge (keep charged) your battery(s).

IMHO – you need more battery and more solar in order to keep up with the demand.

I apologize if I have confused anyone on this but this can be a very confusing topic.
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Old 06-12-2019, 05:19 PM   #10
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Some assumptions in this thread are good, others are not so close.

Solar is only used to charge batteries (in low voltage systems) or to directly replace electrons from the power company in high voltage grid-tie systems. They are similar in how they are calculated, but different in utilization.

You are obviously looking for information on the low voltage side of things.

An RV absorption refrigerator on LP uses 12v power (very minimally) to run the control circuits. This is a constant load and is present on 120v power as well.
The LP gas is used to heat the boiler element which creates pressure, makes cold, etc. That part of the system is not important right now. The 120v version of this is an electric resistance heater using 400watts (measured on a Norcold 12 cubic foot 4-door model) and this heater runs pretty much continuously.

Absorption refrigerators are MASSIVELY inefficient.

I replaced a 9 cubic foot Dometic in a different coach after the cooling unit failed for the third time... With a larger Magic Chef "dorm room" compressor driven fridge. The power demand was cut by 75%. Not only was the box colder, but the compressor only ran a fraction of the time where the LP or electric burner was pretty much always on.
I replaced that 12 cubic foot Norcold (NotCold!) with a 23 cubic foot inverter-compressor Frigidaire and watched the power drop from 400 watts to just over 200 watts on full power coming out of storage... And anywhere between zero and 60 watts in an idle mode after it had reached normal temperatures.

I have 440 amp-hours of battery in my coach (4 GC2 batteries) and with a 50% SOC that gives me 220 amps to play with before recharging. At 12 volts, that is 2640 total watts of power. Assuming 100 watts drain continuously, that *should* be about 26 hours to 50%. I use more than that with other things, and anything pulling from the batteries (water pump etc) will affect the total. You can ALWAYS use more batteries.

Now to fill up that 440 amp-hours of battery... I have 1020 watts of solar on my roof in a high voltage array with a Victron controller. Calculating power needs based on 5 hours of sunlight is a good metric, HOWEVER you cannot charge at full rate the entire time! Flooded lead acid batteries charge in an 80/20 methodology... You can stuff 80% of the power into them in 20% of the total time for a full charge... BUT that last 20% must charge so slowly that it takes 80% of the time! Once your batteries switch from "bulk" charging to any other mode, the total charge rate has dropped considerably.

My array can bring the batteries up to 80% SOC in about an hour or maybe 2... Because I'm pushing up to 85 amps at them. Anything I'm using during the day can also utilize this available power because I have such an excess. (this was intentional to account for cloudy days while still producing) Any power that has no place to go (storage or usage) is just lost capture.

With a 100 watt panel, your total amp output will be 8 amps, probably continuously and never get out of bulk mode before the sun goes down. You need more power - connect more panels in either series or parallel to get to the 30 amps that your controller can handle, and at least you will be able to charge that much until the batteries reach their limits.

At a 400 watt draw, even being generous and saying that it only will run for 75% of the time (because absorption units are also slooooooooow in operation) that is still 7200 watts every day that needs to be gotten from somewhere. Even my system couldn't handle that kind of load!

Doing this kind of fridge on electric just isn't a good idea for dry camping.

On LP.... You will be feeding it gas, but gas is a LOT more efficient at being turned to heat than electricity is, and your existing system should EASILY be able to keep your batteries at 95% SOC or above with your fridge on LP mode.
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