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Need advice on under-performing solar system


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I installed a solar system a while back, but have only recently begun to demand much from it.  Here's what I have:

4 panels of Sunmodule SW260 260W Poly V2.0 Module -- nominal 260 W and 30 Volt panels -- flat mounted on the roof of the trailer

---- The panels are wired with 2 sets of 2 panels in series, for an intended nominal 60V @ 7 to 8 Amp feed on 50 ft of 10 AWG wiring

Morningstar TS-MPPT-60 charge controller

8 ea. Crown CR-23 6V deep cycle batteries in a series/parallel configuration = 4 @ 12 V and 235 Amp Hours

Trimetric - T 2030 meter

After two weeks of full-time boondocking, with several rainy days we were at about -385 amp hours.  We got a good sunny day (at Grand Teton - so not too directly overhead) and only picked up about 80 Amp Hours of battery.  All the lights were off, refrig on propane, etc. so there should have been minimal draw.  I would have expected at least 3 or 4 times that amount on a bright sunny day.

Am I crazy?  What am I missing???


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With the panels you are describing (after looking at the spec sheet on line) you should see approximately 60 volts at approximately 16 amps in a series pair configuration.  The specs I found rate the panels 31.4 volts at 8.37 amps.  In a series pair you add the volts, but the amps stay the same.  This gives you approximately 60 volts at 8 amps.  Then when you parallel two of these pairs together, you add the amps to get a total array of approximately 60 volts at 16 amps.

You say you are running this through 50 feet of 10 AWG wire.  I am going to assume that is 50 feet from point A (panels) to point B (solar controller), which for a wire run is really 100 feet because you need to add the return leg (positive and negative run).  According to an internet voltage calculator, that will result in approximately 5% line loss under ideal conditions.  This drops your 60 volts at 16 amps down to 57 volts at 15.2 amps.

This is under ideal conditions.  You will realistically never achieve ideal conditions in an RV environment, so you will actually see something less than that for maximum output.  Your batteries will charge somewhere around 13 volts.  This leaves 44 volts the MPPT controller can convert to amps going to the batteries (57-13=44).  44 volts is approximately 338% of 13 volts (44/13=3.384).  This remaining power is converted to amps.  16 amps plus another 338% is approximately 54 amps (16x3.62= 54.2).  So under ideal conditions you could see 54 amps (approximately) going into your battery bank from your solar array (which is near the maximum your solar controller is capable of producing).

Lets assume for working numbers you get 80% of that output because the sun isn't perpendicular to the panels and it isn't peak sun time all day long and line loss and heat loss and etc. (and this is still probably being generous and assumes you are in the dessert near the southern border of the US - as close to the equator as possible).  This will bring you down somewhere around 43 amps to the batteries per hour of usable sunlight (your numbers will vary based on many conditions, but this is a good working model for the described system).  You said you are in Grand Teton so you are nowhere near the dessert or the southern border of the US so realistically, I would drop that number down another 10 or 15%.  This gets you closer to 35 amps to the batteries per hour.  At 5 solar hours in a day that puts you around 175 amps back into the batteries per day (again your exact solar output is dependent on many conditions and this is only an estimate).  This also assumes you have nothing coming out of the batteries while the solar is recharging them.  Any amps coming out of the batteries will be taken right off the top of this number.  

According to your post, you saw + 80 amps go into your batteries.  This is a little less than half the output of the estimate I made based on some basic calculations.  I have no way of knowing what the draw was on your batteries during the day (but you say it was minimal).  I also don't know over what time period you saw the 80 amp increase in your battery bank.  Was that from the morning to the afternoon.  Was it over a 24 hour period, 12 hour period?????  I also don't know what size wire you have going from the solar controller to the batteries and how long this wire run is (so I can't factor in any possible line loss on this end of the system).  

Without additional information and/or taking exact measurements, I can't say for sure that your system is under-performing.  It does seem to be somewhat low, but there are many factors that could be contributing to this.  It could also be performing at its capabilities based on the limited information I am working with.  If that 80 amp increase was for a 24 hr period then it is right within where I would expect it to be based on average usage of power in a boondocking situation (175 amps into the bank and 95 amps used from the bank over a 24 our period to give you a net gain of 80 amps).

Some things I would check are that there is no shading on any of your panels from AC shrouds, vent covers satellite antennas, TV antennas, etc.  You need to check this under all possible angles of the sun to make sure there isn't an issue in the morning versus afternoon, etc.  I would also make sure the panels are clean (no bird droppings, leaves, etc covering any cells in the panels).  In a series (or series pair like you have) configuration, any small amount of shading will greatly affect the output of the series string of panels.  I would also recommend shortening that 10 AWG wire run.  I would put a combiner box on the roof and run the 10 AWG to that (short run) and then come down to the solar controller from the combiner box with heavier gauge wire to reduce or eliminate line loss on the long run you have.  I would also make sure all your connections are good and tight on the system as a whole.

I know this is a lot of information, but hopefully some of it helps.

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Agree with both.  10 AWG wire is rather small for that long of a run.  Most runs from the roof to the batteries are a minimum of 4 AWG wire and some even go with 2 AWG.  Also, your propane refer takes quite a bite of energy (DC fans, temp sensor, etc).  That far north this time of year is difficult to get much direct sun....also you would be surprised at what a small shade patch will do to your charge capabilities.

Edited by ewacowboy
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To all who replied, thanks for your input and info.

Combiner box is in utility area of RV, so run from panels to combiner is really about 40' for one pair and about 30' for the 2nd pair -- but used 50' as the "safety factor".  From Combiner Box to Solar Controller is about 5' of 1/0 copper.  From the Solar Controller to the DC Buss is about 1' of 1/0, and from the Buss to the batteries is about 4' of 1/0 cable.

The 80 Amp increase was measured from about 10 AM to 6 PM by reading the Trimetric meter and subtracting the deficit Ah from fully charged.

I had just inspected the panels and (while there may be "some" dust on them) they had no big blobs of bird droppings, leaves, etc. and my location allowed for unshadowed sun throughout the heart of the day.

Chad, I would consider your 175 Ah estimate fairly reasonable, and I would have been quite satisfied with that result...

I moved to a 50A electric site and have been getting about 30 Ah charge showing on the meter instead of near the 60 Ah rating of my PD 9260 -- It connects to the buss with 1/0 copper from within 3'.

Thanks for your info.  Any additional insights would be appreciated.

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39 minutes ago, yourpcgeek49 said:


...Combiner box is in utility area of RV, so run from panels to combiner is really about 40' for one pair and about 30' for the 2nd pair -- but used 50' as the "safety factor".  From Combiner Box to Solar Controller is about 5' of 1/0 copper.  From the Solar Controller to the DC Buss is about 1' of 1/0, and from the Buss to the batteries is about 4' of 1/0 cable...


Is that 40' one way for one set of panels, and 30' one way for the other pair?

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Thanks to all who responded.  

I found an interesting web page that allows you to input your panel capacity, angle, location, and system loss to show expected output hour-by-hour.  It actually came up pretty close to my actual results!  It is amazing how much you lose when you get as far north as Yellowstone & Grand Teton!


Hope others find this useful...

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So you have a 1040 watts of solar going to 940AH of battery.  Sounds like a great system. 

One thing which would have been very helpful is to note how many amps are going into your batteries as shown on your Trimetric while the sun is hitting the panels. 

Also it would be good to watch the voltage on the Trimetric.  When the controller starts charging it goes into Bulk charging for an hour or two, maybe more.  In Bulk charging the voltage starts out at I believe the battery voltage and slowly increases.  Once it switches to Absorb charging you should be seeing 14.5V or maybe higher.  The controller has a setting for how long the absorb charging lasts.  You might need to extend the Absorb time.  A lot of the total amps going into the battery is done during the absorb cycle. 

Did you buy the remote panel for your solar controller?  It would show the voltage coming from the panels and also the amps going to the battery from the controller.

What does the Trimetric show for the amps going out when you don't have sun on your panels?  If you leave the inverter on it probably uses 2-3 amps even when no 120V AC devices are on.  If you have a satellite TV receiver and it is not unplugged it pulls 8-9 amps of 12VDC though the inverter.  If any of that stuff is left on, it really reduces the total amount you get into your batteries. 

I have two 325watt panels on my MH.  On a clear day, I see an amp or so before the sun hits my panels increasing as the sun gets on the panels and gets higher. Even between 8 & 9am we will see 5 or more amps going in.  The point here is you should see a total of 20-30AH's put into the batteries before the 10-11am high sun angle.  Also in the late afternoon you should see another 20-30AH's total.  This adds to the 5-6 hours of peak power during mid day.   

Another thing to watch is to look at the total AH's on the Trimetric before the sun comes up, then see how much went in by 10-11am.  Do the same after 4pm. 

When we left Oregon (above the 45th latitude) about Sept 20th at mid day we were seeing a good 25amps on the Trimetric. 

The really nice thing about solar, unlike running the generator for a few hours, is as long as the sun is hitting the panels you are getting amps to the battery. 

Edited by Al F
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Also have you gone up on the roof around 8-9 am or 4-6pm to see if an air conditioner or vent is shading a panel?  Just shading one solar cell on one panel will kill almost all the output from the 2 panels in series.

A neat little test is to go up on the roof and put something over part of one cell on one panel and see just how much power you loose.  It is amazing to see what happens. 


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On 9/18/2017 at 5:51 PM, avvidclif said:

Fridge still draws 12v on propane plus all the other little things.....

The control board on the Dometic or Norcold fridges only pull about 0.1 to 0.2amps when on propane.  Not enough to matter.  There could be other phantom loads though.  That is where a battery monitor like the Trimetric comes in handy.  You can tell at a glance if something is left on that shouldn't be. 

Edited by Al F
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