Voltage and Output Drop High Heat - Las Vegas

tr0y · July 2018

Hey all -

Can some one hep me with what should be expected for voltage and production drop in high heat. Current ambient temp is 106f / 41c, I would be panels on roof are seeing 125f / 50c and batteries and CC are somewhere between the two.

I have four 275 watt panels on the roof of my RV, Midnite solar Classic 150 and 4 12v 200AH agm batteries, the batteries are reasonably new manufactured in march of 2018 and were sitting @ 13.1v when I picked them up.

I am not seeing the batteries getting to the necessary 14.5 + charge voltage wondering if this is due to the heat and the associated losses.

The panels are connected in series and parallel at the out puts from the panels I see about 63V

275W
Vmp 31.5v
Imp 8.74a
Voc 38.6v
Isc 9.28a

Number 10 wire to breaker before charge controller, 10 wire from breaker to CC. Total run length about 16 feet. CC to batteries number 4 welding wire run of about 8 feet.

So with a clear day and at 5 PM my controller says this

42.1V seems like excessive voltage drop and watts seem like they should be more , but maybe I am wrong. I am going to charge with shore power tonight and see where batteries go.

I do also have room for two more panels, same specs, and figure ill throw them in the mix for the heck of it.

Thoughts anyone ? Thanks

Estragon · July 2018

Do you have a remote temp sensor attached from the classic to the batteries? If so, you may want to check the temp compensation vector setting vs actual battery temp. If batteries are at 115ish°f, charging voltage may be appropriately set lower. You'd have to do the arithmetic to see if absorb voltage is matching expected temp compensated setpoint.

With an input voltage of 40ish volts and a 12v nominal bank, I don't think heat related lack of voltage from pv is a problem. With 2 30ish Vmp panels in series, seeing 40ish volts (assuming full sun) does seem odd though. Have you checked and retorqued connections?

tr0y · July 2018

Hey thanks @Estragon I do have the supplied temp sensor mounted to the center battery of the 3 in the bank. I have 4 panels 2 sets of 2 that show about 63v on the roof and 42 at the CC. I will open up the CC and re-torque and I did just get a WhizBang jr I am also going to wire in as well.

The wire down from the roof is 10g but should be enough for the amps in a short run like 14'.

Hey I can't do math ... You never need that stuff in the real world. lol ... ok I can do math, but sure wish I didn't have to.

mcgivor · July 2018

The temperature of the panels will definitely cause a reduction in output, if the ambient is 41°C the temperature of the panels will be significantly higher especially at noon, perhaps as high as 65-70°C, multiply this by a coefficient of -0.4% per deg.C, losses could be as high as 28%. Roof mounted panels experience higher temperatures than a pole mount due to reduced air circulation, which is why a recommended 150 mm space be provided between the panels and roof surface, I've seen temperatures in excess of 60°C, measured on the back at 37°C ambient, ground mount light breeze
Looking at the display image, at 5pm it would seem normal to have a reduced voltage, indirect sun, what stands out however is the controller being in bulk mode, which begs the question are you running loads during the day?

Photowhit · July 2018

tr0y said:

Current ambient temp is 106f / 41c, I would be panels on roof are seeing 125f / 50c and batteries and CC are somewhere between the two.

Ouch! AGM batteries at between 106 and 125f! AGM batteries do not like heat.

Check on them and look for signs of out gassing and distortion of the battery casing.

Are there loads on the batteries during the day? Fridge in the heat running constantly, added a fan or 3 to help?

mike95490 · July 2018

The panels at +40V is plenty to charge a 12V battery at 16V. Your array could drop to 30V and still be able to downconvert to batteries.

As others say, what loads are on the system. And nasty too hot batteries.

jonr · July 2018

277W from a 1100W array at 5PM? Make sure that you haven't lost 1/2 of your array (ie, check that each parallel string is working). Batteries won't last long at that temperature - consider cooling them.

Photowhit · July 2018

jonr said:

277W from a 1100W array at 5PM? Make sure that you haven't lost 1/2 of your array (ie, check that each parallel string is working). Batteries won't last long at that temperature - consider cooling them.

They are, I'm assuming, flat on the roof of his RV. At 5pm he may well have obstructions (roof air) shading them...

tr0y · July 2018

Thanks alI - checked both strings of panels individually both were putting out between 60 and 63v so that would lead me to think they are working. Could the heat cut me down that low ? Mounting the panels 150cm off the roof worries me due to the fact this is an RV and it runs down the highway at 75 MPH. I have two more panels I am going to mount that might help.

Loads on the system; I just had a Norcold RV fridge running as a test case. The inverter was calling that 11% load ( 2000 Watt PSW ) and that was somewhat intermittent. I don't have a kill a watt or clamp meter at the moment so I don't have an exact load for you.

As for the heat and protecting the batteries there is really not much I can do when the ambient temps are 105+, I keep them shaded, I open the compartment they are in and they get a slight breeze. You learn to live with the heat when you are in the Mojave Desert, and yes I don't expect to get the life that someone in a cooler climate will get from batteries. It appears some of the electric car batteries might have liquid cooling capabilities, maybe I could look that way or that might just be a giant expensive pain in the a$$.

Please excuse the wire mess as I pulled everything apart to look for issues; the finished product is much cleaner.

There is also one panel you can't see in this image and the antenna is coming out to make room for the final panel.

Image: https://us.v-cdn.net/6024911/uploads/editor/ts/m9uk8uws6v10.jpg

Photowhit · July 2018

So is it a mixed lot of panels? The center right appears different from the front nd back one that detail can be seen in. Does look like you have shading issues before and after noon... It's an RV so not a lot you can do about it. Having a firm mount for the height doesn't appear to be an option.

I've seen some odd ball setups, all the panels hanging down the drivers side hinged to the roof, and hydraulically lifted once they stop. Wouldn't work with push outs, and still need to be somewhat high, so they could be angled and allow the door to open.

tr0y · July 2018

The panels are 4 of the same 275w Vmp 32.0 Voc 38.2 Imp 8.95, 2 strings, listed above and the new 2 ( 290w Vmp 32.1 Voc 38.8 Imp 9.05) re different manufacturer but the specs are extremely close within a couple percent of the others, All of the strings match each other. I wouldn't call it a mixed lot. Quite a few people have mixed strings of panels in their systems.

I am really not interested in Rube Goldberg panel mounts, I'll lose a bit of power before I go that route. My thought is just over panel to deal with some of my issues. To get above the vents etc .. I would have had to go almost 17" off the roof.

littleharbor2 · July 2018

Just because you are getting 60 and 63 volts from your panels is meaningless. How much CURRENT are you getting from those late afternoon shaded panels?
Snugging panels up against the AC shroud and vent shrouds is asking for substantial reduction in production vs. potential production. Until the shadows cast upon the panels are entirely gone expect your output to be minimal.

Estragon · July 2018

To avoid doing arithmetic on battery temp compensation, you might want to try the android app for Classics. The "messages" tab shows target battery charging voltage. The "day log calendar" tab shows daily peak input voltage, which, while not definitive, would at least show if voltage is getting close to expected at least some of the time.

Looking at the rooftop pic, I'd agree with others suspecting shade issues. Not much you can do about it, but you might want to try different mode settings (default is "solar", but my understanding is "legacy p&o" or "u-set %voc" settings may be worth a shot). You may find more info on these setting in the forum at the MidniteSolar.com site.

tr0y · July 2018

Ok I'll give those suggestions a try. my question is any thoughts on a 20+ volt drop from the roof of the RV down 11' to the charge controller ? I will also be measuring current today to see what the panels are producing in that way.

Estragon · July 2018

A loosened or defective connection is one possibility. Another is shading. Even a shadow from something as small as the little (cell booster?) antenna can make a big difference.

A dud mc4 connector or loose connection might not show up testing open circuit, but may with the circuit loaded. Maybe try one string at a time to see if there's any significant difference in voltage between them (running in full sun)?

tr0y · July 2018

Off to check all of this, but also wondering if how panels are wired might make a difference. Currently they are in a series / parallel configuration and wondering if that may be part of my issue when shade happens. Since I have plenty of voltage, wondering what would be the harm of straight paralleling the panels ?

NANOcontrol · July 2018

I see trees, if that is what you want to call them. My panels respond exactly the same way. It doesn't take much to put them into bypass.

jonr · July 2018

> about 63v on the roof and 42 at the CC

You probably know this, but all voltage drop measurements must be done when loaded. You can't compare open circuit voltage at the panels to loaded voltage at the controller.

If both of your voltage measurements are under the same load, then you have a wiring problem (and lots of heat being generated somewhere).

tr0y · July 2018

Ok so I think things are looking up here. Current time is approximately 1340, ambient temp is between 105f (41C) and 107f (42c). I have the 4 275 watt panels plugged in to the CC. Finally with no shade on anything, and with derating the panels, using 25% off the label and then another 20% or so due to pretty extreme heat panels seeing (laser temp finder says 155f / 68C) this seems OK. I would assume adding another 600 watts of panels might get me another 250 or so in real world desert conditions.

Thoughts ?

Image: https://us.v-cdn.net/6024911/uploads/editor/o9/abhpaj2y0aku.jpg

Estragon · July 2018

~55v seems reasonable for really hot panels and little/no wind. Based on what you're seeing with the existing panels, an extra 250-300w with a couple more panels seems about right.

littleharbor2 · July 2018

63 volts - 1/3 = 42 volts. Hmmmm… Bypass diodes taking out 1/3 of the panels output?

tr0y · July 2018

@littleharbor2
That 63 volts is at the connectors right off the panels, while not connected to anything, could that be why the drop when I look at the LED on the Charge Controller ? Another thought, what about wiring in parallel as to minimize ( maybe ) shading issues ?

Photowhit · July 2018

tr0y said:
That 63 volts is at the connectors right off the panels, while not connected to anything, could that be why the drop when I look at the LED on the Charge Controller ?

Yes you are reading VOC So the Drop to VMP including the wire run will be substantial.

tr0y said:

Another thought, what about wiring in parallel as to minimize ( maybe ) shading issues ?

Yes, That should help a good bit!

tr0y · July 2018

@Photowhit Thanks for the input, is the wire run that important ? I'll be generous and call it 15' basically from the roof of an RV to under the fridge. Seems like voltage drop across that distance should be minimal, but I am willing to learn.

I'll figure out the cabling I need and give straight parallel a try.

mcgivor · July 2018

@trOy You can use a volt drop calculator such as this https://www.calculator.net/voltage-drop-calculator.html to get a reasonably accurate idea.

tr0y · July 2018

Well if that calculator is right it is saying between 1.1 and 2.4v drop ... hmmmmmmm.......

Photowhit · July 2018

tr0y said:

Well if that calculator is right it is saying between 1.1 and 2.4v drop ... hmmmmmmm.......

Using 30 volts, 34 amps (combined on roof) and 10 gauge wire, 15 foot run, 1 volt drop 3.4% drop

You could get 8 gauge wire and end up at .64 volt drop and 2.16% loss.

In theory you would want to put fuses or breakers on each panel as well. I'm not overly concerned with this but if it was a house being inspected they would expect to see each panel with a breaker/fuse.

tr0y · July 2018

thanks @Photowhit I did punch in an incorrect value. ...On a different note earlier this morning, temps were lower only 90 @ 0800 I was making almost as much power as yesterday in full sun @ 110 . Yeah guessing temps make a bit of difference.

tr0y · July 2018

Cool weather update ... Went up into the Utah mountains and found some cooler temps, low 90's, and no trees to directly shade me. Here is a pretty average mid day reading.

Voltage and Output Drop High Heat - Las Vegas

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