Solar question

MD,
Once you get into solar a bit, you will find out how simple it really is. In my case, 3 100 watt ebay panels @ $130 each. And a 20 amp controller @ $15, and a roll of twin lead 10 ga wire, that's it, all works great for near 2 yrs now.

Your idea od a couple panels on the roof that are connected all the time, and a couple more portable panels for shade,,,,,and more effecientcy, is a very good idea.
I went portable panels the first year, worked great, but my set up was more hassle than it was worth, and I found myself not using it when I should have been. So now I have the 3 panels on the roof, hot all the time, no switching off.
Don't let the solar fanatics make it complicated for you, because its NOT ! :)

Muddydogs wrote:
Another question that I am not sure I want to ask as when I did a search on the forum for solar I got more information than I could handle but here it goes. I tend to boondock in the woods and usually try and shade the trailer from noon to evening sun which will affect solar panels on the roof. Would I be better off to have a couple panels mounted on the roof and a couple mobile to chase the sun? I do have a lot of real estate on the toy hauler roof to mount panels.

Not the best roof pic but as you can see I got the room.

I have the same problem. I have 2 60 watt fixed mount on the RV, works fine in a sunny spot. I have a 100 watt panel with lots of cable to put in the sun when in parked in a shady spot. The movable panel is wired directly to the battery. The movable panel is stored on a bracket mounted outside of the RV.

Doug

pianotuna wrote:
Hi BFL,

At what voltage does your mppt start to charge? Does the panel go from zero volts to 37 instantly?

What is the voltage loss per degree C as temperature increases?

Do both the mppt and pwm systems start to charge at identical times? Or does the mppt start sooner? (and correspondingly stop later?)

I don't have all that info, but I did note when first exposing a cool 37Voc panel to the sun it got 37Voc and then right away you could feel the panel warming up and watch voltage drop to about 34Voc whatever.

The voltage and Isc coefficients are listed in the specs for the panels but they are similar between 24v and 12v poly that I have had.

The key to it all is that voltage goes down when temp rises but the amps actually go up a bit. Since PWM gets Isc, this means you still get all your amps even when the Voc has dropped Also for MPPT, note that the power drops along with voltage because the voltage has a greater effect on the power than the amps.

I have not seen any difference in start or stop of charging times. The start depends on how much cloud there is over the sea and mountains to the East at sunrise as to when you first see the sun get above those. By then it is light out but panel amps are still low. The sun here goes down behind trees to the West so it is still light out but the sun has gone down. I turn my panel East then, ready for sunrise next day.

Another question that I am not sure I want to ask as when I did a search on the forum for solar I got more information than I could handle but here it goes. I tend to boondock in the woods and usually try and shade the trailer from noon to evening sun which will affect solar panels on the roof. Would I be better off to have a couple panels mounted on the roof and a couple mobile to chase the sun? I do have a lot of real estate on the toy hauler roof to mount panels.

Not the best roof pic but as you can see I got the room.

When I want to equalize, I just bypass my $20.00 e-bay PWM controller. I then occasionally glance at my e-bay voltmeter, and my e-bay amp meter. My 7-8 year old pair of 6 volt no-name GC love it.

Doug

Monos have a smaller foot print than Polys for the same wattage, so they are more efficient in bright light. That's great on the roof of a house that is grid tied when the clouds roll in. When that happens the VOC and Soc drops hard on both the 220w and 245w mono panels I have. Even light cloud cover will drop my 245w monos down from over 16a to just above 2a each. (heavy cloud cover, forget it) My 230w poly wakes the controller up at dawns early light and produces 4a in pea soup.

I know it isn't what the advertizing says but over 3 years the results have been the same, at least with the three different panels I have. They are consistent enough that I will probably sell all four of my panels to get a matching set of 250w polys for mounting. ( sunny Colorado gets a lot of clouds)

In any event it is my belief that you are looking at this the wrong way. it starts with your roof. Check for the panels you have a available, can afford and see out of those what will fit on your roof. Once you know this you can figure out what is the best controller to run them.

YOu may start small and expand? That is easier with a PWM system than a 24v panel/ MPPT system. On a PWM system you can put a 100w panel here, a 140w there, and a 120w over there. Run them all to the controller and off you go. All of their VOCs are close as will be the panel you upgrade with tomorrow even if it's a different brand or type. With the big panels the VOC goes up as the wattage does. So a 220w panel has a lower voltage than a 250w panel. That difference in voltage can confuse the MPPT controller needed to run them. Even if the wattage is the same, a different brand or type can have a different voltage. This can make it hard to expand in the future.

So look at your system from all angles and design it a few times differently. There are a lot of panels and components that can be combined in different ways to fit your needs in the situations you camp in. You can't get that designing a system around a couple of buzz words.

Hi BFL,

At what voltage does your mppt start to charge? Does the panel go from zero volts to 37 instantly?

What is the voltage loss per degree C as temperature increases?

Do both the mppt and pwm systems start to charge at identical times? Or does the mppt start sooner? (and correspondingly stop later?)

pianotuna wrote:
.......

BFL13 wrote:
And BTW the "urban myth" about MPPT doing something in low light vs PWM has not been seen by me with both running at the same time when I had both set-ups. In fact for most of the summer the MPPT has under-performed PWM due to higher panel temperature lowering panel voltage so get fewer amps with MPPT than would with PWM.

I don't agree. What is happening is that the input voltage is too low for summer time use. If the system is 33 volts the opposite may be true. In fact the higher the input voltage for MPPT the better, for hot weather use. There will be as a result slightly greater losses at the MPPT controller (98% down to 95%)..........

Not understood, please explain. My 230w panel rated for 37voc is doing about 34voc with panel temps at 51C, ambient temp 25C and loses about 10% output seen as amps compared with ambient 16C and panel at 44C.

I have observed about 13.5amps MPPT instead of 14.4a PWM this summer. I got 15.5a vs 14.4 last Spring. It is now coming back some. Today with ambient 18C and panel temp about 45C I saw 14.X amps, so this confirms it is not panel degradation or bad wiring or whatever, it is all about temperature. Also that fits with the standard info on MPPT that you lose 10% output (however measured) when panel temp is 50C vs 25C.

( no idea what ambient would be to get only 25C panel---freezing out?)

I don't understand how more rated output voltage would do better proportionately. Temperature is temperature.

BFL13 wrote:
So to answer the question of overcast day MPPT vs PWM for chance to equalize, my answer is MPPT or PWM has nothing to do with it.

I tend to agree

BFL13 wrote:
And BTW the "urban myth" about MPPT doing something in low light vs PWM has not been seen by me with both running at the same time when I had both set-ups. In fact for most of the summer the MPPT has under-performed PWM due to higher panel temperature lowering panel voltage so get fewer amps with MPPT than would with PWM.

I don't agree. What is happening is that the input voltage is too low for summer time use. If the system is 33 volts the opposite may be true. In fact the higher the input voltage for MPPT the better, for hot weather use. There will be as a result slightly greater losses at the MPPT controller (98% down to 95%).

BFL13 wrote:
Even if there were a slight MPPT advantage on overcast days it is too small to matter in daily AH compared with daily usage. Anyway from April to October around here it sure can't be seen. Maybe if you did solar in winter, but then solar is hurting then no matter which set-up you have.

I agree for a 24 volt MPPT vs a 12 volt PWM. I don't agree with a higher voltage MPPT (say 100 volts).

No one is going to attempt to equalize in poor solar conditions.

Solar in winter is going to need every bit of advantage possible, including tilting panels, or if there is snow cover a vertical (90 degrees) installation to get a "bounce" off the white surface.

35v seems low for 960 watts.
I would try to go all series with MPPT.