From Freightliner ambulance to RV - we hope!

[dwh]

Huh...from a quicky eyeball of the manual for an Outback FLEXmax:

http://www.outbackpower.com/downloads/documents/charge_controllers/flexmax_6080/owner_manual.pdf


It appears that the thing defaults to "continuous MPP tracking" or you can set it to do a resweep every 1.5 hours

So what does "continuous" mean? Well, it doesn't usually mean continuous. The MPPT has to muck around testing the voltage of the solar, and while it's doing that, you aren't getting useful work out of the solar. So most MPPT units only do a sweep every so often.


Looking at the manual for an Outback MX60:

http://www.umiat.com/network/umiatbase/manuals/MX60_Manual_Rev6_2.pdf

It says (page 25):

Sweep Interval

Sweep Interval selects how often the MX60 does mini-sweep in Autosweep MPPT mode, or how often it acquires a New Voc in U-pick MPPT mode (see Optimization MPPT mode) to track the MPP of the array. When the operational mode display shows MPPT or EQ MPPT, and when the MX60 is configured for AutoSweep MPPT mode, the input source will be periodically swept, (at this sweep interval), up and down in voltage slightly to stay running at the Maximum Power Point operating voltage. How far it sweeps up and down depends upon the output (battery) current at that time. If the output current was less than 5 amps, but greater than the Low Cutoff current setting (see Optimization Low cutoff), then the MPP operating voltage will continue at the Park MPP (see Optimization Park Mpp % Voc). If a mini-sweep finds the MPP outside of the MPP range limit, the MX60 will run the PV input MPP at the selected Park % MPP setting. Adjustable from 0 minutes, (disable mini-sweep), to every 15 minutes. Mini-sweeps are different than the initial wakeup sweep. The initial wakeup sweep will take the PV array input and “sweep” it down to either the battery voltage if minimum MPP range is set to FUL, or down to 1/2 of the Voc if set for 1/2 MPP range Limit.

The initial wakeup sweep looks at the entire IV curve of the PV array input and sets the initial MPP operating point. The mini-sweep looks at a smaller portion of the IV curve starting at the initial MPP. The sweep and MPPT algorithm of the MX60 also works very well at finding the Max Power Point of partially shaded PV arrays, which is found in some systems.


So, when the sun comes up and the solar wakes up the unit, it does a full sweep to zero in on the Vmp, then just does mini-sweeps after that to keep it fine-tuned. The mini-sweeps are on a timer from 0-15 minutes (user programmable). Looks like the default is 7 minutes between mini-sweeps.

Panels rigged in parallel, partial shading, the array voltage stays the same, so no resweep needed.

Panels rigged in series, partial shading lowers array voltage, so a resweep is needed.


In series they didn't wait long enough for the MPPT to adjust to the shading. That's how they screwed up their test.

 

[dwh]

Forgot the point...

The point is, that the conclusion they reached with their faulty test - that parallel is shade tolerant and series isn't - is wrong. Dead wrong.

With lower voltage panels - say 2 @ 18v - series is more shade tolerant than parallel.

 

[Coachgeo]

...There is something hinky about that Gone with the Wynns series vs. parallel test - but I haven't thought it through enough to spot the flaw.

Still...in series, shading one cell should not drop the output of the array of two panels to zero.

I'm pretty sure they goofed up the test somehow.

Edit: Could be that the Outback charge controller only does an MPPT sweep (adjusting array load voltage to determine Vmp) every sixty seconds. If so, they would have to shade a cell, then wait for the next sweep.

In parallel, shading wouldn't change the array voltage (array Vmp), so the MPPT wouldn't need to readjust the load voltage. In series, partial shading would change the array Vmp, so the MPPT would have to find the new Vmp.

So maybe they just didn't wait long enough for the MPPT to adjust...

Dunno. Prolly have to sleep on it.

whattttttttttttt...... you mean everything on the internet is not absolutely correct......

actually I just chose that link when asked about it cause they explained it better than I could attempt to. There is numberous discussions on it one can find elsewhere for more insight. Myself, though I am taking my advice on the subject from an old friend who has 30+years in solar industry.... he is one that originally pointed out the issues with shading to me. It is an absolute for real issue that should be taken into account for.

 

[Britboaters]

Well I could wait for someone else to ask the dumb question, or be that person.....
Thanks for the discussion guys and the knowledge share.
My plan is 4 x 305 watts on the roof, at about 30+ volts each. I thought it would be good to include the opportunity of a 5th panel I could deploy away from the truck if I was in a shady area. The guy at N. Arizona Wind & Sun said I'd have to wire all panels in parrallel so that the 5th panel would be a same voltage as the others - seemed reasonable.
The 5th panel is an idea I may or may not do, so from a shading point of view, is series or parallel better on these panels ?
Thanks guys, and hope I'm not continuing the thread hijack.
Bob

 

[Britboaters]

I found this Canadian / Indian study that concludes parallel better than series for shade, but draws attention to the higher amps that must be considered. In RV / truck situations I'd suggest the cable runs are so short as to not be an issue

6. Conclusion
Series connection of solar cells in an array is essential to get practically utilisable voltage. A number of such strings are
connected in parallel to get the requisite power. As there is a substantial power loss due to non uniform illumination of a
series string care should be taken to see that all the cells connected in series receive the same illumination under
different patterns of shading. Such a care will give a better protection to the array and at the same time the total energy
output will also be higher. In this paper, the series connected and parallel connected SPVA is compared under different
shaded conditions. It is found that parallel connected SPVA is dominant under shaded condition. So, the parallel
connection is the best possible configuration. The problem of high current output in parallel connected system requires
the definition of new configuration.

Ref:
https://www.researchgate.net/public...eries_and_Parallel_Connected_Solar_PV_Modules

 

[Mattersnots]

You guys may be getting bogged down in the details here. Most of you are talking about more than a kilowatt of panels on the roof. What the heck are you going to be using all that energy for that it can't be shaded from time to time. We had a cloudy day here last week and I was happy to see at least 300 and sometimes 500 watts being produced by my system. Unless you are running an air conditioner all day or welding I can't imagine how you couldn't keep up with demands throughout the average day. With just the basics running my batteries are topped off before 10 am. My plan was to put so much wattage up there that I didn't have to worry about shading, cloudy days, tilting, or portable panels.
Says the guy from AZ, the land of Sun.

 

[Coachgeo]

You guys may be getting bogged down in the details here. Most of you are talking about more than a kilowatt of panels on the roof. What the heck are you going to be using all that energy for....

Plan to use the rig when parked at home to produce power to cut my own energy cost. Possibly grid tie..

 

[patoz]

Mattersnots, that is my plan also. The latest EarthRoamer XV-LTS has 1,200 Watts of Solar on the roof with a 12,000 Watt Hour Battery Bank (their term), and it is totally electric, with no propane on board at all, although it does have diesel heat. However, I'm not sure what a 12,000 Watt Hour battery bank is, when everyone else measures the battery bank by Amp Hours. Anyway, if it can function with that much solar then I figure I can also, plus I will have propane heat and hot water.

 

[Coachgeo]

.... However, I'm not sure what a 12,000 Watt Hour battery bank is, when everyone else measures the battery bank by Amp Hours. ....

uggg.... I have zero respect for a company that pulls that shaitz. Now I know if ever won a lottery, married taylor swift or the likes.. I STILL will NEVER buy anything from EarthRoamer. You see that is just "legal" con man buffoonery probably worked out by their advertising executive.... aka....... head baboon.

12000 watts = 1000 amps at 12v. (watts/volts=amps) so simple math says that is a 1000 Amp Hours battery bank.
12000 watts = 500 amps at 24v ......................................".........................says that is a" 500 Amp Hour battery bank.

 

[dwh]

The key is bypass diodes.

For example, let's assume 2 x 18v panels. 36 1/2v cells per panel. Two bypass diodes per panel, each covering 18 cells.

Further assume charging a 12v battery.


Rigged in parallel, the array voltage is 18v. Partial shading results in a single string bypass on one panel, cutting 18 cells out of the circuit. That reduces the output of that one panel from 18v to 9v. That is below battery voltage, so that panel is no longer contributing to battery charging.

That's a 50% loss in charge current. (That's what the Wynns saw in parallel.)


Rigged in series, the array voltage is 36v. Single bypass drops the array voltage to 27v, which is still higher than battery voltage. But one 18 cell string is not contributing.

That's a 25% loss in charge current. (That's what the Wynns would have seen, had they waited for the MPPT to do a resweep.)

 

[dwh]

My 300w Astroenergy panel has 72 cells, 36v Vmp. Basically 2 x 18v panels in series - all in one panel. But it has 6 bypass diodes, each covering 12 cells/6v.

0 bypass - 36v
1 bypass - 30v
2 bypass - 24v
3 bypass - 18v
4 bypass - 12v

It can suffer up to 3 bypasses (50% loss) and still have a high enough voltage to charge a 12v battery.

 

[dwh]

I found this Canadian / Indian study that concludes parallel better than series for shade, but draws attention to the higher amps that must be considered. In RV / truck situations I'd suggest the cable runs are so short as to not be an issue

6. Conclusion
Series connection of solar cells in an array is essential to get practically utilisable voltage. A number of such strings are
connected in parallel to get the requisite power. As there is a substantial power loss due to non uniform illumination of a
series string care should be taken to see that all the cells connected in series receive the same illumination under
different patterns of shading. Such a care will give a better protection to the array and at the same time the total energy
output will also be higher. In this paper, the series connected and parallel connected SPVA is compared under different
shaded conditions. It is found that parallel connected SPVA is dominant under shaded condition. So, the parallel
connection is the best possible configuration. The problem of high current output in parallel connected system requires
the definition of new configuration.

Ref:
https://www.researchgate.net/public...eries_and_Parallel_Connected_Solar_PV_Modules

Again, bypass diodes.

When cells are in series, the cells in light will produce power. The shaded cells will become heating elements. Some of he power being generated by the lighted cells is being burned by the shaded cells.

That paper keeps mentioning the amount of power consumed by the shaded cells.

But preventing that situation is the purpose of the bypass diodes.

When a section of a series string of cells drops below a certain voltage, that section of the string is bypassed.

This protects the shaded cells from overheating from the power they are dissipating as heat, but also drops the voltage of the series string of cells - and the output voltage of the entire PV module (solar panel).

Which is why rigging modules in series to get a higher array voltage is more shade tolerant - the array can tolerate more diode bypasses before the array voltage drops too far to be useful.


And that matters with a couple of 18v panels that probably only have two bypass diodes. With a couple big 36v panels with 6 bypass diodes each, it doesnt matter all that much.

IF you're charging a 12v battery.


Trying to feed a 240v grid-tie inverter with an array rigged to put out >240v...you're gonna have to rig panels in series strings.

 

[Coachgeo]

putting switches to turn of an array in shade so not to drag down the rest...... is essentially a manual version of a diode that Tail-End Charlie discusses above. Myself... have to admit that had no clue panels were even sold with diodes. To my suprise ones I bought have diodes..... but still the fellow who brought this all up to me...... with the 30yr's of solar experience knew which panels I was getting so he knew they had diodes and still told me it was best to install switches to close out an array. Granted I am going 24v.... so I will be series parallel (pairs wired in series to create a 24v array {along with a switch to cut it out if needed} and each array wired in parallel with the other arrays) that feed the charge controller that feeds the battery bank and grid tie inverter. sometjhing like that.. still trying to get my head around it. sorry for hijacking the thread.

 

[Ozrockrat]

About time we had an update mate.