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Old 09-29-2014, 02:46 PM   #61
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Re: Best Solar Charge Controller? And Series vs Parallel Wi

Viva: Just a head's up again re your potential purchase of 32-cell panels.

This is what Scalff77's link to the Bogart Engineering page says re the type of panels required:

"High efficiency charging at lower cost than MPPT chargers--but must be used only with 12V or 24V (36 or 72 cell) solar panels for high efficiency."

This is what the Blue Sky "Product Overview" says on the last page regarding its controllers (which includes the Blue Ski Solar Boost 3024i you are considering):

"12V PV refers to typical 36 cell module providing VMP/VOC of ?18V/22V at STC."
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Old 09-29-2014, 03:24 PM   #62
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Re: Best Solar Charge Controller? And Series vs Parallel Wi

I have checked into this, and I don't think the 32-cell vs. 36-cell is going to be an issue in my case. The Vmpp is similar in either case. The reason the 32-cell are preferable for my installation is that they are physically slightly smaller (narrower and shorter), which will allow me to fit more wattage.

Here is part of the information I have:

About 32-cell versus 36-cell 100 watt solar panels: The 32-cells are a little hotter, in terms of panel output current, with a resultant lower operating voltage and a smaller footprint than the 36-cell. The 36-cell panel is not quite as hot, with a resultant higher operating voltage. The charge current coming out of an MPPT controller tends to even out between the two types of panels, because the 32-cell already starts out with a higher current output, yet the 36-cell will yield a little greater boost because of a slightly bigger voltage delta (between battery and panel). The biggest advantage of the 32-cell is its footprint, which is 6" shorter and 1" thinner [narrower] than the corresponding 36-cell 100 watt panel.

For me, the smaller footprint is important, because of the available space on my roof. With the 36-cell panels I would only be able to fit much fewer watts (and even that would not fit as well due to being wider); with the 32-cell I can fit more panels, and it is a better fit due to the panels being narrower.

That said, I'm not recommending anything for anyone else, necessarily. There are a lot of variables, clearly (including physical space available), and the technology is ever evolving (such as how the document you previously linked to said that all 32-cell panels would be 14 volts or less, which has clearly changed, since the 32-cell panels I'm speaking of are nearly 18 volts Vmpp).

I can see definite advantages to higher voltage panels for various reasons (24v, etc.) but for me I just haven't found any that will physically fit on the available roof area I have. That's the "real world" part of the equation I have to work with. However, I think I will be able to get along fine with my usage and the wattage of 12-volt panels I can fit.

Viva

PS: The place where I am buying the 32-cell panels also sells Blue Sky controllers to go along with them (they sell other ones too, but I mean they do also sell Blue Sky). So I think where you see Blue Sky saying something like "12 volt refers to the typical 36-cell panel" that's just a reference, not a limitation. I don't believe there would be any problem or "mismatch" using one of the Blue Sky MPPT controllers (appropriately sized for wattage) with the panels I'm speaking of.
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Old 10-04-2014, 11:07 PM   #63
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Re: Best Solar Charge Controller? And Series vs Parallel Wi

While waiting for Midnite Solar's "Brat" PWM solar charge controller to be released for sale, I came across this succinct statement regarding the concerns which started this thread (see item 2, below):

"Frequently Asked Questions

How do I size my charge controller?
Take the total wattage of your array (number of panels x watts per panel) and divide it by the voltage of your battery bank to get amps, then round up. The amp rating of your charge controller must exceed this number. If you need any assistance sizing your charge controller, please don't hesitate to call us at 888-899-3509 ext 1.

Should I get an MPPT or PWM charge controller?
Simply put, if the voltage of your panels is close to the voltage of your battery bank, a PWM charge controller is an affordable choice. However, if your panel voltage is much higher than your battery bank, you should use an MPPT controller because it is designed to convert that higher voltage into amps, which will charge your batteries faster and provide you with a more efficient system."

from:

http://www.ecodirect.com/Midnite-Solar- ... r-brat.htm

I know this has been beaten to death in this thread, but...

Given my previous unsatisfactory experience with an MPPT controller for which I had to wire my 17.6Vmp polycristaline panels in series to increase the voltage high enough for the MPPT controller to have excess volts to convert into amps*), but then losing all or some of my panels effectiveness due to tree and other obstacle shading which is an unfortunate effect of wiring in series, I will likely try a PWM controller for my 17.6Vmp panels which I will wire in parallel because when parallel-wired panels are shaded by a tree limb, etc. the panel is not "turned off" by the diodes in the panel like when the sae panels are wired in series. **

* Because when wired in parallel (i.e., although when wired in series volts are additive, in parallel the volts remain the same) my 17.6Vmp panels would only provide a couple of volts over the 14.2v? necessary to charge a 12v battery for the MPPT to convert to amps.

** I was told over at the Midniteforum.com that because my panels are older they probably only have one or two diodes per panel, which means that if a portion of a panel is shaded the diode will turn off a full one-half of that panel. More diodes per panel are better if you are wiring in series because the function of the diode is to turn off the portion of the panel which is shaded. If my panels have two diodes, one diode turns off one-half of the panel. If the shadow of the limb crosses the areas protected by both diodes, each diode then turns off its one-half of the panel for a whopping 100% loss of charging coming from that panel!

*** My goal eventially would be to purchase monocrystaline panels with sufficiently high Vmp voltage to run an MPPT controller.

**** Since I am consolidating my thoughts here, now that I understand Scalff77's statement it bears repeating here because it adds the variable of the affects of ambient temperature on real life panel voltage output:

With a Vmp of 17.6 you are close to getting something out of a MPPT controller, but a lot will depend on the installation, voltage loss from panel to controller and the temperature and location that you are in. The voltage of a panel will drop in higher temp, so as the voltage goes down there is little room for a MPPT controller to squeak and extra power out.

It would be interesting to see if on a 100F day if my panels' voltage would drop below the 14.2v needed to charge a 12v system...

***** I think what all this really means is that: 1. I should wire my current and any future panels in parallel; 2. For cooler Winter temps, I should run a 30amp MPPT controller; 3. in the 95F to 100F Sacramento Summers I should run a 30amp PWM controller.
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Old 10-05-2014, 01:07 PM   #64
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Re: Best Solar Charge Controller? And Series vs Parallel Wi

Sorry I just don't see the issue. In full sun the panels will get hot but unless you're trying to run a specific set load why worry? It's like you're trying to squeeze out an extra amp or so in full sun. My daytime load is usually not more than 5 amps after the battery system is charged and that load cycles between 3 and 5 amps as the refrigerator runs. So what exactly are you wanting to run off your panels? It gets hot here in Turlock for sure and I've never had an issue even when I had a single 135 panel.

In my case it's being shut out of the sun and that usually indicates cooler panel temps no matter what the temp is outside. My goal is to charge the batteries within a reasonable time in the morning while it's cool and handle a small daily load. Maybe I'm missing something but I could use both controllers and still run fine. I'd go for the best performance in shadowed weather but no matter what, very poor skies equal poor solar output using any controller you have.
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Old 10-05-2014, 01:20 PM   #65
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Re: Best Solar Charge Controller? And Series vs Parallel Wi

Thanks daveb! The post was not to highlight any issue. I have learned a lot from you guys and others in a very short amount of time. Thanks to all of you!
So, I was just consolidating what I have learned in one post. (For instance, I thought how the diodes actually work on a shaded panel when the panels are wired in series was interesting.)
My specific load is a continual 3.5-5amps for the Engel Combi MT60 Fridge and Freezer (those who don't have a freezer may have less of an amperage draw or more of a load that cycles on and off) during very hot days. I think I will buy the Brat PWM controller when it comes out and see how it goes.
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Old 10-05-2014, 02:43 PM   #66
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Re: Best Solar Charge Controller? And Series vs Parallel Wi

The way I see it is unless your matching the panel wattage closely to the normal load, I'd think either controller would be fine. Sounds like you're installing a substantial array.
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Old 10-07-2014, 10:38 AM   #67
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Re: Best Solar Charge Controller? And Series vs Parallel Wi

For those of you interested in peeling this infinite onion further...

Based on my research so far re MPPT vs PWM, I assumed naively that the more voltage headroom [i.e., the difference between battery bank voltage (12v house battery in my case) and the solar panel maximum voltage "Vmp" (17.6Vmp in my case)] the better for an MPPT to work with.

Not so! There are apparently sweet spots/compromises with everything in engineering (I assume that's why engineers are so hard to get a direct and definitive answer out of).

The more panel Vmp the more "downconverting" to amps the MPPT controller does. (That is what it is designed to do -- convert excess voltage to more amps for faster more thorough charging.)

But downconverting itself creates heat. And to protect itself, the MPPT controller limits its downconverting to an amount which won't fry its circuits. So in a grid-tied or larger off-grid pv installation I can see why a MPPT controller would need a fan.

Because I stupidly thought that more volts is better for MPPT, I of course made the mistake of wiring my four 85w panels in series to increase as much as possible the voltage headroom between my 12v 250Ah house battery and my 17.6Vmp panels. Just like NewKidOntheBlock did in the following excellent and current thread:

NewKidOntheBlock: "I thought MPPT worked better the higher the difference between the PV voltage and the battery bank voltage."

technodave: "In your case parallel would better for two reasons ......the charge controller will be more efficient at a 1.4 to 1 conversion ratio than it will be at 5 to 1 conversion........more work to do....more heat..less efficient and with your RV you will be much more likely to have shading issues. Parallel configurations will be less affected as only the shaded panel will have a power loss where if you are all in series all panels in the string will be down to the same power as the shaded panel."

From:

http://midniteforum.com/index.php?topic=2002.0

More research is warranted comparing the spec sheets and real world experience across various 30amp controllers, but the good news is that the smaller, fan-less MPPT controllers designed to work for small off-grid installations and rv's appear to work efficiently with a smaller voltage headroom conversion ratio. For instance mine is:

17.6 / 12 = 1.46666666666667

Yippie!!!
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