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Old 02-03-2013, 11:57 AM   #11
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First the system that I put together-
• 2 55W panels
• MPPT controller
• 25' - 50' cable connecting panel to trailer

Design requirements-
I was putting together 2 systems; one for me and another for a friend. We both use a bit of power ranging from a CPAP machine, computers, and other assorted electronics. We camp under trees most of the time so have to deal with less-than-optimum charging conditions. Our trailer has 2 6v batteries and LED lights.
Design decisions-
I took a swag and thought I needed around 100w of power. I chose 2 movable panels because-
• Easier to store; I fold them in half.
• I wired them in parallel so when shade is on one half, the other half has the potential of producing power.
• I wanted to optimize my power by the ability of getting the panels out of the shade (where the trailer usually is) and pointed at the sun.
Why the MPPT controller-
• Efficiency- the MPPT controller runs about 97% and the PWM about 80%
• Can still charge the battery when voltage goes below 12v (think panels in shade shade)
• Has an output to monitor operation (I ended up buying an external monitor)
Trade-offs
I was actually going to go with 80w panels and MPPT as opposed to 100w and PWM (both systems would give relatively the same output power) because the panels are smaller and lighter (easier to store and handle) and a little more versatile. I got a good price on 55w panels (only $15 more than 40w panels) so went with those for the added power.

Actual use
For the most part, the system is over designed BUT there has been a few occasions where we needed everything I had. Last summer we went to Yellowstone for 5 days. The site we had nestled in trees and because of its orientation, the trailer was always in shade. In fact, it was hard to find a spot to move the panels to get much sun throughout the day. In the evening (when home from sightseeing), I would play with the panels, chasing the sun to get all that I could. We never ran out of power but we were never full. During the stay, we both used our computers and phones and ran the heater at night. After this trip, I added an extension cable to give me up to 50’ (as opposed to 25’) to roam.
Summary
I am very happy with my system; I know that I can camp among the trees in winter and have enough power that I don’t have to worry. The PWM controller with the larger panels that I chose would probably work fine (and I could have saved some money) but the (electronic) gearhead (I was an electrical engineer) in me pushed me to the MPPT choice. The owner of the other system that I set up is happy as well.
As a side, one reason to correctly size the MPPT controller as opposed to getting one which handles much more power that you currently need (besides the cost) is that they are slightly less efficient at the lower power ranges. The Morningstar SunSaver MPPT is 97% efficient at around 75-100w and 93% at 25w (really not a big deal) and the larger they are, the more current it takes just to run, leading to being less efficient.

Ben
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Old 02-03-2013, 01:27 PM   #12
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Ben, good write up. You put very well into layman terms your various considerations. I never realized the difference in efficiency with MPPT over PWM. Is this true for all makes and models?

I just bought (well, got a raincheck for) a couple of the $100 Canadian Tire 40W systems (on sale from $250). I know they have their own controllers, and I could just hook them both in parallel, but am considering hinging them for better storage, and combining the feed to the batteries through one new charge controller. Still just a thought, as I might just keep one though until I am in a situation where I can use more charging capacity, because as of yet, I have never really needed topping up my batteries, as I have only done 5 days in a row off the grid.
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Old 02-03-2013, 01:58 PM   #13
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Depending on how reliable you consider the information at forums, you might want to look at this discussion on comparing PWM & MPPT controllers for small systems at the Northern Arizona Wind & Solar site. The website also has some good information on panels & controllers.

While the efficiency of an MPPT controller is higher with high voltage panels, it does not make as much a difference with a single 12V panel. I'm not saying that a PWM controller is better than a MPPT; in general it isn't since the MPPT is more efficient, but it might be better overall to spend more on the panel to increase output mixed with a less expensive controller rather than the other way around with small ( <200 watt) systems.

Since panels designed for 12V produce their power at lower voltages, you do not lose as much with a PWM controller. On the other hand, if you wire a couple of them in series or purchase high voltage panels, the losses created by a PWM controller are excessive - much better to go with a MPPT controller that better matches the panel voltage to the battery voltage.

If you stick with a major manufacturer such as Morningstar, the quality of either type of controller is OK, however there are poorly designed controllers of both types, particularly unknown manufacturers selling on eBay, etc. A poorly designed MPPT controller can be less efficient than a well designed PWM.
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Old 02-03-2013, 02:02 PM   #14
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I'm trying to build our system from scratch, sort of. A few years ago, I managed to pick up a bunch of Siemens individual solar panels. These are about 6" wide and 48" tall. They were used at a commercial mountain-top radio repeater site. Due to a requirement that the panels be replaced every 4 years, regardless of condition, these became available and are in great shape. There are definitely the older technology but they will still do the job. Each individual panel puts out 6 volts at 3.2 amps for a total of about 19 watts. I plan to make up two main panels, each using three of the individual panels. Each of these main panels will then be capable of 18 volts at 3.2 amps for a total of about 58 watts. Two wired in parallel will give me 18 volts at 3.2 amps or just over 100 watts in full sunlight. This setup should be adequate to keep out two 6-volt batteries charged in most light conditions. Since we didn't get the solar option in our 19, these panels will be portable and placed on the ground which will provide more flexibility in keeping them in the sun. I have about 30-feet of 2/8 cable to use with the panels so voltage drop should be minimal.


In reading all of the posts, I'm still not sure whether to go with an MPPT controller or a PWM controller. Since my panels are older technology, maybe the PWM controller would be adequate. And, since I have more power than I really need, maybe the difference in efficiency between the two wouldn't matter here. The price difference is pretty significant. I really don't know. I need more convincing <g>.


Doug
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Old 02-03-2013, 07:38 PM   #15
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Ben, thank you for the explanation. We would want the 50' cable as you have and I guess the MPPT has better efficiency for that as well as for the temperature extremes. I am also drawn to an MPPT and I am not an electrical engineer. I saw a diagram with the greater efficiency and that stays in my head.

The one advantage to a bigger panel might be that it makes it a little harder to steal! (but probably not hard enough). You are very trusting to go off and leave the panels all day.

Of course, the Escape is supposed to be in shade as the enemy of fiberglass is UV, so being able to place panels away from it is a good option.
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Old 02-03-2013, 08:23 PM   #16
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For the Morningstar SunSaver MPPT controller, you can hook up a temp senser to the battery which will help optimize the battery charge voltage. Since I have the 17B with the batteries outside, I bought it but I don't know if it makes much of a difference.
Another reason to have movable panels is that they are more efficient when cool and heat degrades the cells. Having their back free allows air to circulate better, keeping them cooler.
My friend with the other system locks the panels to trees or his car using a bike lock. I figure if someone wants something bad enough they will take it no matter how I try to prevent it; what will be will be...

Ben
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Old 02-04-2013, 03:08 PM   #17
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Using the sun saver you need to think high voltage. Bank the panels for more voltage because batteries require current to charge. The sun saver can accept up to 75 volts and drops it down to 12 V. Power = Voltage X Current. Raising voltage on the panel side will drop the current and allow smaller wiring to transmit the power to the controller. At the battery the controller will produce lower voltage and higher current. When in low sun conditions the panels will produce lower voltage (based on amount of sun) if voltage produced is 14V or better you will still be charging batteries.
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Old 02-04-2013, 03:36 PM   #18
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The problem with 'banking' the panels (putting them in series, nominally 24v) is that you will need more direct sun. If either panel has shade, then your entire output will drop to near 0v. If they are in parallel (nominal 12v), one panel can be completely shaded and the other can still deliver its full power.
You are correct in that twice the voltage will have half the current but if you look at the big picture, it isn't much. For my system, I have 25' of 12AWG cable. According to tables I have seen, this cable is 0.04Ω total producing a 0.28v drop from the panel to the trailer (0.04Ω x 7amp- the max I output). If I arranged them in series, my drop would be 1/2 that or 0.14v. I could go with thicker wire to decrease the drop, but to be real, there probably is more resistance in the connections to the controller which will overshadow any cable improvement.
It's all about compromises. I personally prefer to sacrifice a higher voltage drop for the ability of producing power when part of panel is in the shade.

Ben
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Old 02-05-2013, 05:20 PM   #19
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We just checked the batteries and they are at 12.62. Been in outside storage since October and we have not charged them a bit. We don't need to be in a hurry for solar panels for that use. We did clear snow and hook up and move slightly.

BUT do you know that the camping season is almost upon us?

I did look at where panels might be put while traveling and I am thinking smaller is better as some of you have.
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Old 02-06-2013, 05:07 PM   #20
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In checking out the Morningstar website, it appears that the Tri Star TS 45 controller would best fit our needs. However, I see they only sell through distributers and they don't list any in Canada. Has anyone in Canada purchased a controller from these guys? If so, where did you get it? It would be nice if there was a dealer in BC.

Doug
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