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  #11  
Old 12-09-2013, 12:06 PM
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Yeah, panels may be better now. Mine's a Sharp 24V, 240W from 2011. I don't get close to 240W from it unless I get the incident light is perfect. I don't have a tracker or anything, on the old house it was just bolted on a south facing roof on the garage and here I just use a 2x4 frame and set it out in the backyard when I need it.

Since I had it flat mounted on the roof I was limited to the slope, which was pretty steep at 8-in-12, roughly 34 degrees and not too bad on tilt. The way I do it now, with the frame I put it at 40 degrees in the back yard.

Anyway, point is that for every 10 degrees of elevation or azimuth misalignment my panel output drops 5%. If I leave it set up directly due solar south all day I might get on average about 135 W-hr and get full output for about an hour or so total (which incidentally isn't noon but about 9~10AM and 2~3PM and above 80% efficiency for around 3 hours total. If go out and rotate it every hour or two it does better (this would be 1-axis rotation), about 165 W-hr, but my tilt is non-ideal really. I tore it all apart this fall and plan to make once that I can adjust the tilt and dispense with the cinder blocks and guessing.

I have not done year-round detailed measurements since I don't leave the panel set up all the time. But what I have done indicates to me that 75% is the best you can do and to get it you'd need to do 2-axis alignment and even then the 240W rating is during ideal temperature, too. In the summer my panel gets really hot during the day and efficiency drops a ton. The output rating is for 25*C and Sharp gives a temperature coefficient of -0.51%, which means for every degree over 25*C the panel output drops 0.51%. So if my panel gets to 40*C the output will have dropped 7.65%. That's not all that hot, 104*F, and in full sunlight during the summer the panel will get much hotter than that, 50 or 55*C isn't unusual. You can give up 10% or 15% efficiency there alone.

That the reason why with a fixed panel I was getting less than 50% capacity and why I suggested the 2x capacity. Just the amount of sun missing it took ~20% easily and then the heat zapped a bunch more. OTOH in the winter the temperature coefficient works to your favor and efficiency goes up as it gets cooler. Which is good because you start to loose a lot because the lower sun angle and more energy is absorbed and refracted by the atmosphere.

Also sizing the panel much larger allows the load to be comfortably accommodated simultaneous with charging at the same time in all weather. There are a lot of places to lose solar energy and panel capacity itself is really a small part of the equation.
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  #12  
Old 12-09-2013, 06:29 PM
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I crunched the numbers for a solar system, before springing for a honda generator, and came up with close to what Dave posted. I even re-ran them at 20 watt output (giving up ~1 S unit at the other end of the string) and concluded the generator was still more cost effected.

Couple of the "gotchya's" Dave didn't mention:
1) Ancillary equipment drain:

A) I'm running a RT-100 and need 3 amps* injected at the Bias Tee for a few seconds at beginning of each transmission to feed the tuner.

B) it's takes 12 watts/hr* to keep an iPad charged for logging and running PSK31 and 5 watts/hr* to keep an iPhone charged up so you can tether to the iPad for QRZ look ups and checking DX Sherlock for spots, E paths and MUF.

C) A small 12V low wattage CFL lantern for night ops.

D) FT meter, draws only a watt or so*

*All numbers are from memory but the point is they all add up and must be taken into account.

2) The BIG Gotchya: The price of the solar controller. You can't use the el-cheap-o Pulse Width Modulated Controllers as they put out way to much hash. You'll need to run a MPPT controller. When I was pricing them out a quality (read no Chinese knock offs) 200 watt MPPT controller was half the price of what I paid for the genny.

Last edited by Seldom Seen; 12-10-2013 at 12:06 AM.
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  #13  
Old 12-10-2013, 06:07 AM
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My controller is a Morningstar Sunsaver MPPT 15 which is supposed to hit up to 97% efficient at getting energy from the panel to the battery. I've never personally measured it, though.

You can find MPPT controllers a little cheaper than this one, especially if you don't use 24V panels. I went with a 24V PV system for a couple of reasons. One is that if in the future I want to expand my system beyond 1 battery to use an inverter 24V makes sense, reduced DC current and allows a wider selection of heavy duty inverters.

Even if I just add more storage and stick with a 12V battery bus, running a 24V panel and MPPT controller you get a higher overall energy recovery since the controller is rarely running near its max duty cycle and current. The max panel power rating for the controller doubles at 24V to 400W over 12V panels. My single panel and 12V battery sits right in the middle of most of the rating curves for this controller, so it's never seriously taxed.

Plus, the Morningstar allows me to jump up to 36V panels if I wish in the event I want to build a true self-contained system independent of the grid.
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Last edited by DaveInDenver; 12-10-2013 at 06:27 AM.
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Old 12-10-2013, 01:16 PM
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So what's the story then with the $8-$12 solar charge controllers you see on ebay? I get that they may not be the best for base station use, but what about trickle charging a little juice back into a backup car battery?

http://www.ebay.com/itm/30A-Solar-Ch...-/141136355420 for example
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  #15  
Old 12-10-2013, 02:33 PM
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Remember these charts. They are representative of PV panel characteristic curves.



The box you link looks like a simple battery buck charger. It takes an input, slices it into pulses (quantizes) to cut it down to a lower average voltage. It's taking whatever the panel is giving it and making it match the battery by switching from off to on for various lengths of time. In bright sunlight the panel is outputting say 27V it produces a relatively short duty cycle that averages 14ish volts. In lower light the panel might be at 15V or 20V. The duty cycle would increase to keep the average voltage where the charger wants it to be.



But looking at the panel curves at high voltages it's producing relatively low current. You need current to charge a battery. Not to mention that any time the voltage is above or below the knee on the curve your total power made is lower. So there is a small window where the panel is making its full /power/.



They feed the PV output into a non-inverting buck-boost converter that can produce more output current (or voltage) than the input. This is possible because it's converting power. So rather than just chopping a higher DC voltage down to a lower DC voltage they are attempting to get the full power from the panel to feed into the battery.

MPP is an algorithm that matches a solar cell's maximum power point to the battery. The controller attempts to keep the system operating right on the panel's I-V knee so that the most amount of power is extracted and fed into the charger for whatever the solar energy density (on the left) and module voltage (on the right) happen to fall.

The key here is that it's matching the system, so the panel, load, converters all working to find the MPP. The 'T' part is tracking, which means it uses various control principles to figure out the MPP. The Morningstar uses perturb-and-observe, if you remember your controls theory.

At some times the MPPT and buck converter would be close to equal, small windows of time. But over a whole day, particularly during the middle of the day when the panel output is above MPP voltage, but also for some time before and after as the sun rises/sets, the MPP the buck efficiency will lag a lot behind an MPPT converter. When the terminal voltage is above MPP the buck is throwing away a whole lot of power. The software in MPPT controllers can be sophisticated while a buck converter probably doesn't have a microprocessor at all.
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Last edited by DaveInDenver; 12-10-2013 at 03:46 PM.
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Old 12-10-2013, 02:59 PM
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Quote:
Originally Posted by nakman View Post
So what's the story then with the $8-$12 solar charge controllers you see on ebay? I get that they may not be the best for base station use, but what about trickle charging a little juice back into a backup car battery?

http://www.ebay.com/itm/30A-Solar-Ch...-/141136355420 for example
Funny, that's the EXACT one I have on my truck with my 130watt panel charging my two group 31's.

Sounds like it may be time for an upgrade.

Thanks for the explanation Dave, makes a whole lot of sense.
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Old 12-10-2013, 03:44 PM
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There is nothing wrong with a simple battery charger like that. Primarily they protect the battery from over voltage but just realize that they leave a whole lot of energy on the table. With a sophisticated charger you can use a smaller PV array to keep a larger battery fully charged and across deeper discharges. In a word it'll be more efficient.
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Old 12-10-2013, 06:55 PM
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Quote:
Originally Posted by DaveInDenver View Post
There is nothing wrong with a simple battery charger like that. Primarily they protect the battery from over voltage but just realize that they leave a whole lot of energy on the table. With a sophisticated charger you can use a smaller PV array to keep a larger battery fully charged and across deeper discharges. In a word it'll be more efficient.
Wow, thanks. So ok, you're saying a cheapo charge controller essentially leaves energy on the table, compared to a more expensive one. Is there any difference in the way it delivers that energy to the battery, that would affect one adversely? I guess what I'm getting at is, is it cheaper to just add a second panel to offset this, rather than pony up for a better charge controller? Yikes this feels like trying to overcome lack of wheeling skill with more horsepower..

How much is a "good" charge controller anyway? Can you put up a link to a one?
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Old 12-10-2013, 08:03 PM
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Quote:
Originally Posted by nakman View Post
How much is a "good" charge controller anyway? Can you put up a link to a one?
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Last edited by Seldom Seen; 12-10-2013 at 08:33 PM.
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  #20  
Old 12-10-2013, 08:34 PM
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Nice thing about nice ones like the Morningstars is that they sense load to balance charging against usage. They also can do equalization which is nice to keep batteries conditioned. Sometimes you don't get a good charge because your load take priority over charging while other times you have excess power during low usage that could be used to condition the battery. They can also remote temperature sense the panels which is one variable used in the MPPT algorithm to determine peak efficiency.

The risk for simple chargers is mainly if they don't do a good job and over/under charge your batteries. Going over voltage is bad, leads to sulfation and ruins the battery.

It's more likely that they starve them perpetually of current. If you remember time and current are the two variables in the battery charging equation. There are specific voltages and currents required to fully satisfy a battery during all phases of charging.

It's mainly during bulk charging that you will need to give the battery the current it wants and this is measured in percentage of capacity (C). So a C of 100A-hr might require constant current for 8 or 10 hours at 0.1*C at 2.35V per cell or 14.1V for a 12V type. If you dip below this you will starve the battery, resulting in insufficient chemistry reversal but exceed this current and you overheat the battery. Heat is a big enemy of batteries especially during charging. Usually once a battery hits stage 2 or 3, what is usually called maintenance or floating, the internal resistance begins to climb and they regulate their current themselves. It's when you are starting to charge a deeply discharged one that you have to be careful to give them neither too much or too little current.

None of this is to say that you /must/ spend a lot to get a high quality charge controller, but often you do get what you pay for.
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