What does having a "balanced" system mean?

I need to buy a new charge controller, and my understanding of electrical fundamentals isn't great. So I'm reading lots of stuff I only partially understand.

One thing I read where they were discussing wiring something one way over another was that it was more balanced, so it would be better for the longevity of the system overall. Now, not understanding what balanced means, I'm looking at what we have and wondering how to go about turning our bits and pieces into an actual system.

We've got four 12V gel batteries and three 100W panels. Everything's wired in parallel and we don't even have the batteries hooked up to the panels most of the time, since many days we don't use any electricity at all.

If we were to stick with a 12V system, is it weird having 3 panels instead of 4? Seems like all the wiring diagrams I see are 1, 2, or 4 panel. Is this because of the inscrutable balance I've heard of?

Think of the scale of other things in your life.  Imagine taking a full-size sedan and trying to power it with a lawnmower engine.  Or taking a 12 cylinder big-rig diesel engine and trying to put in on your son's go-cart.  For Thanksgiving dinner, and you buy a 3lb chicken to feed an extended family of 20 people, or buying a 25lb turkey to feed a family of two with a baby.  All examples of the wrong scale for what you want to accomplish.

The same principles apply to solar.  You buy lots of batteries and then try to charge them with one 100W panel.  Or, you install 10,000W of panels to charge a 50Ah battery.  Stuff needs to be scaled to each other to work properly.

There are rules of thumb that are good to adhere to.  In general, you want to deplete your battery less than 50%.  So, you shouldn't try to get 2000Wh of power out of a battery that only contains 600Wh of electricity.

In general, you want 2X the number of panel watts to run your largest load.  If your largest load is a washing machine that uses 500W, you should have 1000W of panels.  You should expect 2.5X your panel rating in winter and 5X in summer.  That means if you install 300W of panels, you can expect to make 750Wh of power in the winter, and 1500Wh in the summer.

Batteries like being charged and discharged at around 1/10th of their capacity.  So, if you have a 100Ah battery, it wants to be drained at no more than 10A.  Running at 12V, that 10A will equal 120W.  So, you don't want to run a 1500W toaster oven on a little 12V 100Ah battery.

In your own example, what is the Amphour (Ah) capacity of your batteries?  100Ah?  Assuming you want 10A to charge one of them you need at least 10A X 13V(charging) =130W  Let's throw in a fudgefactor of 1.25 for poor conditions  130 X 1.25= 162.5W.  Since you have three batteries though, you multiply the numbers by 3, so you really should have 162.5 X 3 = 487.5W.  Call that 500W.  

So, for a battery bank of your size, you really should have 5 100W panels instead of three.  So, that's an example of some inbalance.  On the other hand, if you don't even use any electricity, and the batteries just stay full all day, less than 5 panels will get you by.

In the olden days of solar, only PWM controllers existed.  They only control the amperage the panels put out.  Today's modern MPPT controllers act as a transformer, changing raw high-voltage grid-tie panels down to battery voltage.  Now you wire multiple panels in series to push up the voltage and send the power hundreds of feet through thin copper wire.  I have panels as far away as 130' that I'm running at 120VDC to my controller.  The controller then transforms the raw solar down to my 24V battery voltage, making more amps in the transformation.

I now have enough solar energy for a completely modern existence, with lights, TV, computer, a refrigerator, and even part-time air-conditioning.

Keep in mind that 12V panels are the single most expensive form of electricity.  You get far more watts per dollar buying high-voltage grid-tie panels.  They are made in vast quantities to be put on roofs everywhere in America.  Expect to get 4-5W/$.  All this happens once you go MPPT.

Thank you! That's helpful.

So there's nothing actually harmful with having 3 panels for 4 batteries; the harm happens when we try to use the system as if we had 5 panels.

So now that I'm looking at charge controllers (mppt), I see that an appropriate size for the number of panels we currently have is 40A. I'm thinking about the future though. So I'm thinking I should get a 60A to allow for panel purchases. Using a 60A charge controller for three 100W panels is probably another example of imbalance, but there isn't any harm done by it, unless we try to use the system more intensively than its limitations (in our case the power going into the batteries). Do I understand?

Side question. Is there a benefit to wiring panels in series, only to drop the voltage down again for our 12V battery bank? You mention getting higher amperage, but does that really matter for our size bank?

Hmmm...

I think our batteries are 90Ah. The only thing it says on them is 220 minutes runtime 25A @ x temp, etc. So 220/60*25=91.6 repeating.

Which, going by your rule of thumb, means 9A charging @ 13V x 4 batteries equals 468. With fudge factor for poor conditions, that would bring us up to 6 panels. A 60A controller won't cover that many on a 12V system. Can I ignore the poor condition fudge factor somewhat if I'm using an mppt controller?

Jan White wrote:Thank you! That's helpful.

So there's nothing actually harmful with having 3 panels for 4 batteries; the harm happens when we try to use the system as if we had 5 panels.

So now that I'm looking at charge controllers (mppt), I see that an appropriate size for the number of panels we currently have is 40A. I'm thinking about the future though. So I'm thinking I should get a 60A to allow for panel purchases. Using a 60A charge controller for three 100W panels is probably another example of imbalance, but there isn't any harm done by it, unless we try to use the system more intensively than its limitations (in our case the power going into the batteries). Do I understand?

Side question. Is there a benefit to wiring panels in series, only to drop the voltage down again for our 12V battery bank? You mention getting higher amperage, but does that really matter for our size bank?

Jan, the main advantages to hooking your panels up in series is to reduce the wire size from the panels to the controller and gain efficiency during less than perfect weather. If you were to hook up your 3 panels in series you would get roughly 50 to 60 volts in full sun at roughly 5-6 amps. Versus needing a wire that can carry 15-18 amps in parallel.... Then on cloudy days when your panels do not generate anything now you will produce roughly 20-30 percent of their rated capacity using an mppt controller. It is the way to go... Maybe not now but eventually.
Cheers,. David

Jan White wrote:Hmmm...

I think our batteries are 90Ah. The only thing it says on them is 220 minutes runtime 25A @ x temp, etc. So 220/60*25=91.6 repeating.

Which, going by your rule of thumb, means 9A charging @ 13V x 4 batteries equals 468. With fudge factor for poor conditions, that would bring us up to 6 panels. A 60A controller won't cover that many on a 12V system. Can I ignore the poor condition fudge factor somewhat if I'm using an mppt controller?

Here is what happens in the real-world.  Your 100W panel will NOT produce 100W unless it is under perfectly ideal conditions, which are almost never present.  If you point your 100W panel directly at the sun and measure it's total output, you are not likely to see more than 85W, or 85% (0.85X).  That's because the panels are all tested in artificial sunlight at 1000W/meter squared.  So, that's where the fudgefactor comes in.  I could have told you to divide by 0.85X, but it seems that most people get confused by math with multiple divisions and multiplications.  So, I try to keep it simple.

So, for your batteries (sorry, I initially missed the 4) the math would be 90Ah X 4 batteries X 0.125C X 13V charging X 1.25X FF = 730W.  Maybe three 245W grid-tie panels.  I just paid 55$ each for those last season.  That works out to be [(245W X 3 panels)/13V charging] X 0.85FF =48A.

A little high for a 40A controller, but for a few $ more you could get a 50A controller.  Look at Epever's 50A Tracer.  They come in both 150 and 200V models.  I'm guessing that most likely the controller you are looking at has a 100V limit.  That's not very high by today's standards.  Let's say you wire a total of three 30Vmp (37.5Voc) 245W panels in series.  You'll get 90Vmp when the panels are producing power, but 112.5Voc when the controller shuts off.  Voltage that high might fry a 100V controller.  But, the Epever 50 would handle that just fine.  Another combination would be six 100W panels (18Vmp; 21Voc).  Six in series would put out 108Vmp.  When the controller switches off the voltage would go back up to 126Voc.  That might fry the cheaper 100V controllers.

You get a lot more flexibility when you go with higher voltage.  You save a lot of money buying thinner copper wire.  You also have more choices of where you can position the panels because you can place them 100-150 feet away from the system.  No, you can't ignore the fudgefactor for MPPT.  In fact, that fudgefacter is better for MPPT.  For PWM controllers, the FF is more like 0.6-0.7 instead of 0.85.

Thanks very much to both of you.

The Epever brand is what I was looking to get anyway, so I'll go ahead with that. I think I'm going to go with a 60A one. It'll take 150V, so I should be good if we decide to add two or three panels at some point and wire in series.

Michael, the way you calculated the necessary amperage of the charge controller was unfamiliar to me. I'm going by combined wattage of panels 300/lowest voltage in system 12*safety factor 1.25.  Do you think that's overkill?

This is all kind of unnecessary at this point anyway. The most use our "system" gets is running a 240W food processor on a sunny day 😆 Most of the time we're just charging phones or a laptop. Maybe running a single LED in the evenings. Really minimal usage.

Thanks again for your help!

Jan White wrote:
The Epever brand is what I was looking to get anyway, so I'll go ahead with that. I think I'm going to go with a 60A one. It'll take 150V, so I should be good if we decide to add two or three panels at some point and wire in series.

Michael, the way you calculated the necessary amperage of the charge controller was unfamiliar to me. I'm going by combined wattage of panels 300/lowest voltage in system 12*safety factor 1.25.  Do you think that's overkill?

There are two ways to do this.  Either base the numbers on what your controller can handle, or base it on what your batteries can handle.  You should select the lower of the two values.  Let's do the math for both....

Sticking with your choice of the 60A controller, 60A X 12V = 720W.  No safety factor is needed, because if you expect the panels to put out only 85%, that would be 60A X 0.85C X 12V = 612W in the real world.  So, you could get three 240W panels.

Sticking with your battery's capacity limit, it would be 90Ah X 4 batteries X 0.125C X 12V = 540W.  Now, accounting for the 85% efficiency that works out to be 540W/0/.85 = 635W.

If you stick with 100W panels, you could get 3 more and wire all in series for your new controller.  Keep in mind that grid-tie panels are dirt-cheap on Craigslist now.  I have gotten 500W of grid-ties for the price of one 100W 12V panel.

Now is the time to jump head-first into 21'st century solar!

One thing to remember is if you do any cold weather charging you want that 1.25 voltage fudge factor. So open circuit voltage as printed on your panels times the number of panels in the circuit time 1.25 will be the voltage your charge controller needs to take. You will only see that number a few times a year but every spring someone would call with a dead charge controller. String over voltage kills mppt controllers...

David makes a good point about cold weather.  Here's a link to Midnight Solar's string calculator, which you can use to determine the voltage of your system at it's winter low.

http://www.midnitesolar.com/sizingTool/index.php

What are your winter lows like?  According to Midnight's calculator, your string will exceed a 150V controller's capacity at about -27F