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Old 09-15-14, 07:06 PM   #1
The_Dreamer
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Question What is the Best Volts/Amps arrangement for Solar Panel Output to Controller

I am brand new to this forum, having joined in hopes that someone with more knowledge and experience of solar design can help me move forward on my project. I am a full-time RVer, living in a converted school bus going on four years. I am doing an upgrade and redesign of my solar auxiliary battery system.

I have four solar panels, 140 watts each. Their specs are as follows:

Voc 20.6V
Vop 17.3V
Short circuit current (Isc) 9.63A
Working current (Iop) 8.1A

I have purchased a Morningstar Tristar 60 Amp PWM controller. I have two Concorde Sun Xtender AGM batteries (153 AH each). I have a Go Power 3000 HD inverter to feed power from my batteries back into my RV's main power controller using a 30 Amp switch from Go Power.

My question relates to how best to wire my solar panels to provide the maximum power to my Morningstar Charge Controller while experiencing a minimum of voltage drop. I should mention I have about a 40 foot run from my panels to my charge controller and I currently have 6 AWG wire to run between them.

My battery bank is arranged as a 12 volt system. I will be using the Morningstar controller to keep the batteries charged at a maximum manufacturer voltage of 14.4 volts. Does it matter what voltage I run from the panels to the charge controller? I am assuming that whatever voltage and amps come into the charge controller are converted to the proper voltage and amps to charge a 12 volt battery since there is a jumper switch in the controller to choose a 12 volt battery system.

I had been reading what others say about setting up a solar system, and I am finding that if I hook all four panels in parallel that my voltage remains steady at a maximum power output of 17.3 volts, while my current quadruples to approximately 32 Amps. From what I am reading, this will cause a tremendous amount of resistance in the wiring. What I am reading is that it is better to have higher voltage and lower amperage to alleviate resistance.

Should I connect all four panels in series to have approximately 68 volts and 8 amps, or do them half parallel and half serial to have 34 volts and 16 amps? What would be the best configuration, and would the Morningstar properly handle the input and deliver it to the batteries without damaging anything?

As you can tell, electrical theory is not my strong suit. I did all the AC and DC wiring in my bus, and I haven't fried anything yet. I want to keep it that way. Any help would be appreciated. If you need the specs on the Morningstar charge controller, it says that it is rated for 12, 24, or 48 volt systems and 60 amps current. It will handle solar overloads of up to 130%, tapering them down.

The manual says "Do not connect a solar input greater than a nominal 48V array for battery charging. Never exceed a Voc of 125V."

I am unclear on the statement above as I am not sure what is meant by "a nominal 48V array for battery charging." Are they referring to the voltage Vop coming from the panels?

Any help to sort out my confusion will be appreciated. Thanks!

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Old 09-15-14, 07:58 PM   #2
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"Do not connect a solar input greater than a nominal 48V array for battery charging. Never exceed a Voc of 125V."

That doesn't sound right. How would you charge a 48V bank?
My four 125w panels are connected in series (about 70vdc).
Small gauge wire* is what I use to feed my TS-45 charger for my 48v bank.

Since the total current in my system never over 7 or 8 amps, the losses are minimal.

I have also used the TS-45 with with Four 200W panels (abt 120vdc) 9A max and it worked fine..

The wire I'm using for this test is RG-8 type coax cable.. It works pretty good.

Currently, my 800w array is feeding directly into a hotwater heater..


My 48v bank is only for back-up, so my solar mostly goes to making hotwater..

Our A7 hotwater heat pump ran about 6 hours since April..
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Old 09-15-14, 08:45 PM   #3
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Quote:
Originally Posted by Xringer View Post
"Do not connect a solar input greater than a nominal 48V array for battery charging. Never exceed a Voc of 125V."

That doesn't sound right. How would you charge a 48V bank?
My four 125w panels are connected in series (about 70vdc).
Small gauge wire* is what I use to feed my TS-45 charger for my 48v bank.

Since the total current in my system never over 7 or 8 amps, the losses are minimal.

I have also used the TS-45 with with Four 200W panels (abt 120vdc) 9A max and it worked fine..

The wire I'm using for this test is RG-8 type coax cable.. It works pretty good.
I think I read somewhere that the Tristar Charge Controllers can handle an input voltage up to 150 volts. Your experience with your two configurations seems to validate this. I do appreciate your feedback. It leads me to think that I should connect all four of my panels in serial. My own set-up would not be much different than your four 125 watt panels hooked in serial.

Thanks for sharing what has worked for you.
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Old 09-15-14, 09:08 PM   #4
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That sounds like something I've read somewhere.
It might even be in my manual. Not sure but it's got to be at least 150v..

When you are charging a 48v bank, your voltage has to be well over 50v.
Otherwise it will take too long to charge and the sun will be gone.
70volts seems to work pretty well for my needs.

A lot of people put their panels in parallel and just charge at 12v..
12v inverters are a lot more common than 48v inverters, is the main reason..
They have to buy a lot of thick copper wire. Big bucks.

But the other reason is the high voltage.. Be VERY careful with high DC voltage!
If you make a mistake, it's worse than being shocked by the AC mains..
It can kill you.. That's why firemen hate solar panels.. Even at night..
I heard one guy say the fire truck lights could make the panels live and shock the firemen.. Crazy..

Use a 10A fuse at the panels and it should never pop, even with a dead short..
It's just for looks mostly.. It will last forever.

A good solar forum is at Solar Electric Power Discussion Forum by Northern Arizona Wind & Sun
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Old 09-15-14, 09:15 PM   #5
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What you are saying about the voltage needed to charge a 48 Volt battery bank is certainly true. That is why I was confused by the quote in the TS-60 manual, "Do not connect a solar input greater than a nominal 48V array for battery charging." Not sure what that means.

I am sure I will have more questions as I put my system together. Thanks for the recommendation on the Northern Arizona forum.
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Old 09-16-14, 05:17 AM   #6
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The manual says the max. input voltage can be up to 125 volts. your panels are 20.6 Voc (Volts Open Circuit) each. Connected in series is less than 125 volts. (84.4 volts) This would be fine if you were charging a 48 volt battery. Not a concern with charging a 12 volt battery..

The charge voltage can be selected via programming (Settings) in the controller. You can choose to charge at any of the three voltages. (12, 24, 48 volts)

In your case, you plan to charge at 12 volts, your panels should be connected in parallel. If you want to connect your panels in series to reduce the wire gauge between the controller and the panels to charge at 12 volts, you need a different type of controller. (MPPT)

What is Maximum Power Point Tracking (MPPT)
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Old 09-16-14, 07:24 AM   #7
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Quote:
Originally Posted by OffGridKindaGuy View Post
The manual says the max. input voltage can be up to 125 volts. your panels are 20.6 Voc (Volts Open Circuit) each. Connected in series is less than 125 volts. (84.4 volts) This would be fine if you were charging a 48 volt battery. Not a concern with charging a 12 volt battery..

In your case, you plan to charge at 12 volts, your panels should be connected in parallel. If you want to connect your panels in series to reduce the wire gauge between the controller and the panels to charge at 12 volts, you need a different type of controller. (MPPT)
Hello OffGridKindaGuy. I appreciate the info on the 125 volts limit for this charge controller. I am not following your reasoning, however. If my charge controller will handle the 84.4 volts my panels could produce, why would I need to wire them in parallel and limit them to 20 volts? This would cause the amperage to balloon to 32 amps, causing a lot of resistance in the line, with very little voltage to push it along.

I have read that MPPT controllers are best for very large voltages, especially when one gets above 150 volts, but I am not clear on why you say I would need a MPPT controller if my volts are only half of that.

You wrote:

"In your case, you plan to charge at 12 volts, your panels should be connected in parallel."

I am not disagreeing with you. I am simply trying to understand the concepts involved as having an understanding of what is going on in electrical theory seems to be prudent when tackling a job like this. Why do you say I need to connect in parallel if I am charging a 12 volt battery bank? Would not a higher voltage, low amperage input do the job just as well, if not better? Doesn't the charge controller regulate the volts and amps being fed into the battery bank?

If you could explain your conclusions I would appreciate it.

Thanks in advance!
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Old 09-16-14, 11:26 AM   #8
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The losses due to heating up the wire increase with current.
Watts of loss= Current Squared x Resistance.

(Notice that voltage isn't in the formula).

~~~

I've never even looked at using a 4 panel series array to charge at 12V..
But, it seems like it would put more demands on the charge controller.
Even with PWM, controlling how hard you hit a 12V battery when starting off with 100vdc, isn't real simple. Something to research there..

~~~~
In addition to (12, 24, 48 volts) battery packs, I believe my CC can be configured for Golf Karts.. 36volts! LOL!!
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Old 09-16-14, 01:27 PM   #9
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Quote:
Originally Posted by Xringer View Post

I've never even looked at using a 4 panel series array to charge at 12V..
But, it seems like it would put more demands on the charge controller.
I need to pay more attention to the details. I neglected to notice you have your batteries in a 48 volt array. That would certainly make a difference. All those extra volts would not be wasted on your system as they would be on my 12 volt battery bank.
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Old 09-16-14, 01:48 PM   #10
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Those volts wouldn't be wasted. Volts are like pressure. Like PSI in a compressor tank.
Amps are 'flow', as in letting the air flow from the tank.. cubic feet per minute
is kinda like Amps, only that's electrons per second..

For nerds, 1 amp is 1 coulomb flowing per second (a coulomb is approximately 6.24110 to the +18th electrons)


Anyways, if you were to design a battery charger for 12V,
you would start off with a DC power source of at least 15 volts.
And it should the ability to provide some good current flow at that voltage.

The tricky part is to sense when the battery is up around 14v and ease back on the current, so as not to over-charge.

But, if I had to design a 12v charger using a 100 volts source,
it's not going to be easy, and the parts will be a lot more expensive.
Yes, there will be a bit of waste, as the output transistors turn off and on, they will run hotter..
My 48v charger never gets hot. Warm maybe.

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