Making sure I understand things

8n-bob

OK another few questions
I think I am starting to understand things A little bit.

this is a what if situation

2 PV panels

Each pannel rated at 36.4V @ 280W that Makes it 7.69A

Moringstar 45 MPPT controler
12V cells Flooded type. 660 amp hrs worth of cells.

Of course negating losses

If charging at
12V Current = 23.33A
13V Current = 21.54A
14V Current = 20.00A

So if 2 pannels are used charging would be
12V = 46.66A
13V = 43.08A
14V = 40.00A

Now the Questions
The morning star is a 45 amp controller. As you can see with 2 panels it will be 46 amps.
Now will this controller still be OK or will it burn up of since it's over 45A or will it just put out 45 amps and the rest goes to heat.

Solar panels seldom operate at 100% their rated power
Cable losses
Losses in controller

Why I am asking is I am looking at one panel to start and getting another panel later down the road.
If the 280's will not work then maybe I go with some 250's and be on the safe side. I think my controller budget is right around $400.00 and I want to hold close to that for the controller.

I can only afford one 280W panel at the present. But looking for future expansion

Also Can you tie different panels together say for instance 280W @ 36.4 and a 235W at 30.1 into the same controller either parallel or series (if the controller can handle 66.5V). or will the lower panel pull down the Voltage or Amps (depending on connnection)

Thanks
BobO.

Side note, from previous answers here
I have 660A/H of batt so I need
660 *.05 = 33A of charging.
I currently have about 10A worth of solar charging (I know not enough that's why I am only running one 220A/H cell set). Thats metered and observed not specification. Plus I do have some wind power to the tune of about 45Watts (3.75A @ 12V) average, in a day, also metered not liseted
So one 280W panel will give me 20A at 12V so lest say 18A so that worth of current
Finished mat
18A+10A+3.75A = 31.42A A little under the 33 needed. But with a second 280W panel that would be power to spare for more cells if I find the 660A/H charges quickly.

Cariboocoot

Re: Making sure I understand things

Let's look at it from a standard expression point of view:

A 280 Watt solar panel would have a Vmp of around 30 (not 36.4 - that's probably the Voc) and an Imp of around 9. It is important to be sure you use the Vmp rating for power and not the Voc rating.

Even so, since it is (or they are) being used on an MPPT controller the result on the output is more of a function of the Watts alone, as the controller will adjust V and I as it sees fit. So two of these panels total 560 Watts. The output will be averaged and include losses. This typically comes to a factor of 77%. So the output Watts becomes (560 * 0.77) 431 Watts.

This you can divide by a given Voltage to determine the maximum likely current at that Voltage. I use system nominal Voltage for this because ideally that is as low as the batteries would be allowed to get and thus the point when the maximum current would be needed for full charging.

431 Watts / 12 Volts = 35.9 Amps.

There are many things that will make that worse: high panel temps, excess wiring losses, etc.
There are a few things that will make it better: low panel temps, high altitude, reflective light, etc.

Ultimately the power out is dependent on the power in and the demand: if the combination of battery charging and loads does not need 400 Watts the panels will not produce 400 Watts no matter how much light falls on them. The current out is always a function of V/R, both of which vary.

Mixing panels on the same controller is problematic. If the specs of the panels are too far apart you start to lose more power than you gain. For example with parallel panels a large difference in Vmp will cause the lower Voltage panel to operate too far above its Vmp, reducing its current output. When placed in series the Voltage adds together but the current will be limited by the lower Imp panel. This is why it is best to use all matching panels on any one controller.

Your 5% charge rate, by the way, is net minimum. In other words 5% at the battery with no loads drawing. This is why it is better to target a higher percentage to make up for loads and losses. The sun does not always shine so brightly every day!

8n-bob

Re: Making sure I understand things

I was able to get some more information on the panels (see below).
So the 36.4 is actually Vmp 35.8 and Imp 7.8
Voc is 44.6 and Isc 8.43
with a 280W max power and a "listed power" of 251. This would be 502 for 2 panels.
Now is that the comparison number you got 431 for, this would be a 71 watt difference.

502 is 89.6% of 560
where
431 is 77% of 560

Do you think that it's a different power factor.
I am not doubting your figures Just trying to wrap my mind around this.
and with that What exactly is the CEC PTC Listed Power mean, and is that the same with the addition of wire losses etc. as your 77%

And even using their figures of 501 that's still only
501/12= 41.75amps to fairly depleted cells, less then the 45 for the controller. :cool:

Thanks for you help.
This is fun. :-)


Max Peak Power Pmax
280 W (0% / +3%)
CEC PTC Listed Power
251.0 W
Maximum Power Point Voltage Vmpp
35.8 V
Maximum Power Point Current Impp
7.82 A
Open Circuit Voltage Voc
44.6 V
Short Circuit Current Isc
8.43 A
Module Efficiency (%)
14.2%
Temperature Coefficient of Voc
-0.156 V / ºC (-0.35% /ºC)
Temperature Coefficient of Isc
5.06x10-3 A / ºC (0.06% / ºC)
Temperature Coefficient of Pmax
-1.26 W / ºC (-0.45% /ºC)

Cariboocoot

Re: Making sure I understand things

Just different derating systems. The (35.8 Vmp * 7.8 Imp) 280 Watts would be STC - Standard Test Conditions. The 251 Watts is more like NOCT - Normal Operating Cell Temperature rating.
Neither includes the typical loss in a charge controller, which is usually around 5%. When we use the 77% derating factor it is expecting the panels to be at NOCT and the controller to be consuming some power too.

If the panels are cold the Voltage is higher and the power is as well. If the panels are hot the Voltage is lower and the power is as well. You will never see exactly the same output potential every day of the year. It just isn't possible. Erring on the side of caution with a larger derating ensures a certain minimum.

Despite the output derating differences there may not be any difference in output from the controller: Watts in, Watts out.

In my location the actual derating factor for my system is 82 to 84 percent (best Spring and Fall). I have no problem with receiving 5% more power than expected.

If you try to design a system based on absolutes you will be disappointed with the result. Everything changes from day to day and over time, so what works perfectly to a tenth of a point one day may be off by several percent another. Without a margin for error you end up short of power one day.

One example of this:
220 Amp hour batteries. You calculate the charge based on the full capacity, but you calculate the loads based on less than full capacity (round down to 200, say). That way you know you have enough charging and you are already anticipating the inevitable decline in real capacity that batteries suffer over time no matter what.

8n-bob

Re: Making sure I understand things

OK COOL

So use the 77% and extra is a bonus, got you. I was hoping that I did not need to take the 89% and then 77% after that. I did not think so but HEY I'm new to this.

What I am shooting for is one 280W panel, 45amp MPPT controler, to charge 440 A/H of batt's and then additional kick of my old Harbor Freight panels talked about before lets say at 7Amps, and the wind turbine on top of that.

So final math

Solar stuff
280W*.77 factor = 215
215/12V = 17.9A
17.9A + 7A = 24.6A total charging not counting the wind power.

Batt stuff
440A batts *.05 min charge 22A

So I should be good for this.
Just parallel the 2 MPPT controllers outputs together and I should be good to go Actaully I'll probably cross connect the charge points to let the batteries buffer things a little bit.
when I get more cash I can add another 280watt panel and that will bring me up to about 42.5 amps, more then enough for the 33A needed for 660 A/H of batt's.

So do I understand it now a little bit at least. :roll:

Thanks for you great help.
And a positive side note is the controllers and cells can be bumped up to use with 24V Batts if I ever get to that stage, just need a new inverter.

Thanks
BobO.

Cariboocoot

Re: Making sure I understand things

Just so long as you understand that 5% is a minimum and a net; if all sources don't kick in together at the same time (wind is a variable) you won't have your minimum.

Personally I'd prefer >5% (even with no loads) from solar completely, and take advantage of the wind if possible (but not count on it).

If you have a 440 Amp hour battery bank then you probably have some significant loads. If they run at the same time as charging then the charging is compromised.

The main point of the minimum is to be able to charge the batteries fully in the amount of sun hours available on a good day.

8n-bob

Re: Making sure I understand things

I understand about 5% min.
Also that's why in the math the wind was not brought into play.

Most of my load is at night with just a little Wine Cooler fridge that I keep pop etc in, that draws about 1.5A when it's on.
At night add lights, computers, charging phones, tablets, and a TV.

I am replacing my HF 2000 watt inverter that draws 4 amps at idle for a Xantrex Prowatt SW2000 which is a full wave inverter which draws less then 800mW at idle. LOL.

Once I get my 2nd panel I'll have a little more breathing room.

One question I still have is if I put like 46 or 47 amps through the output of the 45 am controller will it just limit it to 45 amps or will it blow the controller? I am talking about a Sunsaver 45. Say my batt's are down and the panels put out advertisied wattage. I know I know it will not happen but just suppose.

Thanks
BobO.

Cariboocoot

Re: Making sure I understand things

MPPT controller will just clip the extra input power. Quite often these are "over paneled" to make up for some power loss on less-than-perfect days.

PWM controller will only pass what current the panels produce and may/may not clip or pass any current over their rating.

With either controller type you need to pay attention to maximum input Voltage as that is what is more likely to cause damage (components not designed to handle higher Voltages will 'pop' instantly on Voc, current or no).

8n-bob

Re: Making sure I understand things

I must be understanding more and more since I think I get it :cool:

Thanks for the replies, It's all upstairs now.