# charge controller sizing



## scatyb (Jan 20, 2009)

Got another question for yall's. I have a bunch of panels that total 1200w. I will be hooking them up in a 24v system(2 panels in series then those sets in parallel).

Question is, what sized controller should I get for this array?
specs for each panel are as follows:
120w each
* VOC 29.60V
* VMP 17.60V
* ISC 7.48amp
* IMP 6.82amp 

I feel I might be over thinking things.

Thanks again folks.


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## wy_white_wolf (Oct 14, 2004)

For 24v operation you would have 5 strings of 2 panels each. 

That would be a VOC of 59.2 and a ISC of 7.48 per string. 5 strings would put the total array ISC at 37.4. You would want a 24v charge controller that can handle at least 40 amps. A 60 amp charge controller would not be considered too large for that array.

BTW - are you sure about the VOC of 29.60? Just seems to far out there as 12 volt panels usually have a VOC of 21-22 with a VMP of 17-18.


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## Jim-mi (May 15, 2002)

This would be a good place to use a Outback MX 60 or 80 and go for higher voltage strings into the MMPT.

pricy but worth it.................


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## scatyb (Jan 20, 2009)

answered my own question. Thanks for the suggestion Jim. Do you have one?

Oh, and I'm not sure about that 29.6. My panels are one of 4 different 120w models and I THINK that they are the 29v ones. I will go out and look tomorrow, see if I can't find the specs printed on them.


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## WisJim (Jan 14, 2004)

Another Jim answering about MPPT charge controllers. I have been using an Outback MX60 for 4 or 5 years now, and am very impressed with the extra power that I get from my panels with it. The panels are about 250 feet from the controller and batteries, and using that type of controller enable me to use #2 cable instead of #00 cable from the panels to the controller because I can connect them for a higher voltage and let the MX60 convert it to the proper voltage to charge my 24 volt batteries. I originally set it up with 72 volt strings, but the output from the panels exceeded 130 volts in full sun in the fall (increased voltage due to cooler temps) and I could have damaged the MX60 on a sunny cold day in Dec or Jan. Reconnecting at 60 volts (5 panels in series) would have been a good solution, but since I have 12 matching panels, I reconnected for 48 volts and still get acceptable voltage drop through the cables to the controller. I figure that I saved the cost of the MX60 (about $600, I think) in the savings of using smaller copper cables for the 250 foot run, and that the additional power that I get into my batteries because of the MPPT feature is like having the output of another PV panel (which cost about $600 at the time I bought them), so the MX60 really paid for itself twice, in my opinion. And the MX60 (and other similar high end controllers) also has a lot of nice metering features in it, that sometimes cost hundreds of dollars extra with other controllers. It tells voltage, amps, kw-hr out put, etc. And I have seen my 1500 watts of PVs produce over 1600 watts for an hour or more on a clear sunny day, with peak momentary output of 2000 watts.

One important thing to consider when connecting higher voltage strings of PVs for a system using one of the MPPT charge controllers is the maximum full sun open circuit output of the string in your coldest winter temps. They often can put out TWICE their label voltage on a bitterly cold but sunny winter day--I have seen over 100 volts momentarily from my 48 volt string under those conditions! You don't want to take a chance on exceeding the controllers maximum input voltage specs.


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## wy_white_wolf (Oct 14, 2004)

WisJim said:


> .... I originally set it up with 72 volt strings, but the output from the panels exceeded 130 volts in full sun in the fall (increased voltage due to cooler temps) and I could have damaged the MX60 on a sunny cold day in Dec or Jan. Reconnecting at 60 volts (5 panels in series) would have been a good solution, but since I have 12 matching panels, I reconnected for 48 volts and still get acceptable voltage drop through the cables to the controller....


I've seen a few setups around here that people didn't realize this problem. By code here we can only string panels togethor with a rated VOC of under 100V to keep below the rated 150V input of the MX60 when you calculate in for cold temps. That means a max of 4 - 12v panels per string. The thin film Kaneka's cannot be configured in series at all. VOC of 91.8, but that is just as good (if not better) as we can get out of 4 - 12v panels. 

That's why I was asking about the 29.6V rating. Here that would limit him to 3 panels in series using the MX60. Hard to use all his panels without buying 2 more.


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## Jim-mi (May 15, 2002)

No I do not have a MX60 . . .Tho I have installed several.
My panels were/are up and racked before the MX came out. . . . .So yes I have a lot of copper in the ground.
And because I have several different kinds of PV panels I can not stack them properly for the MX . . . darn. . . .

So . . . .any one starting fresh would be well advised to go the MPPT route.


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## scatyb (Jan 20, 2009)

Thanks folks. I really appreciate the help. I have learned a few things from this thread alone. I haven't had a chance to head out to the shed and look at the panels, but will do so at lunch.


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## TnAndy (Sep 15, 2005)

Another vote here for the Outbacks.....I have an FX and an MX-60 ( different arrays added to the system at different times ) and thinking about adding an MX-80 on the next 'go round'.

I'm running strings of 2--175w panels in on them, typically seeing 75ish volts input on a normal sunny day.

In addition the the number you can string together, also note the MX-60 has a 1600watt nominal limit on your panels @24v output.


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## scatyb (Jan 20, 2009)

I'm seriously looking at the fm60. I now understand why companies claim mppt's give you an extra 30%. really a dishonest claim (IMO), but oh well.

the specs for the panels are as follows(if anyone cares):
Vmp - 17.6v
Imp - 6.82a
Voc - 21.6v
Isc - 7.4a


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## scatyb (Jan 20, 2009)

Ok, so assuming that I got the fm-80, could I connect my current panels and add a few of the kaneka's?


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## trkarl (Dec 15, 2009)

You "can" do it but that doesn't mean it will work efficiently.
The panels in each string need to be as close to the same Imp or amperage as possible.
Then each paralleled string needs to have the Vmp and Imp as close as possible. Any difference and you start to get inefficiency.

For example one of my arrays consists of 4 BP590 and 2 BP 585 panels for a total of 530 watts wired as 1 string.
BP 590 : Vmp 18.5 Imp 4.86 Voc 22.30
BP 585 : Vmp 18.0 Imp 4.72 Voc 22.1

Total: Vmp 110 Voc 133.4 Imp 4.72 ( I believe the lower amp rating drags the whole string down to it's level and this is where some of the inefficiency comes in ). Or approximately 530 watts.

This is connected to my MX60. Even in 28 degree weather which we had a few mornings like that this year the highest Voc I got was 135.

I added 6 more panels wired two strings of three of BPSX175NQ in parallel with the first string connected to the MX60

BP SX 175 NQ: Vmp 36.1 Imp 4.85 Voc 43.6
Total each string: Vmp 108.3 Voc 130.8 Vmp 4.85
Or 525 watts
As you can see the voltage and amperage of each string is very close to the same. I could have paralleled different "530 watt" strings in with the first configuration but with way different voltage and amperage ratings but this would throw the whole array out of sync and confuse the MX60 as it is looking for the best voltage to run the array at to get the maximum amperage out of.

The total "array" size is 1580 watts but is a little less efficient than if all the panels were exact. I have regularly seen it between 1300 and 1500 watts almost daily. With the cold weather and since it is angled for spring and fall I have seen it over 1600 watts several times.

Here is a pic of the arrays. The two to the right are the ones I just described above.


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