Midnite Classic - Maximum Voltage?

mjp24coho

I'm a bit confused regarding the midnite classic (250) and the maximum voltage it can accept. I'm planning for a system that will require a 300' run from the array to the charge controller/batteries. I'm planning to do 18 x 230W Kyocera panels (29.79 Vmp and 36.9 Voc). I'd like to run it in one of two ways: eiether 3 strings of 6 each to the combiner, and then one combined run from the combiner to the charge controller, or 2 strings of 8 each to the combiner, adn then one combined run from the combiner to the charge controller. I'm obviously trying to get the voltage as high as I can, while still making sure I'm not overloading the wiring or the charge controller. My plan is to use the Midnite 250 to charge a 24V battery bank (this is an off-grid setup). But I'm not sure how much the Midnite 250 can take, while factoring in temperatue swings (cold climate in the winter) and power spikes. I'm planning on using THHN wiring inside of buried conduit.

Another question - ofr using the Voltage Drop Calculator, do I use the Vmp of the panels (29.79), Voc (36.9), or just 24V? Multiplied by the number of panels, it makes a difference in voltage drop.

Also, my original plan was to use all Outback components, for the integrated communications/data logging capabilities. However, I need the higher voltage capacity of the Midnote Classic. Anyone have experience with using all outback components except for the Midnite Classic controller, and then still utilizing integrated communication functionality?

Thanks!

BB.

Re: Midnite Classic - Maximum Voltage?

Hopefully you will get your answers--But if not, Midnite has their own (active) forum too:

http://midnitesolar.com/smf_forum/index.php

Halfcrazy (Ryan), boB, and Robin from Midnite do participate here too.

-Bill

boB

Re: Midnite Classic - Maximum Voltage?

mjp24coho wrote: »

Another question - ofr using the Voltage Drop Calculator, do I use the Vmp of the panels (29.79), Voc (36.9), or just 24V? Multiplied by the number of panels, it makes a difference in voltage drop.

Thanks!

What matters for this is the highest voltage that the controller will see, which is Voc or Open Circuit voltage. Colder weather makes that Voc go higher.

If you take the 36.9 Voc number (I'd just round it up to 37V) and then multiply that by a number to make it higher for cold days, and then multiply by
as many PV modules in series as you can without going above 250V, you should be good with that number in series.

However that number of modules in the string to be less than 250V may not let you divide up the total number of modules you have to build the array from.

But, first thing to do is to try it out. Let's say it doesn't get too cold where you are, say, it gets down to 0 degrees C. You could multiply that
Voc by 1.1 to get a cold Voc of 37 X 1.1 = 40.7 V. Again, I will round up to 41V here which will take into account it being even a bit colder than 0 C.

Now, divide 250V by 41V which gives you a maximum of 6.1 modules. Great, so you could go with 6 modules in series with that number.
Since you have 18 modules, you can have 3 strings of 6 modules and you are done. This would be perfect for your situation because it
divides up perfectly into 3 strings IF your weather doesn't usually get too cold.

With the Classic's HyperVOC limit of 274 Voc when using a 24V battery bank, that means that IF the weather got REAL cold due to extreme
weather conditions, (global warming ?), you'd most likely be good for that too. The Classic just won't turn on until the PV array heats up
until the Voc drops back down to 250 Volts again, which it can do after the sun shines on it for a while in cold ambient temperatures.

Now let's see if the Classic is good for 18 -- 230W modules. 18 X 230W = 4,140 Watts. If you got 100% rating out of those modules,
that would be a battery (output) current of 4,140W divided by 24 volts = 172 Amps !! That's WAY too high for one Classic controller
and the Classic 250 is good for about 60 Amps at that input voltage with 24V battery bank which just over 1600 watts. If you had a
48V system instead of 24V, you could get around 53 Amps or over 3000 watts from one controller. Close but not quite enough for
your 18 modules. With each charge controller, at the moment, you will need separate 300 foot wire pairs for each CC.

So for 24V battery and 4140 Watts of PV and 1600 (or so) watts per controller, you would need 3 Classic 250s for maximum benefit.
That would be one string per charge controller and a more comfortable 1380 watts per controller, ~IF~ the array put out 100 % nameplate rating.
Usually, you will get more like 75% to 85% of the nameplate rating our of the modules and system so you might be able to play some math games with that.

You ~could~ use larger 300 foot wire and just go for lower voltage and 200 V controller and re-arrange those modules and possibly get away with 2 controllers.

At least 3 X 300 foot wire pair would not have to be as large as one bigger wire pair for the whole 4140 Watts but the wire size is yet another discussion.

boB

niel

Re: Midnite Classic - Maximum Voltage?

mjp24coho,
aside from sizing the right number of controllers/pvs, i see your next biggest problem here as that long wire run. is there any way to shorten that run? i am assuming you are using my v drop calculator and although i prefer nominal panel ratings to view things at their worst case, it is too difficult to determine the nominal voltage with many odd ball pv voltage configurations so go with the total vmp of the strings. do remember after down-converting that there is now a new voltage to calculate voltage drops with and it will be a higher current being outputted too and could present a high v drop %. this output to the batteries is critical on its v drop and it should never be above 1%, but keeping it under .5% is even more preferred. this is due to the fact that a v drop here can fool the controller into thinking that the batteries are fully charged when they are not.

boB's suggestion of going multi-classics and running them with their own individual wire run from their own pv string of 6 is a possibility, but it still presents a large cost for wire as you now using 3 times the run length in wire.

btw boB, arkansas can go below freezing quite a ways so that could be pushing things in the 6 pv string arrangement with those pvs. hyper voc would handle it, but it would still be non-productive during the time that the voltage might exceed the classic operating limits.

boB

Re: Midnite Classic - Maximum Voltage?

Niel, I just wish I had parallel mode finished ! That would certainly reduce the number of wires in the 300 foot run, but those wires
would have to be just about as big as the separate runs for separate strings. Where parallel operation will really shine is when an
old system with say, 4/0 wire was run because MPPT wasn't available at the time AND copper wire was relatively "cheap".
There seem to be quite a few of those old installations out there.

boB

offgrid me

Re: Midnite Classic - Maximum Voltage?

mjp
Why 24 volts with so much panel. If you were to switch to 48 volt you could run one xantrex 80 600 with two strings of 9 panels. Even in cold temps you would not be close to over volting the 80 600. At 300' you want as high a voltage as you can get to reduce drop. You could probably use one run of #8. I run 4200w of panel 100' and have no problems. I have even seen production over the manufacture stc rating in 60 degree weather

boB

Re: Midnite Classic - Maximum Voltage?

offgrid me wrote: »

mjp
Why 24 volts with so much panel. If you were to switch to 48 volt you could run one xantrex 80 600 with two strings of 9 panels. Even in cold temps you would not be close to over volting the 80 600. At 300' you want as high a voltage as you can get to reduce drop. You could probably use one run of #8. I run 4200w of panel 100' and have no problems. I have even seen production over the manufacture stc rating in 60 degree weather

Yes, that controller would be just about right. At this length and more is where a 600V unit can really help. 80 amps is barely on the edge for 48.0 Volts but when the batteries are in the 50+ range, not on the edge at all. Yeah, going to 48V is a pretty good idea for that much available solar power.
boB

mjp24coho

Re: Midnite Classic - Maximum Voltage?

Thanks all forthe input - very good advice. It gives me a lot to think about.

I'll look into the larger controller.

Unfortunately, I'm limited on where I can put the panels. My cabin is at the base of a hill, and the only good place to putthe panels with no obstructions is on top of the hill.

vtmaps

Re: Midnite Classic - Maximum Voltage?

you don't say where you are located. If you are not in an extremely cold place, you could put your batteries & electronics near the panels and send 230 volts down to your cabin. --vtMaps

mjp24coho

Re: Midnite Classic - Maximum Voltage?

The cabin is in Southern Utah at 6000'. It gets a decent amount of snow - rarely under 0 degrees, but it may happen once in a while.

plongson

Re: Midnite Classic - Maximum Voltage?

Like somewhere around 37 N 113 W LOL...