Morningstar's TriStar MPPT TS-45/ best way to wire up panels

[sunpower]

hi

i have a Morningstar's TriStar MPPT TS-45 and 4 Suniva 270 Silver Mono OPT-270-60-4-100 panels
the panels have a VOC of 38.5 volts and i am going to use a 24 volt battery bank that will have 2 agm 12 volt 250 amp hour batts .so my question is it best if i take and hook make  2 sets with 2 panels each in series and then hook the 2 sets of 2 panels in parallel to my mppt   .. would i have better charge for the 24 volt batt bank ?
thanks

[XeonPony]

How far will the panels be from the controller?

My system the panels are 50 feet from the controller going to a 12v bank

the panels are 235W at 44V voc, they're wired parallel to the controller via 6 gauge, then 2 gauge to the combiner box that runs 1/0 gauge to the batteries.

The Vmp is 32V which gives me a happy window between the panel Vs Battery Voltage, so if possible hook up a panel as is to the battery and see what your Vmp will be.

Better the match the more efficient the controller will be, but we need to consider the wattage and length of wire run to figure out best array voltage as well.

4*270 = 1080W @ panel volts

1080/ 38.5V = 28.05A (That's a fair bit to push, So now how far from the panel to the controler and with what AWG size of wire do you plan to use?)

[sunpower]

How far will the panels be from the controller?

about 15 to 20 feet.
.i will need to double check that
thanks

[XeonPony]

Well using 25 feet as a safe number you'd be fine doing parallel all the panels and run it through 6 guage.

But for growing room I'd use marine rated 4 guage. but the most important number is the Vmp of the panels now.

[sunpower]

i have a vmpp number of 31.2 volts  ?

http://www.wholesalesolar.com/products.folder/module-folder/Suniva/OPT-270-60-4-100.html

thanks

[XeonPony]

just hook them up parallel. You'll be fine with such a short wire run, using 4 guage allows some growing room as well.

[sunpower]

ok thanks for the help on this

[Mary B]

Series! Might gain some low light performance that way. Right now I am seeing 40 volts off my panels. Mostly cloudy and sun starting to get low in the west. In full sun I see 86 volts. So with a 24 volt battery bank in low light conditions you gain available charge with a higher input voltage.

[dave ames]

Hello Terry,

Not sure of your location but if I were to put that system in at my location I would have to go with two parallel strings of two modules in series.

The reason being that the temperature can get over 70*f here - sometimes even 80, 90 or 100*f

Those modules, just like all panels tend to have a lower Vmp when they get warm....I looked your panels up and the Vmp at 70*f is 28.35V..@80*f = 27.63V..@90*f = 26.92.. And @100*f = 26.20V

-Nowhere near enough headroom to maintain a 24V bank of batteries.
Cheers, Dave

http://www.morningstarcorp.com/string-calculator#manufacturer=871&module=18632&product=23&vmin=25&vmax=30&tmin=0&tmax=90&tminunits=f&tmaxunits=f

[Flux]

I have found that to get best use of a mppt controller you need more than nominal panel volts, especially in hot weather.

I agree with Dave and would go for the series parallel option. Your cable run is short but losses will be eased at the higher voltage.

The full series option will bring you close to the controller input limit on clear cold sunny days so the series parallel option looks to be safe and convenient.

Flux

[XeonPony]

The morning star controllers like a narrow differential, so closer the match higher their efficiency. (Their data not mine)

As the batteries charge the panels will creep up to their voc rating, even in hot weather I find my panels still do fine as far as voltage is concerned. (Very little deviation from label at +40c n then some)

His wire loss will be less then 3% with 6 guage! Any future growth will be on the battery side as any more panels he'll be needing a second controller any ways.

I'd say 6 of one half dozen of the other, do what ever is easiest for you! series para, or just para, either will do fine IMO.

[Rob Beckers]

The Voltage from a 60-cell like yours is not enough to reliably charge a 24V battery bank. When the panels get warm (and in the sun they're generally 25 - 30C above ambient much of the time) their Voltage drops, in fact Vmp drops by 0.5% per degree Centigrade! Then there are partial-light conditions such as clouds etc. There's a reason why the PV modules for direct battery charging with an older-style PWM controller had/have 36 cells in them for a 12V battery; that's really the minimum to make it work reliably (in particular if you ever need to equalize the batteries in case these are regular flooded lead-acid). For a 24V battery system that would require 72-cell modules to get enough Voltage.

The TriStar MPPT TS-45 can handle up to 150V, which in our climate would mean a maximum of 3 panels in series (a 60-cell panel can go up to 44V Voc in winter here). Having 4 panels in your case the normal way to hook them up is two strings of two panels.

Since efficiency was mentioned: While it's true that any buck-controller such as the TriStar MPPT is more efficient when battery and PV Voltages match more closely to each other, the difference is really very small. In this case, it's less than a percent for a single panel vs. two panels in series, and that's taken directly from the TriStar manual. Even with 3 panels in series and 24V batteries there's only a 2% difference in conversion loss vs. a single panel.

-RoB-

[Mary B]

2% conversion loss offset by how much gain in low light conditions?