Is oversized PV array OK for the charge controller?

[alexf1852]

I'm currently planning a 4.2KW PV array (3 modules/string, 5 strings total, 15 panels total feeding into a 150V, 80A MPPT CC for 48V battery bank). But where I will be using this system, it rains quite a bit during the rainy season (5 to 6 months out of the year). I'm wondering if I would be able to compensate the lower output of the array during cloudy/rainy days with a couple of more PV strings which I already have. I realize that the 2 extra strings would go to waste during clear skies. So basically, instead of using a 4.2KW array, I would use a 5.8KW array (2 additional strings of 3 modules in series) to feed the charge controller.

Would the extra power in the PV array damage the charge controller during sunny days? The reason I ask is because when I use the Midnite Solar sizing tool and put in 7 strings of 3 modules in series instead of 5 strings, it tells me that my array wattage is excessive but all else is OK.

[Vic]

Hi alex,   Welcome to the Forum.

Some Charge Controllers behave better than others with excess PV power is available.   If you use the CC manufacturer's PV String Calculator,  you can find what the manufacture expects is the maximum.

Some CCs have reliable current-limiting capability as a setting -- where the Maximum current can be set.   BUT,  sometimes this current-limiting for some CCs is NOT fast enough to always protect the CC from damage.

What CC are you using?
What is the STC String Vmp of each string?
What is the part number of the PVs that you are using?
What is the coldest temperatures that you would expect,  for the location of  this system?

There is no real guaranteed answered for over-PVing a system. My opinions.  Thanks.   Vic

[alexf1852]

Thanks for the quick response.

Planning to use Classic 150.
Not sure what you mean by "STC String Vmp". Vmp of each string will be about 108V
Panels are Chaori CRM280S (Pmax = 280W, Voc = 44.9V, Isc = 8.28A, Vmp = 36.0V, Imp = 7.77A, TC of VOC = -0.33209%/K, TC of Pmax = -0.47669%/K)
At night, it can drop to 45-50F. But temps during day are 70 to 95 average I think.

[Vic]

Hi Alex,

Thanks for the added information.

One thing about your solar array,  is that the input voltage to the Charge Controller is fairly high.   This high string voltage will reduce the CC efficiency.   This causes the CC (Classic in this case) to run warmer than it would with somewhat lower string voltage.

When the CC gets to the later part of the Absorption stage,  and,  especially in Float  the input voltage rises.   This voltage rise causes the CC to run HOT,  even when the CC is delivering a small amount of power.   Heat is not good for electronics.

The reduced efficiency mentioned above,   would hold true for almost any MPPT CC.

The Classic String Sizer takes into account the reduced efficiency with high string voltages,  and reduces the Maximum PV power recommended.

There can be some allowance for several arrays,  with somewhat different azimuth orientation,   can permit some added PV,  because not all of the PVs produce peak power at the same time.

Some of the MidNite Tech folks may be able to give more specific guidance.   But the bottom line,  to me,  is to not expect power electronics to run at or near their maximum ratings for an extended period of time,   even the MidNite Classic.

FWIW.  My opinions.     Vic

[vtmaps]

Welcome to the forum,

What you are proposing is getting more and more common because panels are historically cheap and batteries are historically expensive.  I will refer to it as "over-paneling".

The Classic can be set to limit its output current to a level that is safe for your battery bank capacity.

For example, if your battery bank capacity was 400 ah, you might want to limit the current to the batteries to 40 amps.  If your solar array were 3000 watts, it would be capable of pushing 62.5 amps (at 48 volts) - 50 amps (at 60 volts) into the batteries.  Of course, you would rarely get full nameplate power out of your panels because as they heat up their output drops.  You would set the current limit on the Classic to 40 amps, even though the current would rarely be that high. 

The Classic 150 can output 76 amps with a string voltage of 110 volts, so a 3000 watt array would be well within its capability.  If you "over-panel" the 400 ah battery with a 4000+ watt array, The system will work, but you will be pushing the limits of heat production.  by the way, you did not mention your battery capacity... in my example above I used 400 ah.  Over paneling can be relative to the battery bank capacity, or relative to the controller's capacity (or both)

Vic's replies are right on the mark... do not design a system where you may be maxing out your controller for hours on end.

A better design for your system would be to split your array and have 2 controllers.  You may also wish to do "virtual tracking", i.e. have a south-east array on one controller and a south-west array on the other.

There is another way to accomplish "over-paneling" with just one controller...  some folks manually turn off a string or two at their combiner box on sunny days.   That can be done automatically with a relay, (controlled by the Classic).

--vtMaps

[Vic]

alex,

So,  you are "planning"  your array at this point.   If you do not already have the PV modules that you have specified,  it would probably be a bit better if you were to use 60-cell PVs,  as the Vmp of these will be a bit lower -- often around 31 V for 275-ish watt PVs.  This will lower your string voltage a bit -- three modules would be about 93 V.   This is a good operating point  for Classics and many other MPPT CCs.

As vtMaps noted,   if you over PVed for the rainy season,   Combiner breakers could be switched off for the sunniest part of the year.  Have read here,  from MidNite Engineer,  boB,  that excess PV modules running at high-ish voltages into the Classic could run the risk of damage,  as CC current-limiting at high input voltages must be very fast to protect the CC - IIRC.

One system in use here runs at 106 V input to a Classic,  the Classic can run warm,  and even a bit hot,  especially when in Float with light loads on the CC.

The Classic string sizer gives good guidance on PC power vs input voltage.   There are variables,  such as shading and differing azimuths or elevation angles of the PVs that must be considered individually by the user,   but the sizer's recommendations are generally safe to use.

Charging batteries can often be a long slog,  takings a number of hours with larger banks,  so giving CC some operating margin (not running fully loaded),  is  a good thing to do.

It is good that you are asking these questions now.    Good Luck,   Vic

[alexf1852]

Thank all! Some good info here. I already have the panels (21 of them). I haven't bought anything else as I'm still doing research on brands/models and best setup for my scenario. I'm planning on at least a 400 to 450 ah bank.

Having some panels in a different orientation sounds like a good idea.

So it sounds like the Classic controllers have passive cooling?

[Vic]

Hi alex,

Great that you have your PVs. 

Classics have a lot of on-board communication capability,  and have the ability to use the WhizBang jr to measure the actual charge current into the battery.   The WBjr is included with standard Classics (not the Classic Lite).  AND,  Classics can coordinate charge-states with other Classics by using two telephone cables.   I know of no other CC that has this capability,  built-in,   if you need two CCs.

Classics have temperature-controlled fans that enhance cooling.   For larger systems (like you are thinking about)  the Classic is a good fit,  as it can deliver a lot of output current for battery charging and your loads.

I am biased toward Classics,  because it is quite advanced in capability,  and the MidNite crew is constantly adding great functions and capability via FirmWare updates.

MidNite has the best support,  and service ...  you probably know much of this.
Am just a very happy customer of MidNite products.   My opinions,   FWIW,    Vic

[dgd]

WTF?
keeps timing out on posts    will this even post
dgd

[Westbranch]

system has been doing this a lot lately

[Vic]

WTF?
keeps timing out on posts    will this even post
dgd

David,  it will only if you are fast ...   Vic

[dgd]

WTF?
keeps timing out on posts    will this even post
dgd

David,  it will only if you are fast ...   Vic

Vic and Wb
Thanks. So its not just me    still its bloody annoying after crafting a decent reply to see it vanish.
Maybe Ryan can get this fixed, no doubt there is an error log somewhere noting these timeouts.

dgd

[Vic]

Hi David,  this site is at times quite slow to access the index page and other pages as well ...
Ryan may well be on vacation ...  it is Summer here,  and 7/4 is a big Holiday,   often folks will tandem vacation time onto this holiday.   Guessing.

Have started copying any post,  posting it,  and usually it has timed-out.   Sometimes I forget to copy ......  GGGGGGGGRRRRRRRRRRRRRRrrrrrrrr.  Kind of a pain,  but someone will figger it out.    Good Luck,    Vic

[dgd]

Alex,

As others have said you can switch out part of the PV array when the Classic no longer needs the power.
but oversizing the PV array is quite common so that in adverse weather the input power available is increased.
You could also divert power from the array, or part of it, to another load. For example to a water heating element using an SSR controlled by the Classic.
With your PV configuration of 3 PVS making 108v mpv then this would be a good match for a 120v element. Two strings diverted to a 2Kw element should pre-heat or completely heat a tank of water.
The Classic's AUX1 control FLOAT HIGH is ideal for automatically managing an SSR or two.

dgd

[Vic]

Oh,  there is a Show July 08 - 10 .. perhaps some of the important MN folks are there ...   
July 08 - 10  Inter Solar North America    San Francisco

Ryan may be there ...    guessing.     Vic