Why are panels rated in 12 Volt units?

[Alaska Man]

I've seen modules rated at 12V, 24V etc. Yet they all put out somewhere around 36.7VOC. So there is really no such thing as a 12v OR 24v Module Correct?

Another head scratcher, as the Volts the MN classics are rated for go up the Amps go down. i.e. 150 rated for 98A the 200 rated for 79A,  250 rated for 63A. If I add more panels to my array, my VOC and Max power current will both go up not down.

[vtmaps]

I've seen modules rated at 12V, 24V etc. Yet they all put out somewhere around 36.7VOC. So there is really no such thing as a 12v OR 24v Module Correct? 

Voc occurs when there is no load on a panel.  Vmp is the panels voltage when it is producing power. 

The so called "12 volt" and "24 volt" panels have a Vmp of about 17 (or 34) volts.   Corresponding Voc is a bit over 20 (or 40) volts.  If you have such a panel, you can use a cheap PWM controller to charge your battery.

The panels you are looking at are 60-cell 'grid-tie' panels.  Their Vmp is about 30 volts and they will NOT reliable charge a 24 volt battery.  These 'grid-tie' panels are much cheaper than the so called "12 volt" and "24 volt" panels.  BUT in order to use them effectively you need to use an expensive MPPT controller.

If I add more panels to my array, my VOC and Max power current will both go up not down.

That depends entirely on how you have your panels configured (how many panels per string, and how many strings).

Have you tried out Midnite's string sizing tool?

--vtMaps

[Alaska Man]

I did I bought the 150 MPPT, but I maxed it out at 110V input from my panels. Since we regulary get -40F up here in the winter and it can get down -60°F I hesitate to add another panel to the three I have in series now.

HyperVOC and all that.

I'm adding a second string (4) 250W Canadian Black on Black modules for 73amps of Power (1040w). I plan on using the Classic 200 this time.

Wish I had went with the 200 on the first string................ Live and learn.

I still don't understand why the Classic Line Rating for AMPs goes down at its Rating for Volts goes up.

[vtmaps]

I still don't understand why the Classic Line Rating for AMPs goes down at its Rating for Volts goes up.

Simple... All the classics become less efficient as the ratio of voltage in to voltage out becomes greater.  In other words, the more down conversion of voltage that the classic has to perform, the less efficient it is. 

Less efficiency means more heat production in the Classic.  More heat production means lower ratings.

btw, the rating of the classic 150 is not 96 amps at all input and output voltages.

--vtMaps

[dgd]

I still don't understand why the Classic Line Rating for AMPs goes down at its Rating for Volts goes up.

Simple... All the classics become less efficient as the ratio of voltage in to voltage out becomes greater.  In other words, the more down conversion of voltage that the classic has to perform, the less efficient it is. 

Less efficiency means more heat production in the Classic.  More heat production means lower ratings.

btw, the rating of the classic 150 is not 96 amps at all input and output voltages.

--vtMaps

I'm not actually sure that this is really a significant issue. What are we talking about? perhaps a variation in efficiency of under a half percent with input voltage to battery voltage rations of 1.5:1 up to 4:1

When I increased the pv voltage from my arrays from about 54v (3 by 18v mpv panels) to 90v (5 by 18v mpv) where the pv array was about 35metres from the controller, I never noticed any Classic temp increase or loss of efficiency.
I did notice a slight improvement when I went from tin 6mm cable to twin 4G cable when it was at the 54v. (24v battery system)

I also see absolutely no loss of efficieny in rewiring my array of 20 pvs into two series strings of 10 pvs giving 180v mpv into my Classic 250 and charging the same 24v battery bank. Problem with 250 was it current limited at 63A with this jnput.

Logic would indicate that what you say is true, but I don't think MN has ever produced an efficiency table showing different input voltages vs %efficient for any of the Classic controllers. The nearest I remember is the table that was in the docs for the MX60 and even then the range of efficiency variation was really really low.

It seems to me that the buck conversion in the Classic is pretty darn efficient. No complaints here 

Anyway just IMHO

dgd

[vtmaps]

I'm not actually sure that this is really a significant issue. What are we talking about? perhaps a variation in efficiency of under a half percent with input voltage to battery voltage rations of 1.5:1 up to 4:1

The OP asked about the Classic's ratings.  The ratings ARE the limits.  And when you're at the limit, there is no capacity to handle extra heat production.

When you are handling 1000s of watts, a half percent (or more) extra heat production does matter if you are already at the limit.   

--vtMaps

[dgd]

Vtmaps,

Yes you are correct. The current limits are just that.

However, I was just not sure with the concept of trying to make input voltage closer to output voltage, or at least at a level to let mppt work.  I was also not sure that the greater the input voltage then the hotter the Classic runs. Hasn't been my experience or its so minimal that its not that noticeable.

dgd

[boB]

The reason that each classic has the limit that it does has entirely to do with how hot the components get.

Usually either the FETs or the inductors.  In the last 10 to 15 years or so, inductors have pretty much
been the limiting factor.

Given a set output current and input voltage along with a higher input voltage specified controller,
(150 --> 200 --> 250) the efficiency must be lower but not a lot

Just a few extra watts of dissipation means you have to reduce the output current if
it is already at the temperature limit.

[Alaska Man]

So when in my head I'm thinking, "Okay I Wired 3 "12 volt" panels is series for an input voltage of 36V" is just not right.

What I actually did was wired 3 modules with a VMP @ 29.7V in series for what is really 89V coming into the MN Controller?

So then these are really "24V" panels is this correct?

[Vic]

Hi Alaska..,

NO,  29 - 31 Vmp PVs are NOT 24 Volt PVs.

Real 24 V PVs have a Vmp of about 35.4 Volts,  or a bit more.  This is because a 24 volt battery is seldom 24 volts.  A fully charged Flooded 24 V battery at 77 degrees F (25C),  which has had NO charge or discharge for 3 -8  hours (Rested)  is about 25.4 V.   On  charge,   this voltage must be higher.   Often the Absorption voltage is about 28.8 to about 30 volts,  depending on the exact battery construction.   A 30-ish Vmp PV cannot fully charge this battery,  due to the voltage drops in cable and the CC,   plus temperature effects on the Vmp of the PV,  etc.

The majority of the PVs being sold these days are for Grid-Tied systems,  and have 60-cells -- 30-ish Vmp.   Recently,  as the power output levels of large PVs have risen,   there are some PVs available with a 36 or so Vmp.    These could be referred to as a real 24 V PV.

When using MPPT CCs,  like the MN Classic and KID,  the distinction of weather a PV is 12 or 24 V,  etc has little meaning,  as these controllers allow converting string voltages higher than the nominal 12 V multiples efficiently.    It is common to see two or three 30-ish V PVs per string on 24 and 48 volt system respectively.

When using PWM CCs,  it is important to match the nominal PV Vmp to the system battery voltage for efficiency.   MPPT CCs allow a much wider difference between  string Vmp and battery voltage.

Vic

[Alaska Man]

Ah.............  Okay, I get it.  Thanks

[mahendra]

just out of curiosity and i should point out i have no intention of trying what i am about to ask.

Theoretically any Midnite classic 150...200...250....250ks can put out a larger amount of amps if inductors are kept cool.....within operating temperature range ?

[boB]

just out of curiosity and i should point out i have no intention of trying what i am about to ask.

Theoretically any Midnite classic 150...200...250....250ks can put out a larger amount of amps if inductors are kept cool.....within operating temperature range ?

Yep.  That's where heat sinks and fans come in.

[Alaska Man]

With a VOC of 36.7 per panel and 3 panels in series producing 110.1V into the MN Classic 150. How many more volts do you think the 150 could handle before I got in HYPERVOC trouble?

I'd really like to add one more PV to the string, even if it's a small one.

[vtmaps]

With a VOC of 36.7 per panel and 3 panels in series producing 110.1V into the MN Classic 150. How many more volts do you think the 150 could handle before I got in HYPERVOC trouble?

I'd really like to add one more PV to the string, even if it's a small one.

With the low temps you get in the winter, you're at the max.  btw, if you were to add a "small" panel to the string, you must be sure that its Isc is the same as the other panels in the string.

If you have a 24 volt system, why not buy a fourth panel and configure them as two parallel strings with two panels per string?

--vtMaps