310w panel Vmp issue

Here is a link to the specifications:

http://www.solardesigntool.com/components/module-panel-solar/Jinko/2952/JKM310P-72/specification-data-sheet.html

You have run into the fact that solar panels are "very poor batteries" (technically, they are current sources--Save for another discussion).

Basically, solar panels have two voltage related specifications. One is Vmp (voltage maximum power) and Voc (voltage open circuit)... And these voltages are quite temperature dependent. Cold temperatures cause the voltages to rise from STC (standard test conditions, or the "marketing" numbers). And hot temperatures (when the sun is beating down on panels on a hot day, now wind) the two voltages fall. (note, Voc is no current being drawn, Vmp is "optimum voltage and current "Imp" being drawn).

Two points we usually worry about when designing a system (especially when using MPPT charge controllers). One is Voc-cold (voltage open circuit cold). Depending on how cold it gets in your area (usually first thing in the morning before charge controller has turned on). That is what is causing your "issue".

Your panel Voc is 45.9 Volts at ~25C / 77F.

• 4 * 45.9 volts Voc = 183.9 VDC with zero current draw
• 4 * 37.0 volts Vmp = 148 VDC with "optimum" full sun at Pmp=Vmp*Imp

Guessing you live somewhere around Esteli Nicaraga--You live in a warm (on average) country, and will never (or almost never) see freezing weather/frost on panels.

So--The fix for your panels is to put 3x panels in series (and this does mean that you have 6 or 9 panels, not 8).

Depending on your battery bank voltage, (12 or 24 volt), you can also have 2x panels in series (x 4 parallel strings). Two in parallel may work OK on a 48 volt battery bank--But it is close.

Which leads us to the other issue... Vmp-hot... As the array gets hot, Vmp (and Voc) fall. you can lose 20% of Vmp on a very hot day (if you are in a marine layer area, you may not get very hot weather either):

• 2 * 37 Volts Vmp * 0.80 (generic) temperature derating for very hot panels = 59.2 volts (high noon, no wind, hot weather)
  

You need around 58-59 VDC to fully/quickly recharge a 48 volt Lead acid battery bank, and need ~60-62 volts to equalize your battery bank (but at a lower current, so it is possible your array as 2s*4p would work fine).

In either case, with a lower array voltage, you may need more (heavier) copper cabling... And with 3 or more parallel strings, you should have a series fuse/breaker per string to reduce the chances of fire if there is a short circuit/broken solar panel.

There is a "string calculator" for Midnite solar charge controllers because it is a (relatively) complicated set of trade-offs:

Classic String Sizing Tool

Of course, there is the option of using a higher Array Voltage rated charge controller too.

Is this making sense?

-Bill

I am on my phone right now. Will have to get back on a real computer to check.

What is your battery back amphour rating (20 Hour rate)? Agm or flooded cell?

Would an extra panel be useful for you (9 total)?

-Bill

2s x 4p can work fine, and before mppt controllers existed, Vmp=72 volts were the standard. So it can work, and you are not in a hot desert.

But with mppt controller and 15 meter to array, a Vmp=111 volts at stc would be a better fit. When I get back, I will look at the voltage drop for 15 meters and the size of battery bank.

What is your typical daily load in watt hours (or amp hours)?

-Bill

Thank you for the complements.

Note, this is probably the last time I will feel the need to go into such detail because it probably causes more confusion and consternation for users than it really needs too... Because I have never done the details like this for Midnite, I believe it is worthwhile to understand where rules of thumbs like 130% come from--But to quickly do a paper design, it becomes one of our "just do it, because it works and there is a whole bunch of details why". And an "exact calculation" is not going to give a much better answer than a (relatively) conservative rule of thumb that will give you a well designed/working solar power system that will work anywhere in the world (Don't get me wrong, I am not upset or anything--But short of a college class on theoretical solar power system design),

But, knowing the reasons for these rules of thumbs help us for specific installations and what can be done (or not) for specific users to give them a working system that may "violate" a world wide design set of guide-lines.

So why 130%? Basically, if you assume for full power solar array power output at 30C ambient temperature, then:

• 1/0.84 panel temp Vmp correction (see my formula previous post) * 1/0.97  cable drop (3% wire drop) * 1/0.89 for Controller drop (18v for Vmp-array * 16v batt charge) = 1.39 = 139% (for "12 volt system with minimum controller drop"
• 1/0.84 panel temp Vmp correction (see my formula previous post) * 1/0.97  cable drop (3% wire drop) * 1/0.92 for Controller drop (65v Vmp array / 60v Batt charge) = 1.33 = 133% for 48 volt battery with "higher minimum controller drop voltage"
  

Also remember that the Pmp "curve", while it "peaks" at the Vmp panel voltage, it is flat enough that (roughly) if you are + 5% away from the peak, you lose about 5% power. If you are + 10% away, you lose about 10% power.

And then some hand waving, while I am not one to "toss" 5% to 10% of a system's output power, it really does not matter "that much" because of some real life issues... If you are deeply discharging your battery bank, then Vbulk is much less than the 60 volts (15 volts @ 12 volt bank) until the battery is ~80% state of charge or less. Once the battery reaches 80%, the charging current (acceptance by the battery) slowly causes charging current to drop (not using array full current/power). And when you get >60 volt (15v @ 12v bank), you are approaching (for most batteries) the equalization voltage--Where the charging current should be around 2.5% to 5% rate of charge vs the 5% to 13% rate of charge needed for a discharged Lead Acid battery bank and good system design (10%+ rate of charge needed for a full off grid system). And a hot battery bank (from all that charging) is going to have a reduced charging set point anyway (another 1+ volt drop for a "warm" 48 volt bank).

So, I can understand where boB (as he occasionally posts here) gets his 130% number (for a nominal 48 volt design) or a 140% (for a minimum 12 volt design).

This is the "problem" with engineering solutions... Rules of thumbs do not always "map well" to a single "factor" (2-5-7 volt controller drop vs 12 to 48 volt battery bank) is a fixed drop, not a percentage based relative drop). Also, when you start putting "all factors" for your derating in a row--You can end up with an "over designed system" (i.e., not all worst case examples happen at the same time for most people--I.e., a -40C battery bank and a +40C solar array).

And to Dapago's questions (if I can find them all)...

The typical MPPT controller will "shut down" if the input voltage is exceeded. For a typical MPPT controller at 150 VDC, if exceeded, will put a "flag" in the memory to say that warranty terms have been exceeded. The Midnite's "warranty" flag is 150+Vbatt. The Midnite will still shut down as it is > 150 VDC, but it will not be damaged by <150+Vbatt (maximum "safe" input array voltage--Remember, I do not work for any solar company or seller--Just my guess of what is going on with the hardware and software).

As to what is happening, you need to understand solar cells too... Voc means that a solar cell (with light, including first direct light from morning sun, just casting a weak shadow) is enough to raise the cell voltage to Voc (voltage open circuit/no current flow). However, in real life, we have to have some current flow, so Voc (which is temperature sensitive/cold panels gives higher Voc) will cause Vpanel to fall from Voc.

And solar panels are "current sources" (more or less). The output current is proportional to the amount of sunlight hitting the panel. You can take a "shorted solar cell or panel", connect it to a current meter (current meter looks like "short") and measure the output current of the cell/panel and it will represent the amount of sunlight hitting the panel to within 10%. So for an MPPT controller, it has to take current from the panel, and the MPPT (Maximum Power Point Tracking) needs to take Vpanel*Ipanel (by drawing variable current and "doing the math") to figure out what Pmp, Vmp, and Imp are for that instant in time (Vmp is temperature dependent, Imp is less temperature dependent and we usually "ignore" Imp-temp as it is << vs Vmp-temp sensitivity). So, for an MPPT controller, it will rarely be drawing zero input current, and it will rarely "see" Voc--However, we design for Voc to prevent controller damage or shutdown when you have a "cold snap" or other less than often event (shutdown controller for service then turn back on or whatever else may happen--I do not design MPPT controllers--just guessing).

Regarding the term "over paneling"--I use it here to indicate when you are charging the battery bank with >~13% rate of charge (100 AH battery at "20 Hour Rate" charging with >13 amps of current). Lead acid batteries are remarkable things. They have been around for >>100 years relatively unchanged and relatively cheap--However they have some limitations. A Lead Acid battery that is less than 80% state of charge can actually take quite high charging currents--Much higher than 13% (or C/8) rate of charge without overheating (very near 100% charging efficiency). However, when the battery is >~80% state of charge, it starts to do things that generate heat (not all energy is going into chemical reaction of "charging"). One of the major items (besides internal resistance P=I^2R) is "gassing" or when Hydrogen and Oxygen gasses are being generated... Part of the energy goes into breaking the water molecule, the other goes into waste heat. It is very easy to get a battery hot (and even overheat/cause a fire) by feeding a 100% charged lead acid battery with 2-2.5% steady state current "forever" (2.5% to 5% current is recommended for "equalization").

In general, if you have 13% or less rate of charge, the battery bank will not get "hot enough" to worry about overheating during a charging cycle. However, if you have >13% rate of charge, it is possible to overheat the battery bank and in some cases to get thermal runaway (AGMs are more suseptible to thermal runaway). Thermal runaway can happen like this 1. Battery is charged to 59 volt set point and 2. gets hot, battery acceptance voltage falls, 3. charger "thinks" battery needs more charging current and supplies it, 4. battery gets hotter, and 5 acceptance voltage falls more....

If you do "over panel" and have >13% rate of charge, there are two options for you. The first is to use a Remote Battery Temperature Sensor. Most solar charge controllers measure the room air temperature, and estimate the bank temperature to reduce charging voltage set point (usually not for equalization though--another issue). A remote battery temperature sensor (glued to case or bolted to the battery terminal) will give the controller actual bank temperature--Reducing the risk of thermal runaway.

For Midnite MPPT charge controllers, most will accept a "wizbank jr" current shunt (and remote communications board). Midnite appears to one of the few companies to use the remote battery shunt to measure charging current to the battery bank... You can, for example, program a C/8 or 13% maximum rate of charge, but if you have daytime loads, the controller will exceed the output current limit to support those loads (i.e., 13 amp charging  + 10 amp load = 23 amps from charge controller). Very cool.

continue on next post

Photowhit,

Yea, it is probably not clear what 130% really means. I assume 60 VDC * 1.3 = 78 Volts Vmp-array-stc (standard test conditions). Just a "rule of thumb" for including all "typical" design losses (Vmp-hot, wiring drop, controller input-output drop, etc.).

I would not think it means that Vinpu-min is 78 volts... There is nothing that I can see that a Midnite MPPT controller "needs" 78 VDC minimum Vinput (Vmp-hot+wiring voltage drop, etc.) to operate "correctly".

-Bill

The 200 Classic is slightly less efficient (the higher voltage FETs, that is how transistors work), and it also has less output current (a limitation on eventual maximum system capacity--If Dapago ever does build out a much larger system.

But yes, a 4s x 2p on a 200 Classic will take him (operationally) to near 0C (which probably never will happen).

Cannot disagree that it would be a straight swap (and not affect any other wiring/configurations).

Oth