310w panel Vmp issue

dapago

Hello. I just got my 48v solar system installed.
Magnum inverter 4.4kw 48v
8 x 310w Jinko panels
8x6v 225ah Trojan batteries
Midnight Classic MPPT 150

The original design by the company was to connect the panels as following: 2 string parallel of 4 panels.
Vmp for 1 panel is rated 37v (STC) and Voc is 45.9v.
The problem is that when all the system was connected, the midnight controller detected 160 volts (above its 150v capacity) and entered self protection mode. That would mean that each panel was delivering 40v Vmp instead of the 37v rated.
The company propose me to rewire the panels as follow: 4 parallel strings of 2 panels in series.
2 questions here:
What would be the consequence of the new panels wiring to my system and load needs?
Why a rated 37v Vmp panel delivers 40v not even in full sun and at 04pm?

Thanks in advance for your answers.

BB.

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

dapago

Hello Bill. Yes it does make sens (in a way). I live in a hot country. The temp never goes under 20 Celsius during night time and the average temp during day time is 33 Celsius. September until December is when we "only" get an average of 5.2 sun hours per day. The rest of the year it goes over 6 sun hours a day. My system is 48v.
When we did the checking with 4 in serie the temp was around 27 Celsius and sun was mid taped by clouds.

The copper cabling we have used for the connection panels/controller is a 6AWG (13.3mm2). Distance panels/ controller is about 15 meters. Will I need to change the cables?

I need to be sure that the system will work perfectly with no problems in all the situations.
If I read you well, basically, I have 3 choices:
- rewire the panels (2 in serie with 4 strings) but that would imply 1 fuse per string, maybe cable change and not really sure that the voltage will be enough to quickly charge the bank in a mid sunny day.
- get a 200v controller and keep with the original configuration
- buy an other panel and connect 3 in serie, 3 strings, install a fuse per string.

Is that right?

dapago

Today is rainy and cloudy. Time is 01pm. We had to disconnect 2 panels for the system to work so I am running 2 strings of 3 panels. Temp is about 25 Celsius.

BB.

Yep.

Fuse per string for 3 parallel strings is usually a "close call".

Always recommend a conservative installation for safety reasons. Check the panel data sheet for their series fuse requirements.

-Bill

BB.

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

Vic

Hi dapago,

Bill is certainly correct.   Strings of two of those PVs will not work reliably with an MPPT CC,  especially if your batteries are Flooded.

Three PVs will have a bit of a high String  voltage (string Vmp of about 111 V at STC),  but with your mild climate, this is probably OK,  and certainly better than trying to use two of these PV per string.

MidNite Solar advises that the string voltage (using STC ratings)  for Classics be 130% of the highest battery voltage that you will ever need.

The Classic 150 should be fine.  Hope that you can find one additional matching PV.

FWIW,  Vic

dapago

I have 8 x 105RE 225ah Trojan (flooded).
Charge voltage settings per cel (25Celsous) as follow:
Absorption 2,35 - 2,45
Float 2,20
Equalize 2,58

An extra panel will be the best solution yes but I might have some space configuration problem on the roofs. I need to be sure that with 2 in serie and 4 strings I can get an efficient charging pattern and equalize batts. After reading your comment I am a bit worried not being able to get an efficient system and need arguments to oppose to the installer (he is the one who calculated the panels/controller match).

dapago

Hi Vic,
Well. Now I need to argu (and I mean argue) with the installer telling him his 2x4 strings fix will not work properly. What a mess.
Thanks for your inputs guys.

BB.

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

dapago

Thank Bill. We want to start with 3.5kwh day but will need to raise that figure to 4.5 kWh-day in the near future.

dapago

Attached the monthly average radiation incident and temperature for my zone.

Vic

Hi again dapago,

I should have posted the Link to the Classic String Sizing Tool,  previously:
http://www.midnitesolar.com/sizingTool/displaySizing.php

Look at Note 2 under the Results from running your PVs through the Sizer  (believe that that is the right note).

Good Luck,  Vic

dapago

Thank you Vic,
Bill there is something I do not understand. If they rewire the panels as 2s, 4p, I should get (37v*2*1.3)*0.80 (derating due to temp on a hot day*)=76.96v per string. Not reaching the minimum voltage required by the controller to perform equalizing unless I did not fully understand your earlier comment (below) or the Midnite Sizer Note (below).

Midnite Sizer Note says: Most all MPPT controllers will want to see a minimum of 130% of the actual high battery voltage. So if we have a 48v battery and it has an Equalize voltage at 62.3 volts than we would multiply that by 130% and we would need a minimum of 81 volts on the input on the hottest day of the year in order to have enough headroom for the MPPT to work.

Bill wrote: 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).

Important here: *hot days (above 35 Celsius) are standard here during the dry season in Granada (8 month a year). During the green season (from sept to dec), average temp is around 30 Celsius.

The installer has a BlueSolar Victron controller to propose as well. I attached the Sizer for the 150/60A model they propose with the 310w Jinko panels data.

dapago

Sorry to bother you again but I have been doing maths and 2 Jinko 310w en serie will not give me the 80.6VDC needed to equalize the batteries bank. Here are the maths.

In my hot climate, 80% of the year the ambiente temperature is around 33 Celsius witch means the panel surface is reaching most of the time the 65/70 Celsius. With a PMAX of -0.41%/Celsius, the lost in efficiency is about 17%. 
37VDC(rated VMP at STC)-17%=30.71VDC per panel or 61.42VDC in a serie of 2.
I need 62*130%=80.6VDC to be injected into the battery bank to perform equalization in my 48v system. Far away from the 61.42VDC obtained with 2 *310w jinko PV en series.

 Please tell me if I am wrong here.

Photowhit

Yet another option might be to get a Midnite Classic 200 controller. It has a 200 volt max Input so would likely work with 2 strings of 4 panels...

This would be easier than finding and importing another panel...

BB.

The numbers for the solar array/panels are a bit fuzzy... There is not a sharp knee for Pmp--It is a bit rounded and if you are within 10% of Vmp--You are normally close enough. See the power chart here:

http://www.jinkosolar.com/ftp/EN-JKM315P-72(4BB).pdf (note, you will need to copy and paste the "whole URL")

And, the best I can find is for MPPT tracking on the Midnite controllers (minimum Vpanel vs battery voltage), page 92:

https://www.solar-electric.com/lib/wind-sun/classic-manual.pdf

LMX – LoMax. This enables the Classic to track the input voltage all the way down to Battery voltage. When disabled the Classic will stop tracking the input around 5 volts above the battery voltage. When the input voltage is within a couple volts of the battery voltage the inductors can Sing, this is usually not very loud and will do no harm.

So, that sets the "ideal" minimum input voltage around 58 volts (absorb, or less, hot batteries need lower charging voltage) of ~58+5 or 58+2 volts--63 to 60 volts Varray-minimum input voltage.

And from the Solar specs (using the chart), the voltage drop at, say 30C outside temp + 30C rise = 60C cell voltage--Which gives you about:

• Vmp-temp = Vmp (1 + Vmp-temp-factor*(Vcell-Vstc))
• Vm-temp = 37.0v * (1 + (-0.0045/C)*(60C-25C) = 31.1725 Vmp-60c per panel
• 2 * 31.1725 Vmp = 62.3 Volts

And since the Power Curve is relatively rounded, you can expect to run the panel upwards of 10% (5% is "better", less losses) above or below Pmax. 105% of 62.3 volts = 65.4 volts -- Which is over the 63 to 60 volts needed to "optimally" charge your 48 volt battery bank at ~25C (battery bank temp--Hotter battery bank charging temp will drop ~5mVolts per Cell per degree C. (24 cells * -0.005 volts per C = 0.12 volts per C, hot bank at 35 C, or 10C rise, then -1.2 volts over 25C charging voltage for typical lead acid battery).

So--It looks like 2 panels in series would probably work OK... But, as you can see, you are right on the edge of "optimum" operation. It would be much easier to call it 3 panels in series and then we would not be worrying about a couple of volts of headroom here.

And there will be some voltage drop in the cabling from the array--Which will be worst if running 2 panels in series... I will go ahead and estimate what an "ideal" 3x3 array looks like--And you can run the calculations for 2x4 if you are going that way:

• If you run three strings in parallel on one cable, then
  
• Imp*3=8.38a * 3 = 25 amps Imp for array
• 3 * 37 volts Vmp = 111 volts nominal Vmp-array

Using a generic (US) voltage drop calculator (I am going to stay in English/USA units--I am getting tired tonight, and I would not recognize if I made errors in metric):
www.calculator.net/voltage-drop-calculator.html

• 15 meters (~50 feet) one way wire run, 25 amps, 111 volts nominal with 3% and 1% voltage drop (3% try to not exceed, 1% drop don't bother spending more money on copper):

10 AWG:
Voltage drop: 2.50
Voltage drop percentage: 2.25%
Voltage at the end: 108.5

6 AWG
Voltage drop: 0.99
Voltage drop percentage: 0.89%
Voltage at the end: 110.01

Some other quick numbers about your system... 225 AH @ 48 volt flooded cell battery bank--We suggest 1-3 days of storage and 50% maximum discharge. 2 Days of storage is usually a nice number (for various reasons):

• 48 volts * 225 AH * 0.85 AC inverter eff * 1/2 days storage * 0.50 maximum discharge = 2,295 WH per day (no sun).

And based on our rules of thumb for battery bank capacity, roughly 1 kWatt of AC inverter (and maximum solar array) per 100 AH @ 48 volt battery bank would be ~2.25 kWatt maximum AC inverter (and cost effective solar array).

For charging your battery bank (highly suggest using the remote battery temperature sensor option for your MPPT charge controller), based on 5% to 13% rate of charge:

• 225 AH * 59 volts charging * 1/0.77 array+charge controller deratings * 0.05 rate of charge = 826 Watt array minimum (weekend/seasonal usage)
  
• 225 AH * 59 volts charging * 1/0.77 array+charge controller deratings * 0.10 rate of charge = 1,724 Watt array nominal (full time off grid)
  
• 225 AH * 59 volts charging * 1/0.77 array+charge controller deratings * 0.13 rate of charge = 2,241 Watt array "cost effective" maximum

And for your array, predicted output for 5.2 hours of sun per day:

• 8 * 310 Watt panels = 2,480 Watt array
• 2,480 Watt array * 0.52 typical off grid AC system eff * 5.2 hours of sun = 6,706 Watt*Hours per day

If you 'over panel' the battery bank, you may need to go into the Midnite MPPT controller and "dial back" the maximum output current if you add more to your solar array. Too much charging current may take the battery >>59 volts charging for short periods of time and give you battery bus over voltage errors.

The above numbers are "conservative" as we want the system to work for many years (as panels get dirty, batteries age, etc.).

If much of your loads are during the day, and you really do not need much "dark weather" storage (you don't have days of dark weather)--Your battery bank is small, but workable. If your surge current/maximum inverter loads are >2,250 Watts (continuous) or >4,500 Watts surge (2x rated Inverter output), you should look at a larger battery bank.

You have lots of sun, so your daily harvest even with 8x panels is very good vs your desired output. Your battery bank may be on the small size--But if it meets your needs, see how it works out for you.

I will stop here--Probably enough for this post.

-Bill

mcgivor

Wow, read through this and my mind is spinning, Bill must be a very patient guy. But one question, when doing research for tropical climates, the information I gathered was that the advantages  of mppt over pwm were not significant to warrant the extra expense. Correct me if I'm wrong. Temperature is the factor which changes everything, perhaps a new "Tropical installation " catagory should be included as it has its it's own set of variables and networking between people who live in areas common to one another can share information, just a thought.  

dapago

Good morning Bill. I am lucky to have found you on this forum. Based on your post (thank you very much for it), I will first take the time to fully understand it, contact the installer then and post again.

There is something you said that got my attention: About adding one more panel (3s,3p) and over paneling my battery bank, does that mean the initial installer configuration 4s,2p would have during sunny days (most of the time) creating bus over voltaje errors?

Photowhit

BB. said:

The numbers for the solar array/panels are a bit fuzzy... There is not a sharp knee for Pmp--It is a bit rounded and if you are within 10% of Vmp--You are normally close enough. See the power chart here:

http://www.jinkosolar.com/ftp/EN-JKM315P-72(4BB).pdf

And, the best I can find is for MPPT tracking on the Midnite controllers (minimum Vpanel vs battery voltage), page 92:

https://www.solar-electric.com/lib/wind-sun/classic-manual.pdf

LMX – LoMax. This enables the Classic to track the input voltage all the way down to Battery voltage. When disabled the Classic will stop tracking the input around 5 volts above the battery voltage. When the input voltage is within a couple volts of the battery voltage the inductors can Sing, this is usually not very loud and will do no harm.

So, that sets the "ideal" minimum input voltage around 58 volts (absorb, or less, hot batteries need lower charging voltage) of ~58+5 or 58+2 volts--63 to 60 volts Varray-minimum input voltage.

Hi Bill, you understand the electronics better than I ever will, but are you saying the Classic's do NOT need a 130% above the charging voltage, but rather a fixed amount of voltage above the charging voltage?

If so, why wouldn't they just state that they need 5 or 7 volts above the charging voltage rather than stating that they need;

•          Midnite Sizing tool; "Most all MPPT controllers will want to see a minimum of 130% of the actual high battery voltage. So if we have a 48v battery and it has an Equalize voltage if 62.3 volts than we would multiply that by 130% and we would need a minimum of 81 volts on the input on the hottest day of the year in order to have enough headroom for the MPPT to work.
  
• You will have to enter submit to see this in the "Notes" at the bottom of their "Results" page

animatt

I can not speak for the midnite controller as I have had limited experience with it, and that was awhile ago.
But as bill state lower voltages on mppt can work.

I am in semi tropical climate.  I have 4 12v panels hooked to a 48v bank thru the magnum mppt controller.
It is an under paneled system. Once house it built a permanent array will be bought and installed.


Bulk would obviously be fine.
Then as voltage demand increases current demand lessens which works as panel V increased available Amps decrease.   

And for equalization current demand is dramatically less. 

And because vmp charts do not show a cliff but a smooth shoulder  does not matter much if you operate outside of max vmp for low current events.

Only caveat would be like in midnite controller.  If it did not do a mppt sweep low enough. 

This is a boarder line case and could sacrifice some overall performance. But as long as rest of system is not on the board line it can work.

In my case an extra 12v panel would be nice for the extra wattage on certain occasions.   I see very few days the extra V headroom would be needed.  

My system is a strictly daytime system.  Inverter and controllers get shutoff at around 6pm and turned on around 7:30 am. I spend very little time in bulk. 

dapago

Photowhit. I was asking myself the same question.

Vic

'Morning,

Just my opinion,  but you probably really do NOT want to run strings of two of these PVs on almost any MPPT CC,  especially with Flooded batteries.

The Classic will perform more reliably with Vin closer to 85 V than with Vin in the 74 V range at STC,  on a 48 V Flooded bank.  Realize that the dapago's PVs,  in strings of three will not have 85-90 V STC string voltage (111 STC Vmp for dapago's PVs),   and two of those PVs will have an STC string voltage of 74 V,  which will give a bit better performance than will more standard strings of two older 24 V PVs  --  35.5 Vmp verses  37 V STC Vmps.

Personally,  I would not run a Classic with only two of these PVs in series.   IMO,  the sweet spot for STC Vin is about 85 V (for Flooded batteries),  so strings of three 60-cell PVs is about perfect.

The Classic will be a bit more efficient when it can produce good power from strings of two of the 37 Vmp PVs,  but,  again IMO,  it will also spend more time Resting in marginal conditions than one might like.

However,   dapago could try strings of two of those PVs,  to see how things perform.  Running the Classic in Legacy P&O, or perhaps Hydro  Mode,   should be able to accommodate a lower Vin.

One system here,  uses strings of three 72 cell PVs,  on MPPT CCs  --  106 string Vmp.  This works well with Classics.  These PVs are true,  older PVs with 35.4 STC Vmps.   With Flooded batteries,  71-ish V  would be just too low.   There is a bit of an efficiency hit,  but  this seemed to be the only approach for a reliable system,  that could EQ the battery on any day with good sun,  and the CC has lots of room to find a good Vin for best power production.

Again,  just my opinion,    FWIW,  Vic

BB.

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

BB.

Besides over charging the battery bank, there is another issue with "too much" charging current... It not the way we "think" of the charge controller controls the battery bus voltage. It is actually the battery itself that holds 10/5-15 volts on the battery bus. Solar charge controllers (and car alternators as an example) do not have regulations circuits that are "fast enough" to hold 14.5 volts (or whatever) if there is no load/battery connected. Most will shut down or quickly "over voltage the Voutput to the DC bus (on your car, disconnect the battery while the engine is running, and you will get 20-100+ VDC out of the alternator and blow everything electrical that is turned on for most cars).

If you feed "too much" current to (most) recharge able batteries (that are near 100% state of charge, you will "pull the battery out of regulation). We have reports of >75 VDC (system fault voltage for one major brand of inverter/controllers) on a 48 volt battery bank (small battery bank and large solar array).

Your installation with 4s * 2p array--You are Over Voltageing (potentially) the solar charge controller input (better term than "over paneling" here). Because you are in a warm climate and the MPPT charge controller is pulling some current almost all the time, you are not seeing Voc>150 VDC. But it can happen. Will not damage your controller (because of "hypervolt" design), but you may be left with lots of sun and a charge controller in shutdown during part of the day, or when (if) you get some cooler weather (or even when nobody is there drawing power and the battery bank is full).

One of the issues with "engineering" a system like an engineer--We talk about reducing losses--Designing a system with 3% or 1% voltage (power) losses... However, in real life, with all of the variables (weather, sun position, temperature, equipment accuracy, etc.)--Most people without a laboratory setup and solar references, could not see a 10% change in system performance, let alone 5% or 3% or 1%. When my GT solar array failed, I was not "sure" until I saw a 50% loss of output (before I called for warranty repair).

Rule of Thumb design guidelines keep us in the middle of the road. And work very well in general. In some cases, like Dapago, we can alter the low temperature limit (lives in an area that is "never" under 20C). So he can run a higher Vmp-array (and Voc-array) that somebody like me that gets hard frosts every few years.

Anyway--I will stop typing for now--And my apologies to anyone that has read through to the end of this post. I am trying my best to be clear here--But, obviously, I do not design/install this equipment and doing best here to add light and not confusion to the discussion. English is my only language--And I am not great at that either.

-Bill

Photowhit

1. BB. said:

   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"

Thank you for your detailed reply,

I guess as you are stating and the way I read Midnight's information are different.As I read it, Midnight states that MPPT charge controller want to "See" 130%;

• Midnight "Most all MPPT controllers will want to see a minimum of 130% of the actual high battery voltage."

Not that the VMP should be 130%...

Photowhit

I suspect this has been hashed out at the Midnight forum in the past, though I haven't checked;

http://www.midniteforum.com/index.php

BB.

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

Vic

Two added comments;

Agree with Photowhit,  that,  particularly if the Installer of dapago's system has responsibility for designing the system with the Classic 150 and the Jinko PVs  --  8 PVs in two strings of four,  that having that installer just swap out the 150,  and install a Classic 200,  for no charge,  that this seems to be a straightforward remedy.  A small efficiency reduction for the high string Vmp,  but no need for a ninth PV,   Combiner and breaker addition (if they are not now pressent), and so on.

Also,  it is just my opinion,  that the Classic CCs really do "like"  a reasonable amount of Vin headroom to support the quick Sweep Solar Mode.  Solar Mode establishes a new MPPT Vmp in about one second.   There might be other reasons that Classics could work best with  more Vin headroom,  but am not certain what those things would be. 

Certainly have watched MN Classics find MPP Vins for a number of years,  but not often on systems with MPP Vins very close to battery voltage  (ie with  STC string Vmps very close to max battery voltage) .

To me,  just like choosing optimum battery charge parameters ...   at least start with the manufacturer's recommendations.

Who am I to try to argue the finer points?    Vic

BB.

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

dapago

Thank you all guys (specially Bill here) for all the details explanations and inputs. Now I can sustain my claim to the "professional" installer and ask him why on earth did he design and wire a system that would have never worked properly (4s 2p).
I do not think I will never need more than the 2480w array (8 panels) so the installer has to change the controller for the Midnitgh 200 at no cost. If he refuses the change arguing that the 2s 4 p solution works with the midnight 150 I suppose I would have to accept this "on the edge" configuration.

Adding one panel will cost him more (panel price + roof support extension an installation) so I am not sure he will accept that but will try anyway.

Sorry Bill for all these questions but after saving a bunch of money and hopes to get my solar system I was very upset that the system as installed does not work at all. It might just be a technician error who connected the panels the wrong way but after this experience I have not trust with the installer and wanted to be sure the 2s4p they proposed is correct and a non future problem fix.

Just received an email and they proposed to install a Victron 150/60 bluesolar. Will that make a difference with the 2s 4p configuration I am stuck with now?