Max watt harvest with 400w array

nschizzano
nschizzano Registered Users Posts: 34 ✭✭
in General Solar Power Topics #1
What should be my max watt collection with this set up(I am getting mostly 200w-300w but I have seen it hit 312 alot and sometimes, 320s, MAYBE 330s or very brief 340w)

4, 100 watt PV arrays wired in Series making 68v and a brief 70v at the MPPT, so no apparent voltage drop. Optimum opperating voltage for each panel is 17.1v. 17.1v x 4 = 68.4. And it is often at that number, there are a lot of parameters to check durring a busy day, including variations of weather but it I often look for 68v and see it, along with rare 70v, and voltage around 64v many times overall aswell, and these volt readings sre when my PV watts are displaying high 200s. 

The screen parameter reading displayed on my 60A LItime MPPT that reads "AMPS - Batt" Is That the amp charge Bat my is receiving from the array? Becuase it is at 17-20amps I believe, when my watts are at 200 - 300w and PV voltage right around 68v

So to be clear on the set up. 4, 100 watt panels (optimum operating voltage 17.1) wired all in series. 8awg cable running 75 feet to 60A non Bluetooth MPPT by LITIME brand. 2 foot length of 6g cables running out of the MPPT to the battery. Battery is a 12v lifepo4 LITIME, 200Ah/2560Wh. 

And if it matters, a 2 or 4awg cable running from Bat to 2000w 12v pure sine samlex inverter if that conciders anything 

Thanks alot. I hope that's clean! . I have been very curious, if my system should be making more like 380w. Maybe I have to check my cable over again. Recently I found a damaged peice of shelding. No damaged wires, just, old, I think copper sulphate oxidization, I repaired that section and the watts went right up 20% compared to the prior readings. 

These are late August and the first half of spetember results in Ottertail Minnesota, its been 70 to 85°F. No wildfire smoke. Lots of full sun to test. And panels are at about a 30° tilt! They are not all exactly, exactly the same because two are fixed on a post but 2 panels are new and leaning on items, but, all, almost exactly the same angle. 

Thanks again 
· Share on Twitter

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    In general, the Vmp-array voltage with a MPPT type charge controller should be around 100% (cool day) to 80% of the Vmp-array-standard temperature voltage (cold temperatures, Vmp/Voc rise... Hot solar panels, Vmp/Voc voltages fall).

    Usually, the Vmp-array does not vary much once there is "full sun" on the array.

    It is the Imp/Current from solar array that varies with sun/angle to sun/demand by charge controller to charge the battery bank and supply any DC loads. The array current (with full demand by charge controller) is very proportional to the amount of sunlight... With the array outputting close to 100% * Imp-std ratings.

    My quick rule of thumb for a system... More or less, the charging Wattage to the battery bank (I.e., full energy from the array) is around 75-77% of the panel's name plate Wattage rating (Pmp). Or,

    400 Watts * 77% = 308 Watts to battery bank/DC loads
    308 Watts / 14.5 volts charging = 21.2 Amps to charging battery

    Note that this is typically on a cool/clear day near solar noon with panels pretty much pointing at sun with no shade, or dust on panels, etc.. This is a (more or less) about the best you would see a few times a year.

    And note with an MPPT charge controller, the battery current varies with the battery bus voltage... I.e.:

    308 Watts / 12.0 volts = 25.6 Amps charging discharged battery and/or heavy DC loads
    308 Watts / 14.75 volts = 20.9 Amps with nearly full battery...

    From what you describe, I don't see any issues with your system... Solar power measurements (Voltage, Current, Wattage) are usually around 5% to 10% (for Watts) accurate with standard DVMs (digital volt meters, LCD charge controller display)--And some charge controllers were known for reporting higher than actual power by upwards of 5%--Nobody complains if their system outputs more energy... Add variation in voltage drop of wiring, dust on panels, panel/cell temperatures, variation in sun hitting panel--I don't get terribly worried until the "peak output" of an array on a sunny day is 50% or less of what I expect.

    Just using our eyes to guess at the amount of solar energy--We really cannot tell between 1,000 Watts/sqmeter and 500 Watts/sqmeter. To be more accurate--Really need to measure the solar radiance with a meter and reference solar cell/etc.

    You can get more solar energy (usually just temporarily) with clouds (edge of cloud effect), sub freezing weather, reflection from snow/sand...

    If you want to double check... Measure the Vbatt voltage at the charge controller output and again at the battery (under high charging current). For "accurate/best" charging, you want a maximum of 0.05 to 0.10 volt drop from Vbatt charger output to battery bank terminals. If you have a high wiring voltage drop (small diameter wire, long wire run) from controller to battery bank, that will slow down your charging (I.e., controller measures 14.75 VDC and the battery is at 14.0 volts)....

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
    · Share on Twitter
  • nschizzano
    nschizzano Registered Users Posts: 34 ✭✭
    #3
    My set up hit 312w the other day but I think that was the peak. The numbers were also bouncing around more than I have ever seen, it was right after removing a section of damaged wire.  I only seen it bouce around like that after my wire repair. I'm not sure it hit above 312w from PV - Array for sure, or not, but now I have been watching the number closely for many days and on the MPPT the PV watts are coming in at 250w for the main parts of the day, 260w for a couple hours and one time a momentary max of 283w. It does sit in the 270w range for a bit as well. Is that still okay? Or is that a bit low for 4, clean 400 watt panels. September in Minnesota ottertail. The panels are relatively upright and the sun is rather lowon the horIzen, i can take a picture of the angle. The cable to MPPT is 72 feet long w/ 8awg. I just lowered it to 72f from about 88 feet hoping it kay enhance things. I am just wondering if the system is conducting fully. And/or my underground cable has some sulfate oxidation in the wire from water getting in a pin hole. Because that happened on a section of that cable and I repaired it. Not the best cable. Should I getting a special 8g solar panel cable? Additionally I am slightly concerned about one panel. My oldest renogy from 2018, has some black X marks in the cells, but I believe I have tested it properly to display the appropriate voltage output. So..
    I can send a picture of that aswell. I could not find any images online that looked like the marks in the panel that mine have. They don't seem all that alarming compared to damaged cells I have seen on Google. 

    Thanks. 

    But you do say as long as system generates not below 50% wattage of panels, I'm fine, so in this case, even 200 watts is sufficient? And can the unit go above 77% watt when its very cold? Thanks

    Also I rarely even see my MPPT read my Bat at above 13.4 or 13.5v. Is it pertinent that it reaches at 14v + often? Is there a major point of the Bat going past 13.5, because when the Charging is released from the sun, it always drops to 13.5. Just trying to learn it all, gives me security in understanding. 

    ALSO, I have recently seen my MPPT display my bat voltage above 13.5 and it jumps around fast sometimes, like 14, 14.1, 13.7, 13.5,14 etc. I suppose that is fine and the panel possibily seeing clouds while the bat is at 'float' charge is it?

    Okay and one more major question, I aquired 2 more 100w panels. So I will be adding a total of 6, 100w panels to an 8awg cable at 72 feet to a 12v 60a non blue tooth Litime MPPT and out of that MPPT is 2ft of 6awg to a 200ah 12v lifpo4 Bat

    1.will my cables be thick enough and for parralel or series wiring? Thanks a million 

    2. I'm guessing I can buckle these two extra panels into my unit in series because amps stay the same? but I'm just not sure, and I'm just not sure what is the very best and how it all works. Thanks thanks thanks
    · Share on Twitter
  • nschizzano
    nschizzano Registered Users Posts: 34 ✭✭
    #4
    Also I have done alot of load testing on my 2000w inverter, drawing 1000w draws fast off the battery bank. It never alters or raises the watt input from the PV, I think this is because my bats are never very full, apparently 13.3 - 13.5 still takes full charge rate from PV. And I have seen it accept full amp charge on my generator setup charger, right until the bat reaches 14+v I forget exactly but it does take along time for that charger to cut down from the 40a peak, on the generator charger system that is. But yes I have incorporated trials of drawing heavy loads to find out if my PV arrays incoming wattage changes(increases)

    Thanks
    · Share on Twitter
  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #5
    I am not saying that 50% of rated output of a solar array is "fine"... It is just difficult to say that it is "bad/low" array on an arbitrary day (solar conditions) without actually measuring solar irradiation (how much sunlight there is--Without using a, for example, a reference solar cell measuring actual solar irradiance at that point in time).

    The 80% "peak" solar power from an array is the typical best case for a solar array in full sun on a warm/hot day (at solar noon, charging/DC loading on charge controller to draw "full array power"). get down towards freezing and below, yes you can see more power (Vmp falls as the solar cells get colder--below ~75F)--Even approach 100% of rated 400 Watts in sub freezing weather (remember cells still get heat from sun, even in cold weather).

    If this is your battery:

    https://www.litime.com/products/12v-200ah-plus-lifepo4-battery

    CHARGE
    Charge Method：CC/CV (constant current/constant voltage)
    Charge Voltage：
    14.4V ± 0.2V
    Recommend Charge Current：
    40A (0.2C)

    You are not causing "damage" to the battery to charge below those numbers--It is just not going to charge to 100% SoC.

    Note: Depending on the BMS functions--A BMS may also "balance" the internal battery cells (bring all cells to "matching" voltages during charging). If you do not bring the battery to ~14.6 volts and hold it for a period of time (a few hours per month???), the BMS may not "balance the cells" and you could damage the battery if taken down below ~20% SoC (I.e., a low voltage/unbalanced cell can be taken to zero State of Charge and be damaged).

    Q: How to Charge LiFePO4 lithium battery?

    A:1. Solar PanelsThe battery can be fully charged in one day (with effective sunshine 4.5 hrs/day) by 1600W solar panels. It may take more than one day to fully charge the battery by ≥1600W solar panels since the duration and intensity of light would be a great factor for their charging efficiency.2. Generator/AlternatorIf the alternator or generator supports DC output, a DC-to-DC charger is needed to connect the battery to the generator; if your alternator or generator supports AC output, please add a suitable battery charger to connect the battery and the generator according to our recommendations. The Recommended Charging Voltage: 14.2V - 14.6V.3. LiFePO4 Smart ChargersTo charge 12V battery, it is recommended to use 14.6V battery charger.The Recommended Charging Voltage: 14.2V - 14.6V.Related reading: Can I Charge LiFePO4 batteries with a Normal Charger?LiFePO4 batteries are known for very low charging resistance and can accept full charge rate up to very nearly 100% SoC, unlike lead acid which "slow down charging" over ~80% SoC (to 100% SoC).270 Watts from a 400 Watt array is not unusual...400 Watt array * 0.77 "typical peak" array output (few times a month at best) = 308 WattsCurrent into battery while charging from a "nominal" 400 Watt array (again best case a few times a month or a year):308 Watts / 13.5 Volts Battery voltage = 22.8 Amps (at 13.5 volt battery voltage)308 Watts / 14.6 Volts Battery = 21 Amps into battery (at 14.6 volt battery voltage)Note that it would take (roughly) for a 50% discharged 200 AH Lithium Ion battery to recharge at 20 amps:200 AH * 0.50 state of charge * 1/20 amps = 5 hours best case (approximate).Note that 13.6 volt "resting" (no charge, no loads) is ~100% full for LiFePO4 battery bank... However, especially with BMS, you should be charging to ~14.4 +/- 0.2 VDC (reach 14.4-14.6 volts, then cut off charging--No need to hold for 2-6 hours "absorb" like Lead Acid needs).From a random Website:
    Battery voltage "jumping around" during charging... That could be the solar charger making "adjustments", could be the BMS doing "its thing", or could be both... Or could be poor/dirty/loose wiring connections be affected by current/heat... Don't know. Make sure all connections are tight and clean. No "browning" from overheated connections or wiring.Are your panels "tilted" 30 degrees from horizontal? For Otter Tail Minnesota, your panels should be around 46 degrees (or higher in winter) for optimum solar harvest... Using PV Watts with 30 degrees, the amount of average sun (by month) you would receive:
    In September, "4.8 hours" will almost fully charge a 50% discharged 200 AH @ 12 volt battery. In December, it would take over 2 days to fully charge a 50% discharged battery. (Note using Litime numbers, it could take 2x more days).400 Watts * 4.8 hours of sun for Sept * 0.77 panel+controller deratings * 1/14.4 volts charging = 102 AH est. average harvest per September day.Using LiTime's numbers:1,600 Watt array * 4.5 hours of sun per day * 1/14.4 volts charging = 500 AH of "12 volt harvest"200 AH battery capacity / 500 AH of "STD" harvest = 0.4 "derating" (kind of high derating??)400 Watts * 4.8 hours Sept sun * 0.4 "derating" * 1/14.4 volts charging = 53 AH per Sept day harvest (their numbers)...Overall, I don't see any "obviously bad/wrong" numbers (voltage, current, etc.) from your setup for charging. If the battery bank is "dropping" to 13.5 volts after charging--That is pretty close to full (and you do reach nearly 14.4 volt charging?)."Solar Panel" cable is UV rated insulation. You can use direct burial, cable in conduit to protect from the sun.Your solar charge controller is rated for 150 Vpanel-max voltage:https://www.litime.com/products/litime-60a-mppt-12v-24v-36v-48v-solar-charge-controller
    Using a handy solar array string sizing tool (for another brand of solar charge controller) to estimate working voltages:https://www.midnitesolar.com/sizingTool/index.php
    Depending on your exact panel Voc/Vmp/Isc/Imp ratings, and how cold it gets (do you run in winter?)--The maximum array voltage could exceed 165 Voc-array-cold at 0F...You may wish to run 3 panels in series * 2 parallel strings to keep the Voc-array voltage below 150 VDC.Or, you cold try 5s x 1p array and remove the "questionable" panel from the string... Panels with visual defects can work OK but fail down the road later...It is always "best" to keep the array voltage high and the current low... This keeps the overall voltage drop from the array to the controller low.You can use a voltage drop calculator for wire drop:https://www.calculator.net/voltage-drop-calculator.html
    https://www.calculator.net/voltage-drop-calculator.html?necmaterial=copper&necwiresize=3&necconduit=pvc&necpf=0.99&material=copper&wiresize=0.4066&resistance=1.2&resistanceunit=okm&voltage=68&phase=dc&noofconductor=1&distance=85&distanceunit=feet&amperes=8&x=Calculate&ctype=nec

    1.6% voltage drop (power loss) is great... Ideally try to keep below 3% wire losses.-Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
    · Share on Twitter

Categories