inverter voltage and battery charging

loteklotek Solar Expert Posts: 65 ✭✭✭✭
If I understand correctly a minimum voltage must be produced by solar panels for the solar charge controller to charge a battery bank. 24 for 24 volt sys. etc. So... with say 108volts (6 panels in sieries @ 18 volts) each panel would need to produce 4 volts before the controller could charge the bank. If it is a 48 volt sys Im guessing each pannel would need to produce 8 volts before the batterys would charge
Wouldnt that impact the amount of usable sunlight during a day?

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,717 admin
    Re: inverter voltage and battery charging

    More or less, solar panels will not produce any useful amount of power until there is sufficient sunlight to get near Vmp.

    Once the panel is near Vmp, then the current is, more or less, proportional to the amount of sunlight hitting the panel(s).

    For example, I have a "357 VDC" Vmp array of 3.5 kWh solar panels (two strings, ~5 amps each).... In reality, Vmp ranges from ~290-330 volts, and max max current ranges from 9-10 amps (depending on how clear day is, where sun is in sky).

    When the sun stop falling directly on my array, the array would bounce around ~180-280 VDC at 0.1-0.2 amps (around 18-56 watts)...

    Since my GT inverter probably needs at least 50 watts of solar power to "run" the inverer output stage--I got Zero watts.

    At times, I would harvest a few wants (0-20 watts or so) from the backscatter of open sky (no direct sunlight)--but that is for a relatively short time (less than an hour, so less than 20 Watt*Hours)... Or about 1/1000th or less of the total amount of power I would get on a very good day (20 kWhrs with my system).

    There just is not enough energy when the sun is directly lighting the panels to harvest any energy if the panel voltage is not near Vmp.

    The worst days (really gloomy/rainy days), I would collect less than 1kWhr per day (very few of those days)--still many times the energy that I could collect from "sub Vmp" operating panels.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: inverter voltage and battery charging
    lotek wrote: »
    If I understand correctly a minimum voltage must be produced by solar panels for the solar charge controller to charge a battery bank. 24 for 24 volt sys. etc. So... with say 108volts (6 panels in sieries @ 18 volts) each panel would need to produce 4 volts before the controller could charge the bank. If it is a 48 volt sys Im guessing each pannel would need to produce 8 volts before the batterys would charge
    Wouldnt that impact the amount of usable sunlight during a day?


    firstly, a controller, at this point, can't take the open circuit voltage that would be seen so 6 of those in series would not be a good idea. you are right that a degree of voltage above the charge voltage is needed to operate a controller and that a higher voltage does reap a benefit in more collected and sooner. the bad news is that the controller efficiency drops when the differential between input and output voltages goes up. somebody correct me if i'm wrong, but i wouldn't call it a wash either as i believe the efficiency losses exceed the gains made by the higher voltages. most will go up to the next level over the desired voltage needed by the batteries. goind 12v pvs to 12v batteries works, but can fall short at times as you already figured out. an 18v pv will be that next optimum level to a 12v system when available otherwise one would need either 2 12v pvs in series or one 24v pv for that same 12v output to the batteries.
  • loteklotek Solar Expert Posts: 65 ✭✭✭✭
    Re: inverter voltage and battery charging

    I don't understand? XW solar charger has max PV voltage of 140vdc/max open curcuit of 150vdc 6 18 volt panels in seiries would make 108 vdc what am I missing? What would be the best way to wire 12 panels in a 48 volt sys.
    should I consider a 24 volt sys?
  • BB.BB. Super Moderators, Administrators Posts: 29,717 admin
    Re: inverter voltage and battery charging

    Lotek,

    I think you are OK... Best thing, what is the Make/Model of solar panels you are looking at. Also, what controller (XW?) and what battery bank voltage do you want/need? And how hot/cold will it be (when the sun is up) in your area...

    There is the issue that MPPT Charge Controllers waste a couple percent more power at higher voltages than lower voltages (some vendors have a graph of efficiencies).

    However, you have to look at the costs and losses of the copper wiring (double your voltage, 1/2 the current, and 1/4 the wiring losses for the same gauge of wire -- P=I^2 * R ).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • loteklotek Solar Expert Posts: 65 ✭✭✭✭
    Re: inverter voltage and battery charging

    1.what is the Make/Model of solar panels you are looking at..

    .evergreen or sun 200 Watt, Solar Panel, 11.05 Imp, 18.1 Vp

    2.what controller?(XW?)

    Xantrex XW MPPT60-150

    3.what battery bank voltage do you want/need?

    I was thinking 48v for more effecenty. BUT.. now im wondering.. what do I want? If I use a bike to generate extra watts It might be easier to get a lower voltage battery bank to charge on smaller volts. but.. no 6048/4548 only 4024 for Xantrex xw if I want to go 24 volt , It might be all the power I need, with LP water and range. But I might need to run a deep well, if my shallow well idea doesn't pan out.

    4. how hot/cold will it be

    South Ga. 90+ in the sommer( its not the heat , It the humitity),average 50's day time highs in winter
  • Solar GuppySolar Guppy Solar Expert Posts: 1,962 ✭✭✭
    Re: inverter voltage and battery charging

    You can do 4 maximum in series for a 150V charge controller ( XW, Flexmax ect ) , for your 6 do two parrallel strings of 3 which is a 48V nominal array and will have no issues charging a 48V battery bank.

    Keep in mind the voltage in the data sheet is for when the panel is at 25C ... the voltage goes up when the temperature goes down

    As for you logic, its not how panels work. With even the smallest amount of day light, panels will put out a VOC of near the vmp value, it might only be measured in milliamps, but the voltage will not be what limits harvesting energy, I have controllers from sunrise to sunset ...

    Make your system 48V ... the bike thing while maybe a feel good thing won't add squat for system energy
  • BB.BB. Super Moderators, Administrators Posts: 29,717 admin
    Re: inverter voltage and battery charging

    There is a XW charge controller string sizing tool too at Xantrex.

    Plug in the panel info (ES-A-200) and it will show you the Min/Max voltages based on your ambient temperatures.

    And they show you what series/parallel strings will be supported.

    From looking at the results (I am assuming 32F is your minimum temperature at sun rise)--4 panels in series will just barely not equalize your battery bank on a hot day (panels will have ~60 VDC max, drop 2 volts for controller drop--58 volts for equalization--may need to equalize on a "cooler" day). And for 6 panels in series--you are getting upwards of 146 volts open circuit on a 32F day.

    5 panels, however, is right in the middle of everything.

    If you used a 24 volt battery bank--then you can use fewer panels (lower voltage) if you wish.

    Regarding which controller (6 kW 48v, or 4 kW 24v)--from what Solar Guppy said--pick the 6kW 48v inverter--not much more price for much higher output (if you ever need it).

    Since you are already talking about fairly large loads--using the higher voltage bank keeps the DC currents much lower--so if you can get the appropriate batteries for your bank (size/amp*hour) -- go for the 48 volt bank.

    Also, I personally like fewer parallel strings of batteries (I like larger batteries in series)--I think it is easier to debug a batter bank in series, and less issues with sharing current among strings.

    Somebody with more experience than I can tell if the 146v @ 32F in southern Georgia is a real problem for a 150 volt limit controller or not.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Solar GuppySolar Guppy Solar Expert Posts: 1,962 ✭✭✭
    Re: inverter voltage and battery charging

    String tools are OK, but rely on a min and max that at best is something people guess at.

    I current have dual 4kW arrays that are 48V nominal, the vmps for roof mount last week was about 58V include all losses at the hottest point of the day, my temperature probes were showing the panels between 145-150 degrees, ambient was about 92 , close to as hot as it gets here in my location in Florida ... one doesn't have to EQ at that time.
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: inverter voltage and battery charging
    I don't understand? XW solar charger has max PV voltage of 140vdc/max open curcuit of 150vdc 6 18 volt panels in seiries would make 108 vdc what am I missing? What would be the best way to wire 12 panels in a 48 volt sys.
    You're missing two key points. One is that the array's Voc spec, and not the Vmp spec, is used to initially determine compatibility with the charge controller's input voltage limit specs. The other point is that the module's STC Voc is based on a cell temperature of 25 C (77 F), and it must be adjusted for local record low temperature per NEC Article 690.7.

    The Evergreen ES-A 200 W module's specs are here: http://www.evergreensolar.com/upload/011108%20Datasheets%20(SR)/ES-A_200_205_210_US_011108.pdf

    The record low temp for Valdosta in southern GA is -17 C (2 F; ref www.weather.com). The 2005 NEC 690.7 temperature correction factor for this temp is 117%. The low temp adjusted Voc for the ES-A 200 is therefore 22.5 Voc (STC) x 117% = 26.325 V.

    Six of these modules wired in series would make 26.325 V x 6 = 157.95 V. This value is too high for either the Xantrex or the OutBack charge controllers. It's also too high the DC circuit breakers (150 VDC spec) typically used to protect the array wiring.

    The 2008 NEC 690.7 has more precise instructions for determining temperature corrected Vmp, but it'll be close to the 2005 results.

    You should wire five of these modules in series per string for your system with a nominal 48 V battery bank. The array Voc won't get too high in the winter, and the operational Vmp will be high enough in the summer to meet the charging voltage requirements for a 48 V battery system. This PV configuration will work with the Xantrex XW controller or either of the OutBack FLEXmax controllers.

    HTH,
    Jim / crewzer
Sign In or Register to comment.