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Solar Panels

  1. Apr 7, 2010 #1
    I came across a statement that I was hoping you could
    shine some light on.

    It said that the laws of physics determine that when
    panels are connected together (I'm figuring in parallel) each can only
    work as well as the lowest power panel in the system.

    Could you briefly
    explain why this is so?

    so if 5 panels were each producing 200W and a 6th wired to them was only producing 190. All 6 would only produce 190W.
  2. jcsd
  3. Apr 7, 2010 #2
    That would be like saying that if I had 100 solar panels 100 watts each connected in parallel, then I added a 1 watt panel in parallel, that the array power result would be 1-watt.
    Doesn't work that way at all.
  4. Apr 7, 2010 #3
    I suspect your source means that if one cell is weak then the voltage of the group of cells in parallel will be reduced, like having a weak battery in parallel with stronger ones. That makes sense to me.

    There is a basic description of how solar cells work here



    but exactly what happens in a weak solar cell is not apparent to me....but seems to have to do with the balance of drift and diffusion currents in pn junctions.....
    somehow current production is reduced which adversly affects voltage...
    now that I think about it, I'm unsure just what happens with a weak traditional lead acid battery in parallel with stronger partners. ..but they absorb marginal current rather than generate it....and usually get hot as they do....suggesting additional thermal vibrations.....
  5. Apr 7, 2010 #4
    Solar cells are NOT batteries in the sense that one "charges" another if one is "weak"
    That doesn't happen with solar cells. Batteries and solar cells have an analogy only to a certain point. Then, they are very different.

    In a solar cell arrangement, parallel, the power factor will increase regardless if one of the panels is less illuminated(but still illuminated)
  6. Apr 8, 2010 #5
    so what's the process in either case??
  7. Apr 8, 2010 #6
    from http://en.wikipedia.org/wiki/Photovoltaic_effect

    Here http://en.wikipedia.org/wiki/P-n_junction

    reverse and forward bias in a pn junction is described......which is the case when one cell in a parallel array is "weak"...?? Yes, I know there may be diodes to prevent reverse current flow, but apparently that's not what the OP is aksing...or maybe he/she is???
  8. Apr 8, 2010 #7
    Well, I suppose the main diferrence(in this context) is that batteries can hold a charge and solar cells do not.
    That is, remove a battery from it's charging source and the battery, absent from that source, will produce an electrical current in a circuit(up to the charge amount).

    A solar cell, removed from incident light, will not do this. It does not hold a charge.
  9. Apr 8, 2010 #8
    So, a weak battery in parallel with a strong battery will have what effect? The strong battery will "charge the weak battery"

    A week solar cell in parallel with a strong solar cell does not do this, because you cannot charge a solar cell.

    Thus, the way both scenarios behave connected to an external load is different.
  10. Apr 10, 2010 #9
    not really...a weak battery draws down all voltages in parallel with it....
    Likely its sulphation prevents complete charging...the battery might get to, say, 80% full charge and that's it....the current drawn likely produces heat rather than chemical conversion leading to a more complete charging....and the current does diminish as if the battery were fully charged....some batteries actually boil dry, buckle and can actually get so hot they melt.....I don't know exactly what processes are involved....
  11. Apr 10, 2010 #10
    An interesting issue related to Scotty's original question is how variable solar cell lives and power production are. With traditional lead acid wet cell batteries, it somewhat of a hit or miss situation...one weak cell in a bettery will drag down a whole bank of batteries....While a group of batteries tends to age similarly, individual cell deterioriation in my experience varies widely,,,,

    I just took out an auto battery from my wife's car: In the fall about five mos ago all cells tested ok via hydrometer; when her car failed to start I checked them again last week and one was flat dead, two others quite weak...what happened in only five mos???

    With solar cells, there is a question as to how uniformly cells will perform in producing power...they are usually guaranteed for maybe 25 years or more so it would appear the manufacturing results in quite uniform power performance...
  12. Apr 25, 2010 #11
    i have a 20 volt solar panel and am charging up a car battery its made to do it through a ciggarett lighter, voltage of the battery goes up to 12 volts but the battery does not last long enough should i bypass the built in mother bord and wire the panel straight to the battery and how would you recomend me to do it, not used in a car just for a 12 volt tv that runs off of the battery
  13. Apr 25, 2010 #12
    Myles...too difficult to answer in brief here....in general a solar panel can "cook" a battery via over charging, like an alternator or generator w/o a regulator...if it's powerful.....but if the solar panel is modest amp hour capacity or is not on all the time you have much greater flexibility...a sign your battery is being overcharged is loss of electrolyte (water)....and maybe even a battery that is too warm on a cool day....

    If you don't get a specific answer here, try searching for a solar panel battery charger manufacturer....
  14. Apr 25, 2010 #13
    A 12 volt battery (lead/acid) has to be charged to about 14.4 volts to be completely charged. It won't actually hold that voltage for long as it will settle out to 13.8 while at rest. Your 20 volt panel will only produce peak voltage and amps under optimum conditions:
    1. A solar panel within 7 degrees of square to the sun
    2. Full sunlight - no clouds
    3. 20 volts is too much to use on a twelve volt battery but it should be higher than 13 volts to the battery and up to 18 volts (open circuit volts) in order to complete a charge cycle.
    The amperage is more important in solar charger applications as there are losses in the system and battery. Too much amperage will "boil" the electrolyte and too little will never complete the charge cycle. You will need at least three amps and no more than twenty amps at twelve volts loaded.
    A battery will take more amps when its open circuit voltage is low than it will when the open circuit voltage is high. The internal resistance increases with the charge on the battery.
    You need to have some kind of regulator to limit the voltage to no more than one-half volt per cell higher than the existing battery open circuit voltage.
  15. Apr 25, 2010 #14
    If you connect two 12v solar panels in series where one produces 1 amp while the other produces two amps, the series combination will produce 24 volts at one amp. The OPs concern is correct. The panel that would like to produce two amps will circulate an additional amp through it's internal PN junction and be heated up by the 12 watts that it could have delivered to the load.
  16. Apr 25, 2010 #15
    The OP is talking about a parallel arrangement, not series.
  17. Apr 25, 2010 #16
    It's the same thin there. If two 1 amp panels are connected in parallel where the first has an ocv of 12 but the other is 10, the 12 volt panel will supply all the power.
  18. Apr 25, 2010 #17
    It's not that simple.

    A solar cell is equivalent to a current source, parallel to a forward biased diode. With an open circuit, all of the current will go through the diode. The voltage drop of a forward biased diode depends only weakly on the current, so the open circuit voltage of a panel made with the same type and the same number of cells in series will not vary much with the amount of light, so putting cells in parallel is generally not a problem.

    If you do connect two panels with different open circuit voltages in parallel, what happens depends on the load resistance. Suppose panel 1 can deliver a current I_1 with an o.c.v. of V_1 and panel 2 can deliver I_2 with an ocv of V_2, and V_1 > V_2 and a load resitance of R is used.
    now if (I_1+I_2) * R > V_2 then the load will only get a current of V_2/R through it, and the rest of the current goes through the diodes of the panel with the smaller open circuit voltage.
    If however (I_1+I_2) * R <= V_2 all of the current will go through the load resistance. There is still some waste, because the panel with the higher voltage, will now have the lower voltage across it. The panels produce (I_1+I_2) min (V1, V2) instead of I_1*V_1 + I_2*V_2

    The voltage drop across a solar cell pn junction is about 1V, this is an impractically low voltage, so a number of cells is placed in series. Bypass diodes have to be used here to stop brightly lit cells from powering the shaded cells. If this wasn't done, the current couldn't get larger than the current through the darkest cell.

    There's a lot about solar cells in chapter 1 of this ebook

    Last edited by a moderator: Apr 25, 2017
  19. May 2, 2010 #18
    what if i take the solar panel, by pass the motherbord wiring the battery to the solar panel, adding a 12v 3 amp diode and a voltage control switch that automaticly stops current flow after the battery reaches the proper charge
    P.S. my battery wouldnt hold charge because Advanced Auto charged it with the amperage to high and messed the life of it up a crap load, so i bought another one and everthing is running fine,this questions just for fun

  20. May 17, 2010 #19
    hey naty1,
    this is myles 7, i now have a charge controller/regulator 12v/24v/10a and a car battery. now i was woundering what type of solar panel would you recommend, i dont know much as i thought i did about this so can you help me out. and also does the amperage have to be 10a for the charge controller,and if so i would need a amperage reducer to charge the battery correct.
  21. May 17, 2010 #20
    also i found a 20WMaximum Power Voltage (Vpm): 17.2 VMaximum Power Current (Ipm): 1.17 AmpOpen Circuit Voltage ( Voc): 21.7VShort Circuit Current ( Isc): 1.25ATolerance: solar panel would this work. i hope you like this stuff because if not its probally allready on your nerves thanx
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