Voltage difference in two solar panels

Discussion in 'The Projects Forum' started by SolarFun, Nov 20, 2012.

  1. SolarFun

    Thread Starter New Member

    Oct 16, 2009
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    I power my cabin with one 80W solar panel and two, 6 volt, deep cycle batteries. I'm having trouble keeping my batteries charged and bought another 85W panel. My original panel is 7 years old. As I was hooking them up (in parallel) I read the voltage on each panel. The new panel read 20V and the old one only 13V. I understand that I'm going to gain charging current by adding the second panel but I'm worried that this disparity in voltage will limit my gain or cause some other trouble.
    Two questions:
    1. Why is there such a difference in voltage from these two, very similar, exactly same sized panels?
    2. Will the low voltage from the old panel reduce the effectiveness of my new configuration or vice versa, will the high voltage from the new panel reduce the total current?

    Thanks
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Put diodes MUR1560G, in series with the panels so they wont feed back into each other,the one with the highest voltage will do most of the charging.
     
  3. wayneh

    Expert

    Sep 9, 2010
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    Are your batteries in parallel or series? If they're in series, and your old panel's open circuit voltage is 13V, there's your problem. You might as well disconnect the old panel.

    If they're in parallel, the batteries pull the voltage down to 6V and the new panel's output may not be much different than the old one. DD is probably right, but you need to know the I-V curve to be certain, if you care.
     
  4. SolarFun

    Thread Starter New Member

    Oct 16, 2009
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    I'm running a 12 volt system, two six volt batteries in series. I'm not sophisticated enough in electronics to understand what I-V curves are so I probably won't pursue them. I appreciate the advice though and I'll investigate the diodes. Am I right to assume they are called blocking diodes?
    I'm pretty bummed about the old (7 years) panel only producing 13V. It had a "limited" warrantee of 25 years. Any guesses as to why it deteriorated so quickly?
     
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    If we assume your first panel had an original output of 20V, then it is only making 13V/20V*80W = 52W of power now. As it is damaged I would suggest just replacing it with the new panel. As to why it is damaged, I suspect it is on the roof of a building 365 days a year exposed to all sorts of weather, hot, cold, wind, and even the occasional lightning strike.

    If you have the model numbers post then so we could check if that blocking diode (the correct common term) is already built into the panel, so you do not need nor want an external diode (it wastes precious power).

    A charge controller not only extends battery life it can increase the charging efficiency. PWM type is good, but the MPPTs type does the power trick. With a MPPT controller you could possible put both panels in series and get all the power both are making. That is the only way I can see you could use both panels.
     
  6. wayneh

    Expert

    Sep 9, 2010
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    That's fine. An I-V curve just describes how the voltage comes down as more current is drawn from the panel. Two points on the curve are typical specs for the PV panel: The maximum, or open circuit voltage is when no current is flowing. The maximum current is obtained when the panel is shorted and the voltage is zero. In between these two extremes is where the panel actually operates and there is an I-V that describes the behavior over the while range. One point of interest is where power, voltage times current, is maximum. A good design will operate near that point.

    It seems that a general rule-of thumb in PV systems that the open circuit voltage is nearly twice the battery voltage. The "extra" voltage accounts for a loss in the blocking diode and allows the system to charge more often, more hours of the day even when not in full direct sun.

    If it was me, I'd be looking for relief under the warranty.
     
  7. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    Your original panels solar cells might still be good but maybe some of the bypass diodes are shorted causing the low voltage. If there is a junction box with terminal lug connections on the panel you might be able to repair it.

    http://www.vishay.com/docs/89398/solarcell.pdf
     
  8. SolarFun

    Thread Starter New Member

    Oct 16, 2009
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    Thanks for the tips. I need to get back to my cabin and investigate specifications and what the junction box on the back of the panel looks like. I'll try to take pics if the problem isn't obvious.
     
  9. SolarFun

    Thread Starter New Member

    Oct 16, 2009
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    I'm back from the cabin and discovered that my solar panel made by Shell (SQ85-P), who is out of the solar panel business now, but is maintained by Solar World. They are checking to see if I have a legitimate warranty claim. Now for the details. There are 2 bypass diodes on my 80 watt panel and, apparently, one blocking diode. I say apparently because I can see and test the two lugs but can't see beneath the bank of connectors. All read about .500 milliohms in one direction and 0 in the other so I guess they are still OK. My charge controller is an ancient 2004 product that basically turns the current to the batteries on and off.
    Should I ditch this old panel or is there hope?
     
  10. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Here's the specs on the panel....
     
  11. wayneh

    Expert

    Sep 9, 2010
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    I wouldn't ditch it. They may want it back before they honor your warranty claim. And it's still possible that the PV panel is fine (although aging) while a diode has failed. That would make it cheap to fix, if they don't honor the warranty.
     
  12. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    How old are the batteries? Part of the issue may be panels, but as the batteries age, especially when charging isn't tightly controlled, they'll either lose capacity or not take a charge.

    The solar panel should be covered by warranty, is the protective glass over the cells clouded, cracked, or hazed?
     
  13. tindel

    Active Member

    Sep 16, 2012
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    My guess is that it's not the batteries, actually. Not if he's reading an open circuit voltage of 13V - but this measurement must be a true open circuit measurement, and I'm assuming it is based on the OP saying the new SA has a Voc of ~20V (which is close to what the datasheet says it should be. I'm assuming the OP had similar lighting conditions on the two panels.

    Below is a typical SA IV curve:
    [​IMG]

    If your open circuit voltage(Voc) is 13V, then your peak power (Vmp) is probably somewhere around 11V (deriving from the datasheet), and in turn you probably won't get much battery charge current into a 12V system (a few hundred mA maybe, assuming your battery is around 12V) - especially if you have a blocking diode anywhere in the line. Also, the datasheet says that Voc should be around 20V, and Vmp around 16V, which would typically be fine for a 12V system - it would leave you on the left hand side (the flat section) of the IV curve.

    Basically your operating on the right-hand side of the SA IV curve. You want to be operating on the left-hand side (or the flat current side) of the IV curve. A quick current measurement could verify this.

    My guess is that it's the SA. You're charge control is going to be wide open, your battery will then accepting current, and if you have a blocking diode anywhere besides the SA, it's just going to make the problem worse. Figure in wire losses, and my guess is that you're just too far on the right-hand side of the IV curve to charge your battery to any sort of usable level.

    I don't typically work with domestic solar array systems - do they instruct you how to clean them off? It could just be dust/dirt on the array that's causing you problems. I'd be very careful cleaning it off though. Thermally speaking, don't clean off a hot array with cold water sort of thing... I also wouldn't clean it off if it has any affect on the warranty.

    I'd think that you are in a good spot to get a refund on your warranty
     
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