What is better, Solar panel with lower current but higher open voltage

Discussion in 'OnBoard Electronics & Controls' started by BertKu, Jun 11, 2017.

  1. BertKu
    Joined: May 2009
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    Location: South Africa Little Brak River

    BertKu Senior Member

    Hi all, who has experience with a 12 Volt solar panel of 250 watt, but has only a max current of 8.03 Ampere and open voltage of 37.2 Volt, ( ARMP6 -60P-250 from ARM-Solar) while I have 240 watt panels with 11.39 Ampere and an open voltage of 25.5 Volt. (SV-L-230-fa1 from sovello) Does that mean that a MPPT unit can squeeze more current out of a higher open voltage panel then a panel with a lower open voltage? I have been advising somebody to use panels with the highest current for our brush-less motors. In view that the current and the magnets will dictate the torque. The magnets are fixed and are the same, but the currents differs. Did I advise him wrongly?
     
  2. alan craig
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    alan craig Senior Member

    BertKu, the type of load to the battery (brushed or brushless motor, heating, anything) is irrelevant. The MPPT (maximum power point tracking) makes the solar panel panel run at its most efficient point, the best combination of amps and volts for the conditions, then converts to a suitable voltage for the battery. So your ARM panel produces about 296W at max efficiency (if these are the figures for max power) and an MPPT charger would convert this to about 14V at 21A for charging a 12V battery.
     
  3. alan craig
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    alan craig Senior Member

    Bertku, I just looked up that ARM panel, you quoted the open circuit voltage - manufacturers figures are 30.6V and 8.17A at max power point so put those figures into the above example.
     
  4. BertKu
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    BertKu Senior Member

    Thank you Alan, 30.6 Volt x 8.17 Amp = 250 watt and that is indeed what the manufacturer quote. Thus 250 : 14.4 Volt charging, means 17.66 Ampere to the motor or battery. But my problem is, nothing for nothing and somehow a MPPT must have somewhere an efficiency percentage, or is it indeed 100% efficiency. I am struggling with believing that the efficiency is 100%. Thus in my case my maximum power is 240 watt, and this thus means 16.66 Ampere versus 11.7 Ampere. I have in that case that person wrongly informed. I will correct my action and approach him. Once again thanks for your response. Bert
     
  5. alan craig
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    alan craig Senior Member

  6. CDK
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    CDK retired engineer

    Conventional design rules put 36 cells in a panel for 12V system voltage; cells were expensive and MPPT didn't exist. Today's line of thinking is that with more cells you can still obtain a useful output at lower light levels, so your solar days are longer.
    But of course there is a catch. Almost 10% of your energy harvest disappears in the MPPT controller, so your 250 watt panel has to struggle for each watt above 225. A 97% efficiency is theory and only valid at one point of the curve and at an unrealistic temperature. I have a grid-tie inverter that is supposed to be 95% efficient, yet I hear the cooling fan already switch on early in the morning and it keeps running until late in the afternoon.
     
  7. BertKu
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    BertKu Senior Member

    Thank you Alan, with all the information I am no longer sure what is the best. Low voltage high current or high voltage and lower current. One should have a better idea if we are in the position to have 2 panels, one at 8 Ampere and one at 11/12 Ampere and compare with the same MPPT unit for each panel and see then what would be the best in summer and winter. Also I charge my 40 lithium ( 10 parallel x 4 in serial batteries) with a solar panel and the advantage of lithium is, that it already start charging at a much lower voltage than the nominal voltage of 14.4 Volt. It basically means that with a lithium battery, with solar one is better off. O.K., O.K your guys think I am an stupid old man, I may agree with you, but I have some very interesting working concepts. which works well for me. My home made charger just checks a maximum voltage of 4 x 3.60 Volt and switch the system off and change over to 14.4 trickle charger of 100 mA for 1 hour. The batteries are now nearly 7 years old and they should be a little degraded, but I really don't notice it . Thanks for the info. Bert
     
  8. BertKu
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    BertKu Senior Member

    Yes those where the days, some even had 37 cells (round ones) to compensate for the prevention of the battery discharge (compensation for diode) during the night. Thanks CDK, I thought already that close to 97% is a dream, but realistically it is more like close to 90%. Thanks for your reply. Bert
     
  9. IronPrice
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    IronPrice Senior Member

    One benefit of higher voltage lower current could be lighter DC cables and/or less voltage drop.
     
  10. BertKu
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    BertKu Senior Member

    It is a consideration, but the real advantage of a higher voltage (same wattage, same working voltage) is when using a MPPT (maximum power point tracking) then you will even late at the afternoon or early in the morning have reasonable power extracted from your panel. I feel if you don't use a MPPT unit, then lower voltage and higher current could be an advantage. Remember, we were discussing open voltage of 2 different make solar panels, both 12 Volt and 250 watt per example, Bert
     
  11. Steve Wright
    Joined: Feb 2018
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    Steve Wright Junior Member

    This is correct.

    There are two ways of connecting PV arrays.
    1. Low PV array voltage (parallel), for better controller efficiency.
    2. High PV array voltage (series), for earlier cut-in, later cut-out, and more usable harvest under very low light levels.
    If you have smaller PV, larger AH, and sunny days - then you want option 1.
    If you have larger PV, smaller AH, and little or no sunny days - then you want option 2.

    Check your battery system is of the efficient type, ie not liquid filled lead acid leaky-bucket type.
     

  12. ArnyLogan
    Joined: Aug 2018
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    Location: Texas,USA

    ArnyLogan New Member

    Hi...i am a new user here. As per my knowledge the MPPT makes the solar panel panel run at its most efficient point, the best combination of amps and volts for the conditions, then converts to a suitable voltage for the battery. So your ARM panel produces about 296W at max efficiency.
     
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