Temperature Coefficient Conversion %/C to %/K

sun_daysun_day Registered Users Posts: 23 ✭✭
Hey all! I am using a software that only allow me to input %/K for the temperature coefficient. The solar module datasheet provided me a Voc temperature coefficient of -0.31%/C.

How can I convert %/C to %/K?

Thanks!

Comments

  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    I think you are asking about *Kelvin, this might give you the answer...

    kel·vin

     (kĕl′vĭn)
    n. pl. kelvin Abbr. K
    1. A unit of absolute temperature equal to 1/273.16 of the absolute temperature of the triple point of water. One kelvin degree is equal to one Celsius degree. See Table at measurement.
    2. Kelvin A temperature scale in which zero occurs at absolute zero and each degree equals one kelvin. Water freezes at 273.15 K and boils at 373.15 K.


     
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  • sun_daysun_day Registered Users Posts: 23 ✭✭
    Hi, thanks for the response. I am asking the about %/C to %/K. Sorry if this is a simple conversion but I am not that good with it. Does this means it is just a direct conversion? As far as I know the -0.31 is in %
  • vtmapsvtmaps Solar Expert Posts: 3,739 ✭✭✭✭
    edited March 2016 #4
    sun_day said:
    Does this means it is just a direct conversion? As far as I know the -0.31 is in %
    A degree Kelvin is the same as a degree Celsius.  If the temp compensation is millivolts per degree K then it is also per degree C (no conversion necessary).  

    Unfortunately the temp coefficient is not linear with temperature... That's why the coefficient for your batteries is given as a percent per degree Kelvin.  In other words, the number of millivolts of compensation per degree depends on the temperature.   

    This is important because 30° C is 150% of 20° C, but 303° K (which is 30° C) is only 103% of 293° K (which is 20° C).  As a practical matter, the temperatures at which we operate our batteries is fairly narrow on the Kelvin scale and the temp compensation in millivolts per cell varies little over those temperatures. 

    Thus if the temp compensation were 4 millivolts per cell at 20 ° C, then it would be reduced by 3.1% (10 degrees X -.31% per degree) to 3.9 millivolts per cell at 30° C. 

    --vtMaps



    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • sun_daysun_day Registered Users Posts: 23 ✭✭
    Thanks for the reply. Does this means that -0.31%/C = -0.31%/K?
  • vtmapsvtmaps Solar Expert Posts: 3,739 ✭✭✭✭
    edited March 2016 #6
    I think so.  I am not too familiar with describing the temperature coefficient as percent per degree.  Consider that the temperature coefficient is also a function of the sulfuric acid concentration... that means that the coefficient changes with the SOC of the battery.  That means that as the battery charges (even if it stays at the same temperature), the temp coefficient is changing.

    I would prefer to set a temperature compensation of millivolts per degree.  In the temperature range we run our batteries, that approach is good enough.  Ultimately, what matters is the SG of your batteries.... if the batteries are cold and the SG is not up to spec, increase your absorb time and/or absorb voltage and/or your temp compensation.   If the batteries are hot and the SG is up to spec, but you are using a lot of distilled water, then reduce your absorb time and/or absorb voltage and/or increase your temp compensation

    --vtMaps

    EDIT: I just reread this thread and realized you are asking about the temp coefficient of Voc of your modules.  I have been writing about temp coefficient of battery charging.   What sort of software are you using that needs the temp coefficient of Voc?
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • sun_daysun_day Registered Users Posts: 23 ✭✭
    edited March 2016 #7
    Hello,

    Thanks for the reply. I am using Tritec TRI-KA software (it is an IV curve equipment and software). I need to input the temperature coefficient from the datasheet but the units they have in the software is only in %/K (or mV/K). However, the solar module datasheet provided the temperature coefficient only in %/C.

    Therefore I don't think I can directly input -0.31%/C into the software as the unit in the software is %/K.

    Really at a lost here... T_T
  • vtmapsvtmaps Solar Expert Posts: 3,739 ✭✭✭✭
    sun_day said:
     the units they have in the software is only in %/K (or mV/K). However, the solar module datasheet provided the temperature coefficient only in %/C.
    I can't help you with the percents... not familiar with that convention. 

    However a K degree and a C degree are the same size, therefore mV/K is the same as mV/C.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • BB.BB. Super Moderators, Administrators Posts: 32,090 admin
    Percent is simply multiplied x Voc of panel. If you have Vmp=21 volts, use 21 volts. If panel is 40 volts, use 40 volts. If testing a cell, then ~0.6 volts.

    If mVolts/C -- The value will change based on Voc of cell/panel. (mV/C will be different when Voc is different between test subjects).
    • Voc * %/C = mV/C
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • sun_daysun_day Registered Users Posts: 23 ✭✭
    Thanks guys for the reply.

    @Bill - I don't think the formula finds the %/C to %/K as I think you are trying to show how to convert %/C to mV/C.

    Does anyone else still have any idea on the unit change from %/C to %/K? @[email protected]
  • BB.BB. Super Moderators, Administrators Posts: 32,090 admin
    edited March 2016 #11
    Delta (K) = Delta (C)

    If you are doing differences in temperatures.

    Absolute temperature conversion:

    °C = K - 273

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • vtmapsvtmaps Solar Expert Posts: 3,739 ✭✭✭✭
    sun_day said:
    Does anyone else still have any idea on the unit change from %/C to %/K? @[email protected]
    Bill explained that the percent (per degree) multiplied by the Voc gives millivolts (per degree).    Since both degrees (K and C) are the same size, it doesn't matter whether you use Kelvin or Celsius.... the coefficient is the same for both.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • sun_daysun_day Registered Users Posts: 23 ✭✭
    Thanks! I get it now. I will convert it to mV to find. Does this applies to Isc too?
  • BB.BB. Super Moderators, Administrators Posts: 32,090 admin
    What exactly are you asking about Isc? Yes, there is a temperature correction for Isc (and Imp). As far as I know, Isc and Imp will rise a little bit with increase in temperature (less than the correction for Vxx vs temperature).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • sun_daysun_day Registered Users Posts: 23 ✭✭
    Hi guys,

    Thanks lot for the help and teaching. I have understood it now.
  • AGTM_DocAGTM_Doc Registered Users Posts: 2
    edited March 2019 #16

    Hello,

    A module I am working with shows the Temp Coeff for Voc as -(80±10)mV/°C How do I convert this to %/C?

    Thank you,

    Ed

  • BB.BB. Super Moderators, Administrators Posts: 32,090 admin

    Here is an example for a Voc~21 volts ("12 volt" solar panel)

    • 80 mV / °C = 0.080 volts / °C = (I think) 0.080 volts per panel per °C = 0.080 volts / (Voc panel * "delta" °C )
    • (0.08 volts per °C) / 21 Volts Voc = 0.00381 per °C = 0.381% per °C

    Note we are missing one variable, the per "what" device ... I am guessing this is per solar panel (i.e., per 21 Voc at standard temperature).

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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