Maximum open circuit voltage calculator

rin67630

Solar Addict
Real World Example:
  • total string voltage is 136.2.V
  • worst case temp is -17.8C
  • temp coefficient is .3%/C
  1. worst case temp differential = 25 - (-17.8)= 42.8C
  2. Voltage increase % = .3%/C * 42.8C = 12.84%
  3. voltage increase = .1284*136.2 V = 17.48V
  4. Max voltage = 136.2+17.48 = 153.68V
One should however mention that this is a theoretical extreme example.
The probability that you get:

-17,8°C
and
- daylight
and
- no clouds, bright sun
and
- the right orientation
and
- a full charged battery that does not require any power, so your panel will deliver Voc

is really tiny at most latitudes.

The minimum recorded temperatures usually happen at night. With a bright sun, the outside temperature usually raises very fast (the panel temperature even faster), normally before your battery is charged.
 

FilterGuy

What, me worry?
One should however mention that this is a theoretical extreme example.
The probability that you get:

-17,8°C
and
- daylight
and
- no clouds, bright sun
and
- the right orientation
and
- a full charged battery that does not require any power, so your panel will deliver Voc

is really tiny at most latitudes.

The minimum recorded temperatures usually happen at night. With a bright sun, the outside temperature usually raises very fast (the panel temperature even faster), normally before your battery is charged.
It may not be quite as rare as you make it out to be. If the battery is full the SCC will look like an open circuit to the panel(s), In an open circuit situation, the panel will reach Voc with very little sun, therefor even in less than ideal solar conditions, Voc is somewhat easy to reach.

That leaves, Daylight, -17,8°C and full batteries as the 3 necessary conditions. -17,8°C is 0°F. There are a lot of places the will stay under 0°F for several days in a row and a system is likely to have full batteries in just one or two days. Consequently I don't consider the example extreme or theoretical for those locations.

Each person needs to decide what temp to calculate the Voc for. Part of this decision includes a risk tolerance that they are willing to live with. I just assume that karma is out to get me so I do conservative calculations. Others might make less conservative calculations.
 

rhino

Solar Addict
Yeah.. I've had -40F with bright sunshine in the morning shining directly on the panels so even the 20% rule mentioned would go over voltage limits. Should be noted that some charge controllers like the Midnite Classic can handle their "rated" max Voc + battery voltage (up to 48V).. so the Midnite Classic 150 is not damaged even with a Voc much higher that 150V.
 

rin67630

Solar Addict
It may not be quite as rare as you make it out to be. If the battery is full the SCC will look like an open circuit to the panel(s), In an open circuit situation, the panel will reach Voc with very little sun, therefor even in less than ideal solar conditions, Voc is somewhat easy to reach.

That leaves, Daylight, -17,8°C and full batteries as the 3 necessary conditions. -17,8°C is 0°F. There are a lot of places the will stay under 0°F for several days in a row and a system is likely to have full batteries in just one or two days. Consequently I don't consider the example extreme or theoretical for those locations.

Each person needs to decide what temp to calculate the Voc for. Part of this decision includes a risk tolerance that they are willing to live with. I just assume that karma is out to get me so I do conservative calculations. Others might make less conservative calculations.
But not -17°C (at the cells) and full sun.
Just daylight with a cloudy weather will not get your batteries fully charged, even less if you have a side consumption.
And as low as 5% of the panel installed power will bring you from Voc to Vmp.

Official temperatures are recorded at a shaded protected location.
Solar panels warm up in full sun very fast, even if the ambient temperature is low.
 
Last edited:

FilterGuy

What, me worry?
But not -17°C (at the batteries) and full sun.
Just daylight with a cloudy weather will not get your batteries fully charged, even less if you have a side consumption.
And as low as 5% of the panel installed power will bring you from Voc to Vmp.

Official temperatures are recorded at a shaded protected location.
Solar panels warm up in full sun very fast, even if the ambiant temperature is low.
OK...... Like I said, to each their own. Everybody has their own risk tolerance and makes their own decision. In my engineering career I learned early on to assume the worst in a design. In one of my first designs I made the mistake of assuming less than the worst and it cost the company several million dollars.... I never made that mistake again.
 
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