Trouble figuring out how to run IV gold standard test cases in pvlib

Describe the bug When pvsystem.singlediode is called with method = 'brentq' or method = 'newton' and ivcurve_pnts > 0, singlediode.bishop88 is called to calculate the voltage and current of the ivcurve points. singlediode.bishop88 expects diode voltage, but pvsystem.singlediode passes it voltage instead.

To Reproduce Steps to reproduce the behavior:

1. Call with method = 'brentq' or method = 'newton',

   pvlib.pvsystem.singlediode(8, 5e-10, 1, 300, 1.8683643583499283, ivcurve_pnts=100, method='brentq')
   

2. The first few voltage values are negative.

Expected behavior The ivcurve points should be on this curve: https://cwhanse.github.io/ivcurves/test_cases.html#case-21

Versions:

• pvlib.__version__: 0.9.3
• pandas.__version__: 1.4.3
• python: 3.10

[adriesse]

I’m not sure what the best place is for this thought, but perhaps it could be useful if IV curve validation data sets actually included the diode voltage as well.

[cwhanse]

I think in this case I may have given wrong direction to @reepoi. We are attempting to use some precisely computed IV pairs as tests for pvlib single diode equation solutions. We want to supply known voltage and get back the corresponding current. When I first looked, I thought that was the intent of

https://github.com/pvlib/pvlib-python/blob/b7768b424d87ff86a863b3565ee708b3470cccbc/pvlib/pvsystem.py#L2876

On second look, that line is passing voltages that are akin to initial guesses at the cell voltages.

To me it suggests that #418 is still relevant: there are uses when e.g. voltage is known and the corresponding current is desired, and pvlib.pvsystem.singlediode doesn’t support that. Maybe it’s a documentation gap, or as @wholmgren pointed out, an incorrect expectation of singlediode raised because of the ivcurve_pnts parameter.