Error in Ixx equation in pvsystem.sapm

[cwhanse]

The SAPM function uses Aisc (temperature coefficient for Isc) in the equation for Ixx, where it should be Aimp (temperature coefficient for Imp). This line.

Equation 10 of the 2004 Sandia report shows Aimp is intended.

Discussion among the Sandia staff concluded that there is a typo in PVLib for Matlab, in this line.

This error was propagated when the Matlab code was ported to python by R. Andrews (see pvl_sapm.py file in the prearizona tag for pvlib).

The comment originated with W. Holmgren, who may have been looking at the Matlab code as the authoritative source.

Expected behavior
Correct this line to use Aimp.

Versions:

• pvlib.__version__: all versions

Additional context
The error only affects Ixx from pvsystem.sapm and nothing else.

PVLib for Matlab issue sandialabs/MATLAB_PV_LIB#37

[kandersolar]

Every reference for these equations that I was able to locate with reasonable effort supports the conclusion that the equation should use Aimp.

I also agree that the error is limited to the line of code linked above.