Move batt stateful sizing to ssc equations

[brtietz]

Convert two functions currently in pysam's battery tools to ssc equations such that they can be used by the API in all languages.

Future pull requests will add additional cmod_battery functions.

[dguittet]

There is an edge case where the original_capacity is 0 and then resized to non-zero. This happens when you size the battery to 0, perhaps as part of a loop across a range of numbers, then the mass and surface area will get set to 0 also. Then for later sizing calls, the function will no longer be able to use prior information about the mass to calculate an appropriate mass, and here it'll cause a divide by 0.

Other cmods are not able to run with a strictly 0 system capacity (pvsamv1, windpower) while others can (pvwatts). While this is a still-unfixed issue in the PySAM code, I think the simplest way to handle this is to not let the desired_capacity be 0.

[brtietz]

Great point! In the latest commit I used the same error convention as cmod_windpower_eqns - writing the error to an "error" variable in the vartable. Am I correct to assume it's the responsibility of the calling code to check this value? When I tried to throw an exec error, it crashed Julia. Given the difficulty of error handling in the SDK, I assume that's not a valid option.

[dguittet]

Yeah, you can't throw exceptions across a DLL boundary so that's not possible. The error will have to be done via a return string. Perhaps we could change the function signature of SSC equations so the return type is bool instead of void. I don't think that would be too difficult and would make error handling easier

[dguittet]

Let me know if you want me to change the function signature to bool. Otherwise you have to check if error len > 0

[brtietz]

That would be super helpful. I tried to do the update just now and ran into a type issue:

Severity Code Description Project File Line Suppression State
Error C2440 'initializing': cannot convert from 'bool (__cdecl *)(ssc_data_t)' to 'ssc_equation_ptr' (compiling source file C:\Users\bmirletz\source\repos\sam_dev\ssc\ssc\core.cpp) ssc C:\Users\bmirletz\source\repos\sam_dev\ssc\ssc\ssc_equations.h 87

It looks like we either need to change them all or define a new signature with bool. I changed the battery stateful equations in the most recent push to get that error, if you're able to do the others I'd appreciate it.

[dguittet]

Apologies for the delay, but I've fixed up all the equations: changing the return type, fixing up the error handling from exceptions to error strings.

[dguittet]

I'll re-review once tests are done