replacing rounded faraday constant with exact value in bpx.py

[dion-w]

Description

The faraday constant has an exact value, calculated by the exact values of the avogadro constant and the elementray charge [see here]. Using a rounded value may introduce systemic errors further down the line that can be easily avoided with this new value.

Fixes # (issue)

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ pre-commit run (or $ nox -s pre-commit) (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: $ python run-tests.py --all (or $ nox -s tests)
• The documentation builds: $ python run-tests.py --doctest (or $ nox -s doctests)

You can run integration tests, unit tests, and doctests together at once, using $ python run-tests.py --quick (or $ nox -s quick).

Further checks:

• Code is commented, particularly in hard-to-understand areas
• [not needed] Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.46%. Comparing base (f255c38) to head (ffb465a).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4290   +/-   ##
========================================
  Coverage    99.45%   99.46%           
========================================
  Files          288      288           
  Lines        22091    22079   -12     
========================================
- Hits         21971    21960   -11     
+ Misses         120      119    -1     

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[arjxn-py]

Thanks @dion-w, looks good

[agriyakhetarpal]

Thanks for the PR, @dion-w! Just a small suggestion on the style for the CHANGELOG entry (maybe we should catch things like this with a Markdown linter as well).

[rtimms]

FYI you can do pybamm.constants.F or pybamm.constants.F.value if you need a number instead of a symbol.

[kratman]

FYI you can do pybamm.constants.F or pybamm.constants.F.value if you need a number instead of a symbol.

I agree, using a constant from a library is better than updating this file.

One other thing to check would be if the BPX standard specifies a value for Faraday constant. Sometimes standards specify the precision to ensure that things are constant between implementations

[agriyakhetarpal]

Re-requested review/approval from @kratman since he previously requested changes