[rtimms]

Description

Closes #5820. Supersedes #5841, whose commits are included here with their original authorship.

From #5841, irreversible heating includes the SEI film drop for the main intercalation reaction. It still covered only that reaction, so the electrical energy lost to side reactions never appeared in "Total heating [W]" or the cell temperature.

This PR adds a BaseThermal._side_reaction_heating helper, called for each porous electrode and particle phase. It adds:

• SEI growth and SEI on cracks: side-reaction current × (delta_phi − SEI open-circuit potential)
• lithium plating: plating current × delta_phi, since plating has zero open-circuit potential

At a planar lithium metal electrode (half cells), the total interfacial current is plating plus SEI growth, but the heat term applied the plating overpotential to all of it. The main term now uses the plating current, and SEI growth adds its current × (delta_phi − SEI open-circuit potential).

delta_phi is the full surface potential difference, so these terms include any SEI film drop. The SEI open-circuit potential is only read when SEI growth is on, so parameter sets without SEI parameters are unaffected.

Energy check

TestEnergyBalance.test_side_reaction_heat_matches_electrical_loss checks that ohmic plus irreversible heat equals the electrical loss: terminal power minus the power stored by every reaction at its open-circuit potential. It runs three DFN cases at 268.15 K for 1800 s, each matching to rtol=1e-3:

case	heat without this PR	electrical loss
OKane2022, 10 A charge, SEI growth + SEI on cracks + partially reversible plating	768.70 J	783.57 J (1.9% missing)
Bonkile2024 composite graphite/silicon, 10 A charge, SEI growth + plating in both phases	coverage for the per-phase path
Xu2019 half cell, 2.4 mA, reaction-limited SEI on the lithium metal	0.0619 J	0.0804 J (23% missing)

Type of change

Bug fix, with an entry under ## Bug fixes in CHANGELOG.md.

Important checks:

• No style issues: uv run pre-commit run --all-files
• Tests added that prove fix is effective: the new energy-balance test fails without the helper
• All tests pass: thermal integration tests plus the lithium-ion integration suite (354), -m unit on tests/unit/test_models (922). Full nox -s tests left to CI.
• The documentation builds: not run, no docs changes
• Code is commented for hard-to-understand areas

🤖 Generated with Claude Code

[github-actions]

Bencher Report

Project	PyBaMM
Branch	fix/side-reaction-heating
Testbed	bare-metal

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.23%. Comparing base (1495fe3) to head (8156c93).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5875   +/-   ##
=======================================
  Coverage   98.23%   98.23%           
=======================================
  Files         348      348           
  Lines       35761    35794   +33     
=======================================
+ Hits        35130    35163   +33     
  Misses        631      631           

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[rtimms]

Consolidation note: this PR contains all of #5841's source changes (the SEI film size-average in base_interface.py and eta_r - eta_sei in the irreversible heating). #5841 lands first, and this gets rebased on it, so the diff shrinks to:

• BaseThermal._side_reaction_heating (SEI, SEI on cracks, lithium plating in porous electrodes)
• the lithium-metal branch, where the current is split into plating j_pl * (eta_r - eta_sei) and SEI growth j_sei * (delta_phi - U_sei)

Also checked against the other thermal PRs:

• fix: broadcast the average SEI film overpotential with a size distribution #5892 is independent. It fixes a DFN particle-size-distribution DomainError that's on main too, and builds with or without this PR.
• Sum the heat of mixing over every particle phase #5769 conflicts only textually, because _side_reaction_heating is inserted right above the _heat_of_mixing block it rewrites. Whichever lands second keeps both.

[github-actions]

Bencher Report

Project	PyBaMM
Branch	fix/side-reaction-heating
Testbed	bare-metal

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