fix: broadcast the average SEI film overpotential with a size distribution

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## Bug fixes
 
 - `pybamm` now installs on Python 3.15. `pybammsolvers` 0.11.0 declared `requires-python <3.15` and shipped no cp315 wheels, so `pybamm`'s solver requirement could not be met there. Released `pybammsolvers` v0.11.1, which allows Python 3.15 and ships cp315 wheels. ([#5909](https://github.com/pybamm-team/PyBaMM/pull/5909))
+- `"SEI film resistance": "average"` now works with `"particle size": "distribution"` in SPMe, DFN and Newman-Tobias, which raised a `DomainError` when building the model. ([#5892](https://github.com/pybamm-team/PyBaMM/pull/5892))
 
 # [v26.10.1.0](https://github.com/pybamm-team/PyBaMM/tree/pybamm-v26.10.1.0) - 2026-10-13
 

packages/pybamm/src/pybamm/models/submodels/interface/kinetics/base_kinetics.py

@@ -132,6 +132,10 @@ def get_coupled_variables(self, variables):
             R_sei = self.phase_param.R_sei
             L_sei_av = variables[f"X-averaged {domain} {phase_name}SEI thickness [m]"]
             eta_sei = -j_tot_av * L_sei_av * R_sei
+            # With a size distribution resolved through the electrode, eta_r has the
+            # electrode as its secondary domain, which a current collector term skips
+            if eta_r.domains["secondary"] == [f"{domain} electrode"]:
+                eta_sei = pybamm.PrimaryBroadcast(eta_sei, f"{domain} electrode")
         elif self.options["SEI film resistance"] == "distributed":
             R_sei = self.phase_param.R_sei
             L_sei = variables[f"{Domain} {phase_name}SEI thickness [m]"]

packages/pybamm/tests/unit/test_models/test_full_battery_models/test_lithium_ion/base_lithium_ion_tests.py

@@ -589,6 +589,17 @@ def test_well_posed_psd_swelling_only(self):
         }
         self.check_well_posedness(options)
 
+    def test_well_posed_psd_sei_film_resistance(self):
+        for film, j_tot_state in [("average", "false"), ("distributed", "true")]:
+            options = {
+                "particle size": "distribution",
+                "surface form": "algebraic",
+                "SEI": "constant",
+                "SEI film resistance": film,
+                "total interfacial current density as a state": j_tot_state,
+            }
+            self.check_well_posedness(options)
+
     def test_well_posed_psd_hysteresis_thermal(self):
         # Regression test: hysteresis OCP + particle size distribution + a
         # non-isothermal thermal submodel previously raised a DomainError in