Add PyBaMM 26.9 support

.github/workflows/test.yml

@@ -31,18 +31,18 @@ jobs:
       matrix:
         include:
           # All Python versions against the latest PyBaMM
-          - {python-version: "3.10", pybamm-version: "26.8"}
-          - {python-version: "3.11", pybamm-version: "26.8"}
-          - {python-version: "3.12", pybamm-version: "26.8"}
-          - {python-version: "3.13", pybamm-version: "26.8"}
-          - {python-version: "3.14", pybamm-version: "26.8"}
-          # Older PyBaMM versions against the latest Python they support
-          - {python-version: "3.13", pybamm-version: "25.12"}
-          - {python-version: "3.14", pybamm-version: "26.3"}
+          - {python-version: "3.10", pybamm-version: "26.9"}
+          - {python-version: "3.11", pybamm-version: "26.9"}
+          - {python-version: "3.12", pybamm-version: "26.9"}
+          - {python-version: "3.13", pybamm-version: "26.9"}
+          - {python-version: "3.14", pybamm-version: "26.9"}
+          # Older PyBaMM versions against the latest Python they support.
+          # Keep exactly the latest 6 releases (see policy in pyproject.toml).
           - {python-version: "3.14", pybamm-version: "26.4"}
           - {python-version: "3.14", pybamm-version: "26.5"}
           - {python-version: "3.14", pybamm-version: "26.6"}
           - {python-version: "3.14", pybamm-version: "26.7"}
+          - {python-version: "3.14", pybamm-version: "26.8"}
 
     steps:
       - uses: actions/checkout@v4

pyproject.toml

@@ -32,7 +32,12 @@ dependencies = [
     # is skipped on Emscripten so the pure-Python parts of the toolbox
     # (`pathsim_batt.thermal`) still install in the browser.
     # Upper-bounded: each new minor release is vetted before being allowed.
-    "pybamm>=25.12,<26.9;sys_platform != 'emscripten'",
+    # Support policy: the latest 6 PyBaMM releases (~6 months). Adding a new
+    # release drops the oldest in the same change — raise the lower bound,
+    # remove its CI row and any workarounds only it needed. A version may be
+    # dropped earlier if it blocks a fix or feature. Raising the lower bound
+    # requires a minor (not patch) pathsim-batt release.
+    "pybamm>=26.4,<26.10; sys_platform != 'emscripten'",
 ]
 
 [project.optional-dependencies]

tests/cells/test_lead_acid.py

@@ -83,7 +83,7 @@ def test_monolithic_electrothermal_raises(self):
     def test_cosim_electrical_smoke(self):
         # On PyBaMM < 26.7, LOQS disables its Jacobian and IDAKLUSolver (the
         # co-sim default) errors without one; CasadiSolver works on every
-        # pinned version (25.12-26.8), so use it explicitly here.
+        # pinned version (26.4-26.9), so use it explicitly here.
         solver = pybamm.CasadiSolver(mode="safe")
         cell = CellCoSimElectrical(
             model=self._model(),