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Fix Q_heat always zero for isothermal cells; add unit tests #9

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DavidMStraub merged 2 commits into from
May 5, 2026
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Fix Q_heat always zero for isothermal cells; add unit tests #9

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ee3f7cb
Fix Q_heat always zero for isothermal cells; add unit tests
DavidMStraub May 5, 2026
ea36e25
Fixes
DavidMStraub May 5, 2026
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18 changes: 14 additions & 4 deletions src/pathsim_batt/cells/pybamm_cell.py
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Original file line number Diff line number Diff line change
Expand Up @@ -62,6 +62,7 @@ class _CellBase(DynamicalSystem):
"""

_thermal_option: str = ""
_thermal_extra_options: dict[str, str] = {}
_pybamm_output_vars: list[str] = []
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def __init__(
Expand All @@ -74,7 +75,9 @@ def __init__(
self._initial_soc = float(initial_soc)

if model is None:
model = pybamm.lithium_ion.SPMe(options={"thermal": self._thermal_option})
model = pybamm.lithium_ion.SPMe(
options={"thermal": self._thermal_option, **self._thermal_extra_options}
)

self._parameter_values = _prepare_parameter_values(parameter_values)

Expand Down Expand Up @@ -180,6 +183,7 @@ class _CoSimCellBase(Wrapper):
"""

_thermal_option: str = ""
_thermal_extra_options: dict[str, str] = {}
_pybamm_output_vars: list[str] = []

def __init__(
Expand All @@ -196,7 +200,9 @@ def __init__(
raise ValueError("dt must be positive")

if model is None:
model = pybamm.lithium_ion.SPMe(options={"thermal": self._thermal_option})
model = pybamm.lithium_ion.SPMe(
options={"thermal": self._thermal_option, **self._thermal_extra_options}
)

self._model = model
self._parameter_values = _prepare_parameter_values(parameter_values)
Expand Down Expand Up @@ -278,7 +284,8 @@ class CellElectrical(_CellBase):
Parameters
----------
model : pybamm.BaseBatteryModel or None
PyBaMM lithium-ion model. Defaults to ``SPMe(thermal="isothermal")``.
PyBaMM lithium-ion model. Defaults to isothermal SPMe with heat
source calculation enabled.
parameter_values : pybamm.ParameterValues or None
PyBaMM parameter set. Defaults to ``Chen2020``.
initial_soc : float
Expand All @@ -300,6 +307,7 @@ class CellElectrical(_CellBase):
"""

_thermal_option = "isothermal"
_thermal_extra_options = {"calculate heat source for isothermal models": "true"}
_pybamm_output_vars = [
"Terminal voltage [V]",
"X-averaged total heating [W.m-3]",
Expand Down Expand Up @@ -371,7 +379,8 @@ class CellCoSimElectrical(_CoSimCellBase):
Parameters
----------
model : pybamm.BaseBatteryModel or None
PyBaMM lithium-ion model. Defaults to ``SPMe(thermal="isothermal")``.
PyBaMM lithium-ion model. Defaults to isothermal SPMe with heat
source calculation enabled.
parameter_values : pybamm.ParameterValues or None
PyBaMM parameter set. Defaults to ``Chen2020``.
initial_soc : float
Expand All @@ -384,6 +393,7 @@ class CellCoSimElectrical(_CoSimCellBase):
"""

_thermal_option = "isothermal"
_thermal_extra_options = {"calculate heat source for isothermal models": "true"}
_pybamm_output_vars = [
"Terminal voltage [V]",
"X-averaged total heating [W.m-3]",
Expand Down
205 changes: 205 additions & 0 deletions tests/cells/test_pybamm_cell.py
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Original file line number Diff line number Diff line change
Expand Up @@ -190,6 +190,64 @@ def test_pathsim_state_advances(self):
self.sim.run(2)
self.assertFalse(np.allclose(self.cell.engine.state, state_before))

def test_q_heat_nonzero_during_discharge(self):
"""Q_heat must be strictly positive when a discharge current flows.

With thermal='isothermal' PyBaMM does not compute heat source terms,
so Q_heat would be identically zero — this test guards against that.
"""
cell = CellElectrical(initial_soc=1.0)
I_src = Constant(5.0) # 1C-ish discharge
T_src = Constant(298.15)
sim = Simulation(
blocks=[I_src, T_src, cell],
connections=[
Connection(I_src, cell["I"]),
Connection(T_src, cell["T_cell"]),
],
dt=10.0,
Solver=ESDIRK43,
)
sim.run(60)
self.assertGreater(
cell.outputs[1],
0.0,
"Q_heat is zero — thermal model may not compute heat sources",
)

def test_temperature_input_affects_voltage(self):
"""T_cell must actually influence the electrochemistry.

Butler-Volmer kinetics are temperature-dependent, so discharging at
a significantly higher temperature must yield a measurably different
terminal voltage after the same duration.
"""

def _run_and_get_voltage(T_cell):
cell = CellElectrical(initial_soc=1.0)
I_src = Constant(5.0)
T_src = Constant(T_cell)
sim = Simulation(
blocks=[I_src, T_src, cell],
connections=[
Connection(I_src, cell["I"]),
Connection(T_src, cell["T_cell"]),
],
dt=10.0,
Solver=ESDIRK43,
)
sim.run(300)
return cell.outputs[0] # terminal voltage [V]

V_cold = _run_and_get_voltage(278.15) # 5 °C
V_hot = _run_and_get_voltage(318.15) # 45 °C
self.assertNotAlmostEqual(
V_cold,
V_hot,
places=3,
msg="T_cell input has no effect on terminal voltage",
)


class TestElectrothermal(unittest.TestCase):
"""Integration tests for CellElectrothermal — PathSim integrates the PyBaMM ODE."""
Expand Down Expand Up @@ -240,6 +298,58 @@ def test_pathsim_state_advances(self):
self.sim.run(2)
self.assertFalse(np.allclose(self.cell.engine.state, state_before))

def test_q_heat_nonzero_during_discharge(self):
"""Q_heat must be strictly positive when a discharge current flows."""
cell = CellElectrothermal(initial_soc=1.0)
I_src = Constant(5.0)
T_src = Constant(298.15)
sim = Simulation(
blocks=[I_src, T_src, cell],
connections=[
Connection(I_src, cell["I"]),
Connection(T_src, cell["T_amb"]),
],
dt=10.0,
Solver=ESDIRK43,
)
sim.run(60)
self.assertGreater(
cell.outputs[2],
0.0,
"Q_heat is zero — thermal model may not compute heat sources",
)

def test_tamb_input_affects_cell_temperature(self):
"""T_amb must influence the output cell temperature.

With a lower ambient temperature the cell should run cooler after
the same discharge duration.
"""

def _run_and_get_T_cell(T_amb):
cell = CellElectrothermal(initial_soc=1.0)
I_src = Constant(5.0)
T_src = Constant(T_amb)
sim = Simulation(
blocks=[I_src, T_src, cell],
connections=[
Connection(I_src, cell["I"]),
Connection(T_src, cell["T_amb"]),
],
dt=10.0,
Solver=ESDIRK43,
)
sim.run(300)
return cell.outputs[1] # cell temperature [K]

T_cell_cold_amb = _run_and_get_T_cell(278.15) # 5 °C ambient
T_cell_hot_amb = _run_and_get_T_cell(318.15) # 45 °C ambient
self.assertLess(
T_cell_cold_amb,
T_cell_hot_amb,
msg="T_amb input has no effect on output cell temperature",
)


class TestCoSimulationElectrical(unittest.TestCase):
"""Integration tests for CellCoSimElectrical — PyBaMM performs the stepping."""
Expand Down Expand Up @@ -293,6 +403,55 @@ def test_dfn_step_outputs_physical(self):
self.assertGreater(cell.outputs[2], 0.0) # SOC
self.assertLessEqual(cell.outputs[2], 1.0)

def test_q_heat_nonzero_during_discharge(self):
"""Q_heat must be strictly positive when a discharge current flows."""
cell = CellCoSimElectrical(initial_soc=1.0, dt=10.0)
I_src = Constant(5.0)
T_src = Constant(298.15)
sim = Simulation(
blocks=[I_src, T_src, cell],
connections=[
Connection(I_src, cell["I"]),
Connection(T_src, cell["T_cell"]),
],
dt=5.0,
Solver=ESDIRK43,
)
sim.run(60)
self.assertGreater(
cell.outputs[1],
0.0,
"Q_heat is zero — thermal model may not compute heat sources",
)

def test_temperature_input_affects_voltage(self):
"""T_cell must actually influence the electrochemistry."""

def _run_and_get_voltage(T_cell):
cell = CellCoSimElectrical(initial_soc=1.0, dt=10.0)
I_src = Constant(5.0)
T_src = Constant(T_cell)
sim = Simulation(
blocks=[I_src, T_src, cell],
connections=[
Connection(I_src, cell["I"]),
Connection(T_src, cell["T_cell"]),
],
dt=5.0,
Solver=ESDIRK43,
)
sim.run(300)
return cell.outputs[0] # terminal voltage [V]

V_cold = _run_and_get_voltage(278.15) # 5 °C
V_hot = _run_and_get_voltage(318.15) # 45 °C
self.assertNotAlmostEqual(
float(V_cold),
float(V_hot),
places=3,
msg="T_cell input has no effect on terminal voltage",
)


class TestCoSimulationElectrothermal(unittest.TestCase):
"""Integration tests for CellCoSimElectrothermal — PyBaMM performs the stepping."""
Expand Down Expand Up @@ -349,6 +508,52 @@ def test_dfn_step_outputs_physical(self):
self.assertGreater(cell.outputs[3], 0.0) # SOC
self.assertLessEqual(cell.outputs[3], 1.0)

def test_q_heat_nonzero_during_discharge(self):
"""Q_heat must be strictly positive when a discharge current flows."""
cell = CellCoSimElectrothermal(initial_soc=1.0, dt=10.0)
I_src = Constant(5.0)
T_src = Constant(298.15)
sim = Simulation(
blocks=[I_src, T_src, cell],
connections=[
Connection(I_src, cell["I"]),
Connection(T_src, cell["T_amb"]),
],
dt=5.0,
Solver=ESDIRK43,
)
sim.run(60)
self.assertGreater(
cell.outputs[2],
0.0,
"Q_heat is zero — thermal model may not compute heat sources",
)

def test_tamb_input_affects_cell_temperature(self):
"""T_amb must influence the output cell temperature."""

def _run_and_get_T_cell(T_amb):
cell = CellCoSimElectrothermal(initial_soc=1.0, dt=10.0)
I_src = Constant(5.0)
T_src = Constant(T_amb)
sim = Simulation(
blocks=[I_src, T_src, cell],
connections=[
Connection(I_src, cell["I"]),
Connection(T_src, cell["T_amb"]),
],
dt=5.0,
Solver=ESDIRK43,
)
sim.run(300)
return cell.outputs[1] # cell temperature [K]

T_cold = _run_and_get_T_cell(278.15)
T_hot = _run_and_get_T_cell(318.15)
self.assertLess(
T_cold, T_hot, msg="T_amb input has no effect on output cell temperature"
)


if __name__ == "__main__":
unittest.main()
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