SEI film resistance loss is not included in heat generation #5820

PyBaMM version: 26.8.0.0

Description

When any SEI option is on, "SEI film resistance" defaults to "distributed". The film's voltage drop (eta_sei = -j * L_sei * R_sei) is added to the reaction overpotential in BaseKinetics (eta_r += eta_sei), so it lowers the terminal voltage. But BaseThermal computes irreversible heat as a_j * eta_r using that reduced overpotential. The energy lost across the film therefore never appears in Total heating [W] or in the cell temperature. Energy is not conserved: electrical loss rises and heat does not.

To reproduce

OKane2022, SPMe, lumped thermal, SEI: "solvent-diffusion limited", 5 A discharge for 3000 s, comparing film resistance "distributed" against "none":

Total heat is 1951 J in every case, so at 50 nm about 10% of the true heat is missing.

import numpy as np, pybamm

def run(film, L):
    m = pybamm.lithium_ion.SPMe(options={
        "thermal": "lumped",
        "SEI": "solvent-diffusion limited",
        "SEI film resistance": "distributed" if film else "none",
    })
    p = pybamm.ParameterValues("OKane2022")
    p.update({"Initial SEI thickness [m]": L, "Current function [A]": 5.0})
    s = pybamm.Simulation(m, parameter_values=p).solve([0, 3000], initial_soc=1.0)
    return s["Time [s]"].entries, s["Voltage [V]"].entries, s["Total heating [W]"].entries

t, Va, Qa = run(True, 50e-9)
_, Vb, Qb = run(False, 50e-9)
print("film loss J:", np.trapezoid(5.0 * (Vb - Va), t))
print("extra heat J:", np.trapezoid(Qa - Qb, t))

Suggested fix

Add a film heating term a_j * eta_sei (and the equivalent for SEI on cracks) to the heat source. It could be a separate "SEI film heating [W]" variable that is included in Total heating [W]. In a local patch, adding film voltage × current to the heat recovered about 95% of the missing energy.