[Bug]: Is it normal for the "Cell temperature [K]" to drop when using the battery model for charging simulation?

[limpid-11]

PyBaMM Version

22.12

Python Version

3.9

Describe the bug

When I did the charging simulation I pulled up the state volume "Cell temperature [K]" to check and it showed that the temperature of the battery was dropping during charging, normally charging should also make the battery temperature rise, right?

Steps to Reproduce

import pybamm as pb

pb.set_logging_level("NOTICE")
model = pb.lithium_ion.DFN(
    {
        "thermal": "lumped",
        "SEI": "ec reaction limited",
        "SEI film resistance": "distributed",
        "SEI porosity change": "true",
        "lithium plating": "irreversible",
        "lithium plating porosity change": "true",
    }
)

param = pb.ParameterValues("Mohtat2020")

experiment = pb.Experiment(
    [
        (
            "Charge at 1 C until 4.2 V",
            "Hold at 4.2 V until C/20"
        )
    ]
)

sim = pb.Simulation(model, experiment=experiment, parameter_values=param)
sim.solve(solver=pb.CasadiSolver(mode="safe"))
sim.plot(
    [
        "Current [A]",
        "Total current density [A.m-2]",
        "Terminal voltage [V]",
        "Cell temperature [K]",
        "X-averaged cell temperature [K]",
        "Discharge capacity [A.h]",
        "Total SEI thickness",
        "Negative electrode porosity",
        "X-averaged negative electrode porosity",
        "SEI interfacial current density [A.m-2]",
        "X-averaged total SEI thickness [m]",
        [
            "Total lithium lost [mol]",
            "Loss of lithium to SEI [mol]",
        ],
    ]
)

Relevant log output

No response

[rtimms]

The reversible heat source term can be negative, leading to cooling. See the plot below

sim.plot(
    [
        "Current [A]",
        "Total current density [A.m-2]",
        "Terminal voltage [V]",
        "Cell temperature [K]",
        "X-averaged cell temperature [K]",
        "Discharge capacity [A.h]",
        "Total SEI thickness",
        "Negative electrode porosity",
        "X-averaged negative electrode porosity",
        "SEI interfacial current density [A.m-2]",
        "X-averaged total SEI thickness [m]",
        [
            "Total lithium lost [mol]",
            "Loss of lithium to SEI [mol]",
        ],
        [
            "Volume-averaged Ohmic heating [W.m-3]",
            "Volume-averaged irreversible electrochemical heating [W.m-3]",
            "Volume-averaged reversible heating [W.m-3]",
        ],
    ]
)