[Bug]: DFN not updating volumetric surface area when particle radius is changed

[stuadler]

PyBaMM Version

25.12.2

Python Version

3.12

Describe the bug

Based on the solutions I am seeing for X gradients of Li concentration and also electrode reaction overpotential, it seems that PyBaMM is not updating the volume-specific area (3*solid_frac/r_p) when the particle radius (r_p) is changed. The concentration polarization changes (as expected), but the volumetric reactivity does not seem to change. Model is P2D (DFN) using Chen 2020 base parameters.

Steps to Reproduce

Model Chen 2020 at 1C, plotting various contributions to cell polarization. Then change positive particle radius to something 10 times smaller and repeat. You'll see the concentration polarization get tiny, but the utilization length stays about the same and the net electrode polarization loss is about the same.

Relevant log output

[rtimms]

Thanks @stuadler, could you please share a minimum working example to test this? The volume-specific area is calculated based on the radius here

PyBaMM/src/pybamm/models/submodels/active_material/base_active_material.py

Line 103 in 6837493

a = 3 * eps_solid / R

[stuadler]

Hi Robert. Now that I’ve worked on this a bit longer, I am questioning my original assertion that PyBaMM is not updating the surface area. I think the problem may actually be lack of convergence of the solution when the particles are made very small. At high surface areas, gradients within the electrode become sigmoidal. As behavior approaches the Newman reaction zone limit, PyBaMM has a really hard time calculating the gradients, at leastr with default mesh and convergence criteria. Maybe you guys have advice how to optimize convergence for electrochemically thick electrodes approaching the reaction zone limit. Stu

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On Mar 3, 2026, at 2:19 AM, Robert Timms ***@***.***> wrote: rtimms left a comment (pybamm-team/PyBaMM#5401) <https://urldefense.com/v3/__https://github.com/pybamm-team/PyBaMM/issues/5401*issuecomment-3990037621__;Iw!!K-Hz7m0Vt54!kj12PAhCqIl83glLHxDkvuObvb91zqowoq_MbLALXLdExv6MkoKHwIEIff7i9myAv85xu8LyL_4XS0v89364TY0$> Thanks @stuadler <https://urldefense.com/v3/__https://github.com/stuadler__;!!K-Hz7m0Vt54!kj12PAhCqIl83glLHxDkvuObvb91zqowoq_MbLALXLdExv6MkoKHwIEIff7i9myAv85xu8LyL_4XS0v8kqI4mhs$>, could you please share a minimum working example to test this? The volume-specific area is calculated based on the radius here https://github.com/pybamm-team/PyBaMM/blob/68374936e8eda32cd88af04517ed41b49d62d796/src/pybamm/models/submodels/active_material/base_active_material.py#L103 <https://urldefense.com/v3/__https://github.com/pybamm-team/PyBaMM/blob/68374936e8eda32cd88af04517ed41b49d62d796/src/pybamm/models/submodels/active_material/base_active_material.py*L103__;Iw!!K-Hz7m0Vt54!kj12PAhCqIl83glLHxDkvuObvb91zqowoq_MbLALXLdExv6MkoKHwIEIff7i9myAv85xu8LyL_4XS0v8_MajQok$> — Reply to this email directly, view it on GitHub <https://urldefense.com/v3/__https://github.com/pybamm-team/PyBaMM/issues/5401*issuecomment-3990037621__;Iw!!K-Hz7m0Vt54!kj12PAhCqIl83glLHxDkvuObvb91zqowoq_MbLALXLdExv6MkoKHwIEIff7i9myAv85xu8LyL_4XS0v89364TY0$>, or unsubscribe <https://urldefense.com/v3/__https://github.com/notifications/unsubscribe-auth/AOP37POECE6F5LBDUCQGXID4O2WTLAVCNFSM6AAAAACWC45DQOVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZTSOJQGAZTONRSGE__;!!K-Hz7m0Vt54!kj12PAhCqIl83glLHxDkvuObvb91zqowoq_MbLALXLdExv6MkoKHwIEIff7i9myAv85xu8LyL_4XS0v8nyb9a9A$>. You are receiving this because you were mentioned.