REVIEW 4 major objections 6 minor 45 references
Koopman Spectral Analysis of Lithium-Ion Battery Dynamics: State of Charge as a Marginally Stable Observable
T0 review · 4 major / 6 minor · reviewed 2026-07-10 · grok-4.5
Pith's one-line read State of charge appears as the battery's slowest, marginally stable Koopman mode, recovered from voltage and current alone without circuit parameters.
desk verdict Solid application of Hankel-DMDc that recovers the charge-conservation integrator as a near-unit eigenvalue; useful for BMS people, but the 0.0043% SOC RMSE and single-trajectory min-max scaling keep it provisional. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
Hankel-DMDc operator: voltage is stacked into a delay-embedded Hankel matrix so that the nonlinear battery map becomes approximately linear; DMDc then yields the pair (A,B) whose eigen-decomposition of A produces the Koopman eigenvalues and modes that separate SOC from faster polarization dynamics.
What would settle it
Re-run the identical Hankel-DMDc pipeline on a second cell of the same chemistry under a different temperature or after measurable aging; if the eigenvalue nearest unity no longer tracks reference SOC (or another mode becomes closer to the unit circle), the spectral identification claim fails.
Extended reading notes
Core claim
When battery voltage and current from HPPC testing are lifted by Hankel delay embedding and the resulting input-output map is identified by DMDc, eigen-decomposition of the learned state-transition matrix isolates SOC dynamics as the unique slowest, marginally stable mode (eigenvalue closest to 1). The associated modal coordinate, after min-max normalization over the operating range, supplies a quantitatively usable SOC estimate without any explicit equivalent-circuit parameter identification.
Load-bearing premise
A single linear operator learned from one HPPC trajectory in a 2000-dimensional delay space remains faithful enough that the eigenvalue nearest unity can be unambiguously labeled the SOC mode and simply rescaled by min-max to give quantitative state of charge.
Editorial extensions
If this is right
- SOC can be read directly from the spectrum of a data-driven operator rather than from an equivalent-circuit model that must be re-identified whenever temperature or age changes.
- Voltage reconstruction and SOC estimation become a single eigen-decomposition step once the Hankel-DMDc operator is known.
- Long-term drift of the same eigenvalue or its associated mode may later serve as a spectral signature of capacity fade.
- The same workflow can be re-applied to other chemistries or packs without rewriting physics-based state equations.
Reading between the lines
- If mode mixing is the main obstacle, sparse or orthogonalized Koopman dictionaries could further separate SOC from hysteresis and diffusion without enlarging the Hankel dimension.
- Adaptive re-identification of the DMDc operator on a sliding window would turn the present offline spectral snapshot into an online BMS estimator that tracks aging.
- The same marginally stable mode idea may transfer to other electrochemical storage devices (fuel cells, supercapacitors) whose charge conservation likewise produces an integrator pole.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes a data-driven SOC estimation framework based on Dynamic Mode Decomposition with control (DMDc) applied to Hankel time-delay embeddings of terminal voltage, with applied current as the control input, using HPPC measurements. After identifying a linear lifted operator, eigen-decomposition is used to argue that SOC appears as the slowest, marginally stable Koopman mode (eigenvalue nearest the unit circle), consistent with the continuous-time integrator pole of charge conservation. The corresponding modal coordinate, after min–max normalization over the operating SOC range, is reported as an SOC estimate (RMSE 0.0043%), while the same model reconstructs terminal voltage with RMSE 0.0131 V, outperforming a 2RC-ECM EKF and Coulomb counting on the same HPPC cycle.
Significance. If the spectral identification of SOC as a robust, physically grounded Koopman mode holds under broader conditions, the work would offer a useful middle path between ECM/Kalman estimators (which require repeated parameter identification) and black-box learning methods (which often lack interpretability). The theoretical link from charge conservation (pole at s=0) to a discrete eigenvalue at unity is standard and correctly stated, and framing SOC as a marginally stable observable is a clear, falsifiable spectral claim. Voltage reconstruction performance is credible and supports the utility of Hankel–DMDc for input–output battery modeling. The contribution is primarily methodological and interpretive rather than a fully validated BMS-ready estimator; its lasting value depends on whether mode selection and quantitative SOC mapping remain reliable beyond a single calibrated HPPC trajectory.
major comments (4)
- Quantitative SOC is obtained only after min–max normalization of the modal coordinate over the experimental operating SOC range (Section 2.5, Eq. 9; Section 3.5). This is an affine calibration that requires knowledge of the SOC bounds (or equivalent end-point labels). The abstract and Table 2 present RMSE 0.0043% as if the spectrum alone supplies a quantitative SOC observable; the manuscript should clearly separate (i) spectral mode identification from (ii) the subsequent data-dependent scaling, and report accuracy of the raw modal coordinate (or a fixed calibration) rather than only the post-normalized signal. The conclusion already flags this limitation; it should be reflected in the claims and tables.
- Table 2 reports SOC RMSE of 0.0043% (DMDc), 0.0047% (Coulomb counting), and 0.0078% (EKF). These values are orders of magnitude below typical laboratory SOC estimation errors and below what sensor noise and Coulombic-efficiency uncertainty usually allow on multi-hour HPPC profiles. The reference is described as “derived from experimental discharge capacity data,” which is often itself an ampere-hour integral of the same current. The paper must specify exactly how the reference SOC is constructed, whether it is independent of the Coulomb-counting baseline, and why all three methods achieve sub-0.01% RMSE. Without that, the performance comparison does not support the claim of a superior SOC-sensitive observable.
- Section 3.4 identifies mode 457 with λ_SOC = 1.000001 among 2000 modes as the unique SOC mode because it is “closest to the unit circle.” The reported eigenvalue lies slightly outside the unit circle (mildly unstable), with no discussion of numerical tolerance, isolation gap to the next-nearest eigenvalues, or sensitivity to Hankel dimension, SVD truncation rank, or train/test split. A load-bearing claim that SOC “naturally emerges” requires evidence that this selection is unique and stable (e.g., eigenvalue gap plot, ablation over embedding dimension/rank, and at least one independent trajectory or operating condition).
- All identification and evaluation use a single HPPC trajectory (≈75% train / held-out remainder of the same test; Sections 3.1–3.3). There is no second cell, temperature, aging state, or drive-cycle hold-out that freezes the learned operator and tests only mode selection and the SOC coordinate. Given the free parameters (embedding dimension d=2000, SVD truncation, min–max bounds), the central claim that the Koopman spectrum itself supplies a robust SOC observable remains under-supported for generalization beyond this dataset.
minor comments (6)
- Table 1: EKF voltage MAE is listed as 0.0007 V while RMSE is 0.0439 V. Such a large RMSE–MAE gap is possible only with rare extreme outliers; please verify the numbers and, if correct, briefly explain the residual structure (e.g., pulse spikes).
- Figure 3A caption refers to “long-timescale capacity fade behavior” for the SOC mode; the experiment is a single HPPC cycle and does not measure capacity fade. Align the caption with the charge-conservation / OCV interpretation used in the text.
- Notation: continuous-time eigenvalue is written λ_c = 0 and discrete λ_d = 1 (Eqs. 7–8), while the identified value is 1.000001; state consistently whether “closest to unity” or “on the unit circle” is the selection criterion.
- Section 2.6 and Eq. (14): EKF noise covariances are given without tuning rationale or sensitivity; a short note would help readers reproduce the baseline.
- References [2] and [3] share nearly identical generic titles (“Review of battery management systems”); expand bibliographic detail for traceability.
- Abstract and keywords: “0.0043%” should be reconciled with whatever unit convention is adopted after addressing the reference-SOC definition (percentage points vs. absolute SOC fraction).
Circularity Check
Theoretical SOC-mode selection (nearest unit-circle eigenvalue) is independent, but quantitative SOC values are obtained by min-max normalization of the modal coordinate over the same experiment’s operating range, forcing absolute scale by construction.
-
fitted input called prediction
[Section 2.5 (SOC Mode Identification), Eq. 9 and following paragraph]
"The row of Y corresponding to the SOC eigenvalue (λ_d ≈1) yields the raw SOC-sensitive modal coordinate. After Min–Max Normalization of this raw coordinate over the operating SOC range, the resulting signal provides the DMDc-based SOC estimate."
Min-max normalization is an affine map whose two free parameters are fixed by the min and max of the modal coordinate and the known SOC endpoints of the same HPPC experiment. Absolute SOC values are therefore forced by construction; only the intermediate shape remains free. The reported RMSE of 0.0043 % consequently evaluates a data-calibrated signal rather than an independent prediction of SOC.
full rationale
The derivation that charge conservation implies a continuous-time pole at s=0 and therefore a discrete eigenvalue of exactly 1 (Eqs. 6–8) is self-contained and non-circular; selecting the empirical eigenvalue nearest the unit circle (reported λ_SOC=1.000001) therefore has independent theoretical grounding. Voltage reconstruction via Hankel-DMDc is a standard least-squares fit evaluated on a held-out portion of the HPPC trajectory and is likewise non-circular. The only load-bearing circular step is the conversion of the raw modal coordinate into a numerical SOC estimate: min-max normalization maps the extrema of that coordinate exactly onto the known SOC operating range of the identical experiment, so absolute SOC values are forced by an affine map fitted to the same data whose shape is then scored. The resulting RMSE (0.0043 %) therefore measures fidelity of a calibrated signal rather than an uncalibrated, parameter-free prediction. No self-citation is load-bearing for the central claim, and no uniqueness theorem or ansatz is smuggled in. The circularity is partial and confined to the final quantitative mapping; the spectral identification itself remains independent.
Assumptions & free parameters
free parameters (5)
- Hankel embedding dimension d =
2000
- SVD truncation rank for DMDc pseudoinverse
- min-max normalization bounds for SOC modal coordinate =
operating SOC range of the HPPC test
- EKF process and measurement noise covariances =
Q=diag(1e-6,1e-3,1e-3), R=1e-4
- train/test split fraction =
~75 %
assumptions (4)
- domain assumption Charge conservation implies a continuous-time pole at s=0, which maps under sampling to a discrete eigenvalue exactly equal to 1.
- domain assumption A finite Hankel delay embedding of terminal voltage yields a Koopman-invariant subspace in which the battery dynamics are approximately linear.
- ad hoc to paper The eigenvalue of the identified DMDc operator that lies closest to the unit circle can be uniquely identified with the SOC mode.
- ad hoc to paper Min-max normalization of the corresponding modal coordinate over the experimental SOC range produces a quantitatively accurate SOC estimate.
invented entities (1)
-
SOC-sensitive modal coordinate (row of Y = Φ^{-1} H_v corresponding to λ≈1)
Cite this review
Pith. "Pith review of Koopman Spectral Analysis of Lithium-Ion Battery Dynamics: State of Charge as a Marginally Stable Observable." pith.science (2026). https://pith.science/paper/FEJHOGPN
@misc{pith2026260707594,
author = {Pith},
title = {Pith review of: Koopman Spectral Analysis of Lithium-Ion Battery Dynamics: State of Charge as a Marginally Stable Observable},
year = {2026},
howpublished = {\url{https://pith.science/paper/FEJHOGPN}},
note = {Machine review of arXiv:2607.07594}
}
read the original abstract
Accurate state-of-charge (SOC) estimation remains a fundamental challenge in lithium-ion battery management systems because battery dynamics are highly nonlinear, operating-condition dependent, and sensitive to parameter variations caused by aging and temperature. Conventional model-based estimators, such as equivalent circuit model (ECM) and Kalman-filter-based approaches, rely heavily on repeated parameter identification and accurate electrochemical modeling, whereas purely data-driven methods often sacrifice physical interpretability. This work proposes a Koopman-theoretic, data-driven framework for SOC estimation using Dynamic Mode Decomposition with control (DMDc) combined with Hankel time-delay embedding. Instead of explicitly identifying ECM parameters, the proposed approach reconstructs a lifted dynamical state space directly from measured terminal voltage and current obtained through Hybrid Pulse Power Characterization (HPPC) testing. Spectral decomposition of the identified DMDc operator reveals intrinsic battery dynamics in terms of Koopman modes and eigenvalues. The SOC dynamics naturally emerge as the slowest marginally stable mode whose eigenvalue lies closest to the unit circle, consistent with the integrator-type behavior of charge conservation. The corresponding modal coordinate is subsequently utilized as an SOC-sensitive observable.
Figures
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Reference graph
Works this paper leans on
-
[1]
Xiong, Rui, Li, Linlin and Tian, Jinpeng. “Towards a smarter battery management system: A critical review on battery state of health monitoring methods.”Journal of Power Sources Vol. 405 (2018): pp. 18–29. DOI 10.1016/j.jpowsour.2018.10.019
-
[2]
Review of battery management systems
Wang, Y et al. “Review of battery management systems.”Applied EnergyVol. 228 (2017): pp. 1512–1525
work page 2017
-
[3]
Review of battery management systems
Feng, X et al. “Review of battery management systems.”Renewable and Sustainable Energy ReviewsVol. 103 (2019): pp. 60–75
work page 2019
-
[4]
State-of-charge estimation of lithium-ion batteries
He, Hongwen et al. “State-of-charge estimation of lithium-ion batteries.”EnergiesVol. 4 No. 4 (2011): pp. 582–598
work page 2011
-
[5]
Critical review of state of health estimation methods
Berecibar, M et al. “Critical review of state of health estimation methods.”Renewable and Sustainable Energy ReviewsVol. 56 (2016): pp. 572–587. 11
work page 2016
-
[6]
State estimation techniques for lithium-ion batteries
Zheng, Y et al. “State estimation techniques for lithium-ion batteries.”EnergiesVol. 11 (2018): p. 1822
work page 2018
-
[7]
A Critical Look at Coulomb Counting Approach for State of Charge Estimation in Batteries
Movassagh, Koosha, Raihan, Ahmed, Bhattacharya, Bhaskar and Pattipati, Krishna. “A Critical Look at Coulomb Counting Approach for State of Charge Estimation in Batteries.” EnergiesVol. 14 No. 14 (2021): p. 4074. DOI 10.3390/en14144074
-
[8]
Methods for state-of-charge determination
Piller, S et al. “Methods for state-of-charge determination.”Journal of Power SourcesVol. 96 (2001): pp. 113–120
work page 2001
Show all 45 references
-
[9]
Enhanced coulomb counting method
Ng, K et al. “Enhanced coulomb counting method.”Applied EnergyVol. 86 (2011): pp. 1506–1511
2011
-
[10]
A comparative study of equivalent circuit models for Li-ion batteries
Hu, Xiaosong, Li, Shengbo and Peng, Huei. “A comparative study of equivalent circuit models for Li-ion batteries.”Journal of Power SourcesVol. 198 (2012): pp. 359–367. DOI 10.1016/j. jpowsour.2011.10.013
2012 doi
-
[11]
A comparative study of different equivalent circuit models for estimating state-of-charge of lithium-ion batteries
Lai, Xin, Zheng, Yuejiu and Sun, Tao. “A comparative study of different equivalent circuit models for estimating state-of-charge of lithium-ion batteries.”Electrochimica ActaVol. 259 (2018): pp. 566–577. DOI 10.1016/j.electacta.2017.10.153
2018 doi
-
[12]
Review on battery modeling methods
Zhang, S. “Review on battery modeling methods.”Renewable and Sustainable Energy Reviews Vol. 15 (2011): pp. 3126–3131
2011
-
[13]
Identification of battery model parameters
Barai, A et al. “Identification of battery model parameters.”Journal of Power SourcesVol. 280 (2015): pp. 74–80
2015
-
[14]
Extended Kalman filtering for battery management systems of LiPB-based HEV battery packs: Part 1. Background
Plett, Gregory L. “Extended Kalman filtering for battery management systems of LiPB-based HEV battery packs: Part 1. Background.”Journal of Power SourcesVol. 134 No. 2 (2004): pp. 252–261. DOI 10.1016/j.jpowsour.2004.02.031
2004 doi
-
[15]
Unscented Kalman filtering for battery management systems
Plett, Gregory L. “Unscented Kalman filtering for battery management systems.”Journal of Power SourcesVol. 134 (2004): pp. 277–292
2004
-
[16]
Sigma-point Kalman filtering for battery management systems
Plett, Gregory L. “Sigma-point Kalman filtering for battery management systems.”Journal of Power SourcesVol. 161 (2004): pp. 1354–1368
2004
-
[17]
Adaptive EKF for lithium-ion battery SOC estimation
Sun, Fengchun et al. “Adaptive EKF for lithium-ion battery SOC estimation.”EnergyVol. 36 (2011): pp. 3531–3540
2011
-
[18]
A Comparative Study of SOC Estimation Based on Equivalent Circuit Models
Wei, Meng, Ye, Min, Li, Jian-Bo, Wang, Qiao and Xu, Xueqiang. “A Comparative Study of SOC Estimation Based on Equivalent Circuit Models.”Frontiers in Energy ResearchVol. 10 (2022): p. 914291. DOI 10.3389/fenrg.2022.914291
2022 doi
-
[19]
Modeling of galvanostatic charge and discharge of the lithium/polymer/insertion cell
Doyle, Marc, Fuller, Thomas F. and Newman, John. “Modeling of galvanostatic charge and discharge of the lithium/polymer/insertion cell.”Journal of the Electrochemical SocietyVol. 140 No. 6 (1993): pp. 1526–1533. DOI 10.1149/1.2221597
1993 doi
-
[20]
Modeling of lithium-ion batteries
Newman, John and Thomas-Alyea, Karen. “Modeling of lithium-ion batteries.”Electrochem- ical Systems(2004)
2004
-
[21]
Review of models for predicting battery performance
Santhanagopalan, S and White, R. “Review of models for predicting battery performance.” Journal of Power SourcesVol. 156 (2006): pp. 620–628. 12
2006
-
[22]
Control oriented battery models
Smith, K and Wang, C. “Control oriented battery models.”Journal of Power SourcesVol. 160 (2007): pp. 662–673
2007
-
[23]
Long short-term memory networks for accurate state-of-charge estimation of Li-ion batteries
Chemali, Ephrem, Kollmeyer, Phillip J., Preindl, Matthias, Ahmed, Ryan and Emadi, Ali. “Long short-term memory networks for accurate state-of-charge estimation of Li-ion batteries.” IEEE Transactions on Industrial ElectronicsVol. 65 No. 8 (2018): pp. 6730–6739. DOI 10.1109/TIE...
2018 doi
-
[24]
Deep learning based battery state estimation
Li, Y et al. “Deep learning based battery state estimation.”Applied EnergyVol. 250 (2019): pp. 983–993
2019
-
[25]
State estimation using deep neural networks
Yang, F et al. “State estimation using deep neural networks.”EnergyVol. 201 (2020): p. 117664
2020
-
[26]
Battery health prediction using machine learning
Sehgal, A et al. “Battery health prediction using machine learning.”Applied EnergyVol. 250 (2019): pp. 1110–1121
2019
-
[27]
Dynamic mode decomposition of numerical and experimental data
Schmid, Peter J. “Dynamic mode decomposition of numerical and experimental data.”Journal of Fluid MechanicsVol. 656 (2010): pp. 5–28. DOI 10.1017/S0022112010001217
2010 doi
-
[28]
Spectral analysis of nonlinear flows
Rowley, Clarence et al. “Spectral analysis of nonlinear flows.”Journal of Fluid Mechanics Vol. 641 (2009): pp. 115–127
2009
-
[29]
Spectral properties of dynamical systems
Mezic, Igor. “Spectral properties of dynamical systems.”Nonlinear DynamicsVol. 41 (2005): pp. 309–325
2005
-
[30]
Analysis of fluid flows via Koopman operator
Mezic, Igor. “Analysis of fluid flows via Koopman operator.”Annual Review of Fluid Me- chanicsVol. 45 (2013): pp. 357–378
2013
-
[31]
On dynamic mode decomposition
Tu, Jonathan H et al. “On dynamic mode decomposition.”Journal of Computational Dynamics Vol. 1 (2014): pp. 391–421
2014
-
[32]
Dynamic Mode Decomposition with Control
Proctor, Joshua L., Brunton, Steven L. and Kutz, J. Nathan. “Dynamic Mode Decomposition with Control.”SIAM Journal on Applied Dynamical SystemsVol. 15 No. 1 (2016): pp. 142–
2016
-
[33]
DOI 10.1137/15M1013857
-
[34]
Springer (2020)
Mauroy, Alexandre et al.The Koopman Operator in Systems and Control. Springer (2020)
2020
-
[35]
Koopman operator based model reduction
Peitz, Sebastian and Klus, Stefan. “Koopman operator based model reduction.”Automatica Vol. 106 (2019): pp. 184–192
2019
-
[36]
Linear observer synthesis via Koopman operator
Surana, Amit and Banaszuk, Andrzej. “Linear observer synthesis via Koopman operator.” IF ACVol. 49 (2016): pp. 716–723
2016
-
[37]
Detecting strange attractors in turbulence
Takens, Floris. “Detecting strange attractors in turbulence.”Lecture Notes in Mathematics Vol. 898 (1981): pp. 366–381
1981
-
[38]
Ergodic theory, Koopman analysis and computation
Arbabi, Hassan and Mezic, Igor. “Ergodic theory, Koopman analysis and computation.”SIAM Journal on Applied Dynamical SystemsVol. 16 (2017): pp. 2096–2126
2017
-
[39]
Chaos as an inter- mittently forced linear system
Brunton, Bingni W., Proctor, Joshua L. and Kutz, J. Nathan. “Chaos as an inter- mittently forced linear system.”Nature CommunicationsVol. 8 (2017): p. 19. DOI 10.1038/s41467-017-00030-8. 13
2017 doi
-
[40]
Time-Delay Observ- ables for Koopman: Theory and Applications
Kamb, Mason, Kaiser, Eurika, Brunton, Steven L. and Kutz, J. Nathan. “Time-Delay Observ- ables for Koopman: Theory and Applications.”SIAM Journal on Applied Dynamical Systems Vol. 19 No. 2 (2020): pp. 886–917. DOI 10.1137/18M1216572
2020 doi
-
[41]
SIAM (2016)
Kutz, Nathan et al.Dynamic Mode Decomposition. SIAM (2016)
2016
-
[42]
Cambridge University Press (2022)
Brunton, Steven and Kutz, Nathan.Data-Driven Science and Engineering, 2nd ed. Cambridge University Press (2022)
2022
-
[43]
Pulse power characterization of lithium-ion batteries
Smith, K et al. “Pulse power characterization of lithium-ion batteries.”Journal of Power SourcesVol. 196 (2010): pp. 8723–8731
2010
-
[44]
Evaluation of commercial lithium-ion cells
Dubarry, M and Liaw, B. “Evaluation of commercial lithium-ion cells.”Journal of Power SourcesVol. 195 (2010): pp. 5415–5425
2010
-
[45]
Modeling of Non-linear Dynamics of Lithium-ion Batteries via Delay-Embedded Dynamic Mode Decomposition
Labib, Khalid Mahmud and Ahmed, Shabbir. “Modeling of Non-linear Dynamics of Lithium-ion Batteries via Delay-Embedded Dynamic Mode Decomposition.”arXiv preprint arXiv:2601.22403(2026)URLhttps://arxiv.org/abs/2601.22403. 14
2026
Reviewed July 10, 2026 · model on record in the stance chip above.
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