REVIEW 4 major objections 7 minor 31 references
Combining CC and CV charging indicators beats either alone for battery health, and ordinary cross-validation overstates accuracy by about 119%.
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
T0 review · grok-4.5
2026-07-30 21:08 UTC pith:4VXGWNJD
load-bearing objection Solid NASA LOBO head-to-head of CC/CV indicators with a clear 5-fold vs LOBO warning; complementarity ranking is real but statistically thin on four folds. the 4 major comments →
Charging Phase Health Indicators for Battery State-of-Health Estimation: A Systematic Comparison of CC, CV, and Combined Approaches under Cross-Battery Validation
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Under Leave-One-Battery-Out validation on four NASA LiCoO2 cells, the combined set of four CV-phase indicators plus CC phase duration achieves the best SOH estimates (R² = 0.874, RMSE 3.68%), beating CV-only (R² = 0.796) and CC-duration alone (R² = 0.845). That ranking shows CC and CV phases capture complementary degradation. Separately, LOBO RMSE averages about 119% higher than 5-fold cross-validation across models, so conventional splits substantially overestimate practical cross-battery accuracy.
What carries the argument
Leave-One-Battery-Out (LOBO) comparison of indicator sets: CV duration, CV-to-CC time ratio, current-decay time constant τ, CV charge throughput, and CC duration, scored with LightGBM (and checked against other gradient-boosting models) plus SHAP importance, which ranks the CV-to-CC ratio highest.
Load-bearing premise
That findings from four room-temperature NASA LiCoO2 18650 cells on one fixed CC–CV protocol are representative enough to guide indicator choice on other chemistries, temperatures, and BMS cutoff settings.
What would settle it
Repeat the same LOBO indicator-set comparison on another public aging set (different chemistry or CC–CV cutoffs); if combined CC+CV no longer beats CC-only and CV-only, or the CV-vs-LOBO gap collapses, the central ranking and the overestimation claim fail.
If this is right
- When full CC–CV logs are available, prefer the five-indicator combined set for highest cross-battery SOH accuracy.
- When only CV is logged or CC is adaptive/unstable, CV-only indicators remain usable without numerical differentiation.
- Expect roughly 4–5% RMSE on unseen batteries of this type, not the ~2% suggested by random 5-fold CV.
- SHAP ranking supports treating the dimensionless CV-to-CC time ratio as a primary, noise-robust health feature in BMS design.
- Simple CV-duration thresholds can support field maintenance alarms without a full SOH model.
Where Pith is reading between the lines
- If the 119% CV–LOBO gap is typical, many published sub-1% SOH numbers from mixed-cycle splits are not deployment-ready until re-checked with battery-held-out protocols.
- The complementarity claim suggests multi-phase feature design may matter more than swapping among similar tree ensembles, which the paper’s model comparison already shows cluster tightly under LOBO.
- A natural next test is whether the same CV-to-CC ratio stays top-ranked after temperature swings or after recalibrating the CV voltage threshold on CALCE/Oxford-style datasets.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript compares health indicators extracted from the constant-current (CC) and constant-voltage (CV) phases of CC–CV charging for battery state-of-health (SOH) estimation. Using four NASA LiCoO2 18650 cells (B0005/6/7/18) and LightGBM (plus XGBoost, CatBoost, Random Forest baselines), the authors evaluate four CV-phase indicators (CV duration, CV/CC time ratio, current-decay time constant, CV charge throughput) and CC duration, individually and combined, under Leave-One-Battery-Out (LOBO) validation. Headline results: the combined set achieves pooled R² = 0.874 (RMSE 3.68%) versus 0.845 for t_CC alone and 0.796 for CV-only (Table 7), supporting a complementarity claim; and LOBO RMSE averages ~119% higher than 5-fold CV (Table 9), quantifying how random splits overestimate deployable accuracy. SHAP analysis ranks the CV/CC time ratio as dominant, and practical indicator-selection guidelines (Table 11) are offered.
Significance. If the results hold, the paper makes a useful, practice-oriented contribution: it quantifies, on public and widely used NASA data, the gap between conventional random-split cross-validation and cross-battery evaluation — a message the SOH literature needs, since many published sub-1% RMSE claims rest on random splits. The work is careful in several respects that deserve explicit credit: a voltage-threshold sensitivity analysis (Appendix C), a nominal-capacity sensitivity check (§3.1), a hyperparameter robustness check (§3.4), per-battery error statistics (Appendix B), a model-parity analysis showing the bottleneck is generalization rather than model capacity (Table 6), and honest limitations including a negative chronological-split result (§5.5). The 98.9% CV-detection success rate and the simple, differentiation-free indicator extraction are genuine practical strengths. The complementarity claim itself, however, currently rests on pooled metrics over only four folds and needs stronger statistical support before the Table 11 guideline built on it can be considered established.
major comments (4)
- [§4.4, Table 7] The central complementarity claim — Combined R²=0.874 > t_CC-only 0.845 > CV-only 0.796 — rests on pooled LOBO metrics over only four folds, with no per-fold indicator-set breakdown. The paper's own Table 5 shows large per-fold heterogeneity (per-battery R² from 0.609 to 0.896). With N=4 folds, a ΔR² of 0.029 between Combined and t_CC-only could be carried by a single fold (B0006 or B0018). The authors should report per-fold metrics for each indicator set in Table 7, plus a paired comparison across folds (e.g., per-fold ΔRMSE with sign consistency), or temper the complementarity conclusion and the 'Complete CC-CV data available → Combined' recommendation in Table 11 accordingly.
- [§4.3.2/§4.4, Tables 5 and 7] The 'Overall R²' values appear to be computed by pooling predictions across folds, where each fold's predictions come from a differently-trained model. Pooled R² credits the model with between-battery SOH variance, which is an easier task than within-battery degradation tracking — the decision-relevant quantity for the maintenance use case the paper targets. The manuscript's own numbers reveal the discrepancy: Table 5 reports per-fold R² = 0.769 ± 0.110 while the text and Table 7 quote 0.796 for the same CV-only configuration. The authors should (i) state explicitly how 'Overall R²' is computed, (ii) report both pooled and mean per-fold (within-battery) R² for all indicator sets in Table 7, and (iii) verify that the Combined > t_CC-only ordering survives under the within-battery metric.
- [§3.2 and Appendix C vs. §4.3.2, Table 5] Numerical inconsistency: the threshold sensitivity analysis (§3.2, Table C1) reports the baseline 4.17 V configuration at LOBO RMSE = 4.235% and R² = 0.808, but Table 5 reports the same CV-only LOBO configuration at RMSE = 4.69% and R² = 0.796. These should be identical experiments. Please explain the discrepancy (different model configuration? different random seed? pooled vs. mean-per-fold computation?) and reconcile the two tables, since Appendix C is the basis for the robustness claim about threshold choice.
- [§3.4] Feature standardization is described as 'z-score normalization before model training' without stating whether the scaler is fit on training folds only. If statistics are computed over all 623 samples before the LOBO split, test-battery information leaks into every fold. The effect is likely small for standardization, but given the paper's central message is evaluation rigor, the pipeline (fit scaler within each training fold, apply to test fold) should be stated explicitly and, if necessary, corrected.
minor comments (7)
- [§3.1 vs. Table 1] Direct contradiction on SOH>100% cycles: the text states these are 'concentrated in B0006 (18 cycles, max 104.6%) and B0007 (20 cycles, max 101.7%)', but Table 1 lists B0006's SOH range as 69.8–99.6 and assigns the 104.6% maximum to B0018. Please correct whichever is wrong.
- [§3.3, Indicator 3] Cycle-count inconsistency: the text states exponential fitting converged for '636 of 637 CV cycles', but §4.1 reports 623 valid cycles out of 630 charge cycles. The 637/630 discrepancy (and per-battery counts summing to 637) should be reconciled.
- [Table 2] t_CV/t_CC and τ are both reported with r = -0.719 to three decimals. If this is a coincidence of rounding, fine, but given that τ is derived from the same current-decay profile, a note on the near-collinearity of the CV indicators (and its implications for the CV-only set) would strengthen the analysis.
- [§4.5, Table 8] Two different SHAP summaries are given (mean |SHAP| = 7.839 for the ratio in Table 8; 6.77 ± 1.56 'under LOBO validation' in the text) without stating which model/validation produces the Table 8 values. Please clarify; presumably Table 8 is from a model trained on all data or under 5-fold CV.
- [§4.3.1/§3.4] The hyperparameter grid search is evaluated with 5-fold CV and the default configuration retained; this is reasonable, but note that selecting hyperparameters on 5-fold CV while reporting LOBO as primary is methodologically slightly mismatched. A sentence acknowledging this would suffice.
- [§5.5] The limitations section is commendably honest. Consider adding that with only four LOBO folds, the ±1.26% RMSE spread and the 119% CV-to-LOBO gap are themselves noisy estimates; cross-dataset replication (already listed as future work) is the natural remedy.
- [General] No code or extracted-indicator data availability statement is given. Given the public NASA dataset and the reproducibility-oriented framing, releasing the extraction and evaluation code would substantially increase the paper's usefulness.
Circularity Check
No circularity: SOH labels are independent discharge measurements; CC/CV indicators are inputs to a supervised fit, not definitional of the target.
full rationale
The paper is a standard empirical comparison of hand-crafted charging-phase features under LOBO validation. SOH is defined from discharge coulomb counting relative to a fixed nominal capacity (Section 3.1), while the predictors (t_CV, t_CV/t_CC, τ, Q_CV, t_CC) are extracted solely from the charge curve (Sections 3.2–3.3). Nothing in the feature definitions algebraically forces the LightGBM (or RF/XGBoost/CatBoost) outputs to match SOH; the reported R²/RMSE values are ordinary out-of-sample supervised metrics. SHAP rankings and Pearson correlations are post-hoc descriptions of the fitted model, not self-defining identities. The 119% CV-vs-LOBO gap is arithmetic from Table 9, not a derived law. There is no load-bearing self-citation, uniqueness theorem, or ansatz smuggled in via prior author work. Methodological choices (4.17 V threshold, 20 mA cutoff, default LightGBM hyperparameters) affect feature extraction but do not make predictions true by construction. The derivation chain is therefore self-contained against external benchmarks; circularity score is 0.
Axiom & Free-Parameter Ledger
free parameters (5)
- CV detection voltage threshold =
4.17 V
- Nominal capacity for SOH =
1.86 Ah
- LightGBM hyperparameters =
100 / depth 6 / lr 0.1
- Exponential τ model and fallback =
NLLS + 36.8% fallback
- CV current cutoff and min points =
20 mA, ≥20 points
axioms (5)
- domain assumption Discharge coulomb-counted capacity over a fixed nominal is the ground-truth SOH label for supervised learning.
- domain assumption CC–CV charging at the NASA protocol (1.5 A to 4.2 V, then CV to 20 mA) yields health-informative phase timings and current decay.
- ad hoc to paper Leave-One-Battery-Out on four cells is a sufficient proxy for realistic cross-battery deployment error.
- domain assumption Tree ensemble regression on z-scored tabular indicators is an adequate estimator for comparing indicator sets (model class not the bottleneck).
- standard math Standard statistical learning and Pearson/SHAP interpretation tools apply to these cycle-level samples.
read the original abstract
Accurate State-of-Health estimation is essential for safe battery operation and cost-effective maintenance. Although numerous health indicators have been derived from constant-current (CC) and constant-voltage (CV) charging phases, their effectiveness under realistic cross-battery validation remains insufficiently studied. This work addresses this gap through a systematic comparison of CC-only, CV-only, and combined indicator sets using rigorous Leave-One-Battery-Out (LOBO) validation on the NASA battery aging dataset. Four CV-phase indicators and CC phase duration are evaluated individually and in combination. Results show that the combined CC+CV approach achieves the best performance (R2 = 0.874), confirming that CC and CV phases capture complementary degradation information. Moreover, a 119% performance gap is observed between standard 5-fold cross-validation and LOBO validation, indicating that conventional evaluation overestimates practical accuracy. Based on these findings, practical guidelines are provided for indicator selection under data and computational constraints.
Figures
Reference graph
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discussion (0)
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