Differential Voltage Analysis and Patterns in Parallel-Connected Pairs of Imbalanced Cells

TL;DR

This work addresses identifying capacity and resistance imbalances in parallel-connected battery cells using only pairwise voltage and current measurements. It develops a differential voltage analysis framework for a 2P cell pair under constant-current discharge and links mid-to-high SOC $dV/dQ$ peak height and skewness to imbalance magnitudes. Through an OCV-R two-cell model and current-splitting dynamics, the study demonstrates that the peak height and skewness jointly inform the product of imbalances, though not the two imbalances uniquely, and shows how joint effects can be decoded via heatmaps. The findings provide a practical approach for fault detection and balancing assessment with limited sensing, and outline future work to incorporate degradation coupling, noise sensitivity, and extensions to larger parallel groups.

Abstract

Diagnosing imbalances in capacity and resistance within parallel-connected cells in battery packs is critical for battery management and fault detection, but it is challenging given that individual currents flowing into each cell are often unmeasured. This work introduces a novel method useful for identifying imbalances in capacity and resistance within a pair of parallel-connected cells using only voltage and current measurements from the pair. Our method utilizes differential voltage analysis (DVA) when the pair is under constant current discharge and demonstrates that features of the pair's differential voltage curve (dV/dQ), namely its mid-to-high SOC dV/dQ peak's height and skewness, are sensitive to imbalances in capacity and resistance. We analyze and explain how and why these dV/dQ peak shape features change in response to these imbalances, highlighting that the underlying current imbalance dynamics resulting from these imbalances contribute to these changes. Ultimately, we demonstrate that dV/dQ peak shape features can identify the product of capacity imbalance and resistance imbalance, but cannot uniquely identify the imbalances. This work lays the groundwork for identifying imbalances in capacity and resistance in parallel-connected cell groups in battery packs, where commonly only a single current sensor is placed for each parallel cell group.

Figures (6)

• Figure 1: Flow diagram of our proposed method useful for identifying imbalances in capacity and resistance within a parallel-connected cell pair using only the pair's voltage, $V_t(t)$, and current, $I(t)$. The method leverages the relationship between features of the pair's dV/dQ peak and imbalances in capacity and resistance.
• Figure 2: (Top) Full Cell Open Circuit Voltage ($OCV_i$) and Half-Cell Potentials ($U_{p_i}$, $U_{n_i}$) of NMC/graphite. (Bottom) Differential Voltage of the Full Cell and Half Cells with respect to Discharge Coulomb Counting, $Q_i$. Figure adapted from Weng2023-lj
• Figure 3: Simulation of a Balanced 2P Cell Group and of an Imbalanced 2P cell group with capacity imbalance ($C_2$/$C_1$ =.5) under C/3 constant current (CC) discharge. A) Individual Cell Current ($I_i$) vs Time, B) 2P Voltage ($V_t$) vs Time, C) Individual Cell Discharge Coulomb Counting ($Q_i$) vs Time, D) Individual Cell dV/dQ ($\frac{dV}{dQ}_i$) vs Time, E) 2P dV/dQ ($\frac{dV}{dQ}$) vs 2P Discharge Coulomb Counting (Q), F) 2P dV/dQ ($\frac{dV}{dQ}$) vs 2P Voltage ($V_t$)
• Figure 4: Simulation of a Balanced 2P Cell Group and of an Imbalanced 2P Cell Group with resistance imbalance ($R_2$/$R_1$ =2) under C/3 CC discharge. A) Individual Cell Current ($I_i$) vs Time, B) 2P Voltage ($V_t$) vs Time, C) Individual Cell Discharge Coulomb Counting ($Q_i$) vs Time, D) Individual Cell dV/dQ ($\frac{dV}{dQ}_i$) vs Time, E) 2P dV/dQ ($\frac{dV}{dQ}$) vs 2P Discharge Coulomb Counting (Q), F) 2P dV/dQ ($\frac{dV}{dQ}$) vs 2P Voltage ($V_t$)
• Figure 5: (Top) Comparison of 2P mid-to-high SOC dV/dQ peak vs 2P Voltage for varying capacity ratios ($C_2$/$C_1$) on the left and resistance ratios ($R_2$/$R_1$) on the right. (Middle) Height of 2P mid-to-high SOC dV/dQ vs $C_2$/$C_1$ on the left and $R_2$/$R_1$ on the right. (Bottom) Skewness of 2P mid-to-high SOC dV/dQ vs $C_2$/$C_1$ on the left and vs $R_2$/$R_1$ on the right.