The effect of cell-to-cell variations and thermal gradients on the performance and degradation of lithium-ion battery packs

Abstract

The performance of lithium-ion battery packs are often extrapolated from single cell performance however uneven currents in parallel strings due to cell-to-cell variations, thermal gradients and/or cell interconnects can reduce the overall performance of a large scale lithium-ion battery pack. In this work, we investigate the performance implications caused by these factors by simulating six parallel connected batteries based on a thermally coupled single particle model with the solid electrolyte interphase growth degradation mechanism modelled. Experimentally validated simulations show that cells closest to the load points of a pack experience higher currents than cells further away due to uneven overpotentials caused by the interconnects. When a cell with a four times greater internal impedance was placed in the location with the higher currents this actually helped to equalise the cell-to-cell current distribution, however if this was placed at a location furthest from the load point this would cause a ~6% reduction in accessible energy at 1.5 C. The influence of thermal gradients can further affect this current heterogeneity leading to accelerated aging. Simulations show that in all cases, cells degrade at different rates in a pack due to the uneven currents, with this being amplified by thermal gradients. In the presented work a 5.2% increase in degradation rate, from-7.71 mWh/cycle (isothermal) to-8.11 mWh/cycle (non-isothermal) can be observed. Therefore, the insights from this paper highlight the highly coupled nature of battery pack performance and can inform designs for higher performance and longer lasting battery packs.

Calibration Optimization Methodology for Lithium-Ion Battery Pack Model for Electric Vehicles in Mining Applications

“…The aim was to investigate the role of the interconnection resistance and thus capture the effect of cell-to-cell extrinsic variations on the overall performance of the pack. Liu et al [29] used the single particle model for Li-ion cells to build a battery pack that consists of six parallelly connected cells to study the impact of cell-to-cell variations and temperature gradients on the performance and aging of the battery pack. However, using the single particle model leads to model inaccuracy in voltage prediction due to neglecting electrolyte dynamics, especially under high current operating conditions (higher than 1C) typically occurring in automotive applications.…”

“…The literature review reveals that the interconnection resistance is an external factor that is responsible for current and consequently temperature and SOC inhomogeneities within highly parallelized packs [3,27,28]. The interconnection resistance can induce significant current distributions in the pack, even under low to moderate operating C-rates (lower than 1.5 C), which causes accelerated aging of the cells subjected to the higher cycling currents or temperatures [29]. Although the lumped battery pack model provides a useful tool for fast analysis of battery packs, especially during system sizing stage, it fails in a detailed assessment of the battery pack performance when considering temperature gradients, localized aging, and cooling circuits' design and evaluation.…”

Measuring Test Bench with Adjustable Thermal Connection of Cells to Their Neighbors and a New Model Approach for Parallel-Connected Cells

“…The cell tabs are connected to the load cable via a screwed aluminum union joint (13). The cell temperature is captured by two Pt100 sensors (12) of each cell side, T Cell,Center and T Cell,Top see Figure 1c. The temperature of the adjacent aluminum plates is measured with one Pt100 sensor T Plate .…”

“…These are regulated by a PI controller implemented in Matlab. Measurement data, including six Pt100 temperature sensors (12), three for each cell side, as well as two voltage signals, are recorded with a frequency of f = 100 Hz. The signals are transferred by an interface via CAN to a PC and real time processed in Matlab.…”

“…There is a variety of articles focusing on modeling [11][12][13][14][15], aging [16][17][18], safety [19][20][21], state estimation [22,23] and measurement [24][25][26] of parallel-connected cells. Mostly qualitative effects like OCV [27], SoC [13], temperature [13,26,28,29] and current differences [13] are demonstrated but quantitative relationships are missing, especially with regard to the thermal connection of the cells to neighboring cells and cooling.…”