Abstract
The utility of a single-point impedance-based technique to monitor the state-of-health of a pack of four 18650 lithium-ion cells wired in series (4S) was demonstrated in a previous publication. This work broadens the applicability of the single-point monitoring technique to identify temperature induced faults within 4S packs at 0 °C by two distinct discharge cut-off thresholds: individual cell cut-off and pack voltage cut-off. The results show how the single-point technique applied to a 4S pack can identify cell faults induced by low temperature degradation when plotted on a unique state-of-health map. Cell degradation is validated through an extensive incremental capacity technique to quantify capacity loss due to low temperature cycling and investigate the underpinnings of cell failure.
Highlights
Lithium-ion batteries are pervasive in applications spanning from small commercial electronics to electrified vehicles
In order to characterize the degradation of the cells and validate Electrochemical impedance spectroscopy (EIS) observations, reference performance tests (RPTs) were undertaken after long-term cycling at 25 ◦ C and 0 ◦ C and compared to the initial RPT1 measurement
The primary conclusions of this work pertaining to the utility of the single-point diagnostic are as follows: (i) a current amplitude perturbation of 0.52 A is sufficient to produce high signal-to-noise impedance response when applied across the 4S pack; (ii) the previously identified single-point state-of-health frequency (f SOH = 316 Hz) for the cell type under test provides the least variance in impedance response from 0–100% SOC and over the temperature range, −10 ◦ C to 60 ◦ C; (iii) the imaginary component of the impedance response at f SOH is temperature dependent and can be expressed through a simple exponential equation; (iv) the single-point technique applied to the 4S pack is able to identify cell faults within the strand during
Summary
Lithium-ion batteries are pervasive in applications spanning from small commercial electronics to electrified vehicles. As lithium-ion battery technologies penetrate additional market segments their performance requirements will inevitably be expanded to include abusive operating environments, such as low and high temperatures, as well as large pack designs composed of many series and parallel-wired cells [1]. As commercial batteries are pushed closer to the edge of their operating limits, there is a need for novel and redundant safety features external to the battery. This includes robust diagnostic and monitoring techniques to improve the effectiveness of battery management systems that provide continual user feedback of the state-of-health (SOH) and of internal stability of the cells within the pack [2]. Full spectrum impedance data collection requires significant computing capabilities and time, especially for low frequency impedance, reducing
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