Mechanical Method of Lithium-Ion Soh and SOC Measurement

Despite the fundamental importance of state of health and state of charge determination in battery management systems, measurement of SOH and SOC is difficult and remains an area of active research. In this presentation we propose a novel approach to lithium-ion battery cell management which makes use of observed relationships between cell expansion and SOH/SOC to determine the SOH and SOC of a cell during operation using simple stack stress measurements.

We first present results from an aging study of constrained commercial lithium-ion pouch cells in which we observed an irreversible increase in stack stress during cycling [1]. The increase in stack stress is due to irreversible expansion of the solid battery electrodes against the rigid constraint of the cell housing. This increase in stack stress is not a result of gas evolution, as the same stress increase is observed in pouch cells with a slit cut in the pouch and cycled under argon atmosphere.

Plotting the stack stress against the SOH of the cell reveals a linear relationship between stack stress and SOH during cycling, as shown in Figure 2. This suggests a fundamental coupling between irreversible electrode expansion and capacity fade. By monitoring this irreversible expansion during cycling—either through stress or strain measurements of the pouch cell—we show that it is possible to determine the cell’s SOH from simple mechanical measurements. The simplicity of these measurements and their relationship to SOH offer many advantages to the current state of the art.

We also show that the same mechanical measurements can be used to determine the cell’s SOC by measuring reversible changes in the cell stress/thickness with cycling. The measured stress can be related to the SOC of the cell if the SOH is known. Stress measurements are advantageous to voltage measurements for determining SOC because voltage measurements are strongly affected by battery usage via overpotentials. This mechanical method of SOC determination is computationally simpler than Coulomb counting and is also able to account for discharge events such as internal short circuiting that Coulomb counting is unable to detect.

FIGURE CAPTIONS

Figure 1. Stress evolution of a constrained pouch cell during cycling showing an irreversible increase in stack stress.

Figure 2. Stack stress plotted against state of health shows a linear relationship between the two parameters.

Figure 3. Stack stress plotted against SOC at different states of health of the constrained pouch cell. 

REFERENCES

[1]. J. Cannarella, C. B. Arnold, “Stress evolution and capacity fade in constrained lithium-ion pouch cells,” J. Power Sources 245(2014) 745-751.

ACKNOWLEDGEMENTS

Support was provided by the DoD through the NDSEG Program and by the Siebel Energy Challenge. J. C. also acknowledges the Rutgers-Princeton IGERT in Nanotechnology for Clean Energy.