Operando Stack Pressure Measurement of LFP/Graphite and LMFP/Graphite Cells to aid in State of Charge Prediction

Abstract One of the biggest issues facing electric vehicles with LiFePO4 (LFP) batteries is state of charge management. Over much of the state of charge window, the voltage of both LFP and LiMnxFe1-xPO4 (LMFP) batteries does not vary. Therefore, state of charge management for LFP batteries relies on coulomb counting, which requires frequent charging to 100% to remain accurate. We propose to use the signal from pressure sensors placed between prismatic cells in battery modules to help determine the state of charge of LFP and LMFP batteries. Using LFP and LMFP pouch cells to demonstrate this principle, the pressure vs. time profiles of cells constrained in fixed volumes reveal distinct and repeatable profiles, which can be used to determine the state of charge in combination with voltage monitoring. Additionally, a simulated LFP battery pack was constructed and the pressure vs. time profile was monitored in the presence of compliant material under low initial force loadings to demonstrate how this could work in an EV battery.