The problems of using performance parameters such as voltage, current and temperature measured with electrical sensors in today’s battery management systems (BMS) are well known. These parameters can be weakly informative about cell state, particularly as cells age, and contribute to over-conservative utilization and oversizing of a battery pack. Fiber optic (FO) sensors can offer an interesting alternative to conventional electrical sensors, with several advantages such as high selective sensitivity to various parameters, light weight, robustness to EMI, and multiplexing capabilities. In this study, a particular type of FO sensors, fiber Bragg grating (FBG) sensors were externally attached to lithium ion pouch cells for monitoring additional informative cell parameter such as strain and temperature. Multiple charge and discharge cycle were performed to examine the qualification of these signals for cell state estimation in BMS. In comparison to corresponding measurements using conventional electrical sensors, the FBG signals showed very promising results for utilization in effective BMS.

A novel and fundamental method was reported to judge states of lithium ion batteries (LIBs) using the capacitance and the voltage of the cells that were estimated from the real-time currents and voltage characteristics of the cells. We measured the differential capacitance, that is, dQ/dV or delta Q/ delta V that is equal to the currents (I) divided by differential voltages (dV/dt) calculated from the current and the voltage characteristics of the cell during the charging/ discharging, where Q is the charge that flows through the cell, V is the voltage of the cell and t is time. It is thought that the capacitance decrease with the degradation of the cell because the effective area of the electrodes is decreasing due to formation of undesirable compounds. The differential capacitance in some specific voltage range for the LIBs was approximately directly proportional to the state of the degradation of the cell. Therefore, it is concluded that the novel method is very useful to judge the state of the LIBs.

Cell aging and state-of-health (SOH) estimation is widely acknowledged as a challenge in state-of-the-art battery management systems deployed today. Towards addressing this issue, gas evolution monitoring from side reactions using embedded sensors was investigated as a parameter of interest for SOH. Li-ion battery cells with a Mn-rich chemistry were subjected to overcharge experiments. Two cells were repeatedly overcharged and the evolution of gaseous CO2 was measured using fiber optic colorimetric sensors, which were incorporated and sealed into the side pouch of the battery pouch cells. A ratiometric read-out principle has been employed for the optical measurements. Initial results indicate a non-reversible gas evolution inside the battery cells during overcharge, wherein the onset of gas evolution is delayed in time relative to the overcharge condition. An increase in the sensing signal can be observed over a time span of 40 – 50 minutes during each overcharge cycle. This investigation provides real-time information on the dynamics of gas evolution in Li-ion pouch cells during overcharge experiments and allows for an early detection of potentially hazardous cell states.