Electric vehicle sensors

As gasoline prices continue to rise, hybrid electric vehicles (HEVs) and electric vehicles (EVs) are becoming more popular and in demand. For such vehicles, accurate estimation of battery state of charge (SOC), which is equivalent to the fuel gauge of conventional combustion vehicles, is required for a power management system and the convenience of the driver. However, it is difficult to estimate the SOC of next generation lithium ion batteries which will be widely used for HEVs, EVs, and other commercial products such as cell phones.

Sensors and Integrated Microsystems Laboratory (SIMSLab) is currently working with General Motors to develop a SOC estimation sensor for their Chevrolet Volt (Fig. 1) lithium ion batteries (Fig. 2).

It is challenging to estimate SOC of a lithium ion battery because the voltage is almost constant through from 35% SOC to 95% SOC (Fig. 3). In order to estimate SOC with a high accuracy, SIMSLab is currently working on identifying properties of the battery which change due variations in SOC. In order to identify the changes inside a lithium ion battery during cycling, a lithium ion battery cell (Fig. 4) was fabricated in-house. The voltage profile due to discharge (Fig. 5) of the in-house battery is comparable to that of a commercial lithium ion cell.

A lithium ion battery cell is cycled to identify and characterize properties which change during cycling such as voltage, temperature, etc. using a Maccor cycler in a temperature chamber (Fig. 6) to characterize the battery properties. SIMSLab is currently identifying mechanical and electrical property changes due to SOC..

• Krishna Iyer
• Jake Rochman
• Mahdi Mahdi
• Ryan Norris
• Ali Najafi Sohi