A detailed look at the cell supervisor unit (CSU)

Figure 3 shows a simplified CSU. The CSU operates closely within the actual cells of the pack, connecting the cell monitor device wiring harnesses and ensuring efficient transmission of the essential pack data back to the host BCU.

Without the CSU, there would be little information available regarding the status of the battery pack. The diagnostic data output from the CSU enables state-of-health and state-of-charge estimations, which directly impact the safety goals of the system. With highly precise monitors, these algorithms can provide extremely accurate estimations to the driver and maximize the most utility from each charge. This action is typically done passively, and at high enough currents where thermal management becomes challenging to maintain and measure. Overall, implementing a sophisticated CSU into the pack benefits the lifetime charge cycles of the vehicle, offering a safer and more improved overall experience.

The CSU provides increasingly detailed cell status measurements to maximize the aforementioned battery pack benefits. For state-of-health and state-of-charge calculations, synchronizing these measurements at the highest possible data rates safely and reliably offers the best estimations. As the trend toward >400-V high-voltage packs rises, an intelligent CSU design facilitates the growing number of cell data transmissions across the pack. To enable the most affordable HEV/EVs, the challenge is how to accomplish these benefits with the lowest possible power consumption and external printed circuit board components.

With LFP becoming more popular, the flat discharge curve compared to NMC (as depicted in Figure 4) requires even more accurate cell voltage measurements to determine the usable mileage of an EV. The Texas Instruments (TI) BQ79718-Q1 stackable battery monitor and cell balancer measures 18 cells in series. It provides cell voltage measurements to ±1-mV accuracy, with passive cell balancing featuring 300-mA current capability. The device also supports voltage and current synchronized measurements together with the BQ79731-Q1 battery monitor to provide more accurate state-of-health and state-of-charge calculations.