Battery pack permanent failure protection
It is critical for the battery management unit to provide conservative means of shutting down the battery pack under abnormal conditions. Permanent failure detection includes safety over current discharge and charge fault, safety over temperature in discharge and in charge, safety over voltage fault (pack voltage), cell imbalance fault, and shorted discharge FET fault and charge MOSFET fault. It is the manufacturer's choice to enable any combination of the above permanent failure detections. When any one of these enabled faults is detected, it will blow the chemical fuse to permanently disable the battery pack. As an extra fail-proof of electronics component failure, the battery management unit is designed to detect if the charge and discharge MOSFETs Q1 & Q2 failed. If either the charge or discharge MOSFETs is shorted, then the chemical fuse will also be blown.
Battery internal micro-short was reported as the main root cause for several recent battery recalls. How can you detect the battery internal micro-short and prevent the battery from catching fire and even exploding? The battery may have an internal micro-short when the metal micro particles and other impurities from the case sealing process contaminate the interior of the cells. The internal micro-short significantly increases the self-discharge rate which results in lower open circuit voltage than that of the normal cells. Impedance track gas gauge monitors the open circuit voltage and, therefore, detects cell imbalance when the open circuit voltage difference between cells exceeds the preset threshold. When this type of failure happens, a permanent failure is signaled and MOSFETs are opened, and the chemical fuse can be configured to blow as well. This will render the pack unusable as a power source and thus screen the pack with the internal micro-short cells, thus, preventing it from causing hazards.
The battery management unit is crucial for the end user's safety. The robust multi-level protections of over voltage, over current, over temperature, cell imbalance, and MOSFET failure detection significantly improve the battery pack safety. Impedance track technology can detect the battery internal micro-short by monitoring the cell open circuit voltage and disable the battery permanently, making the end users safer.
 Spencer Chin, "Battery recall could cost Sony over $170 million" EE Times, August 25, 2006
About the authors
Jinrong Qian is an Applications Engineering Manager and Senior Member of TI's Technical Staff for the Portable Power Battery Management group at Texas Instruments. He has published more than 40 peer-reviewed power electronics transactions and conference papers and holds 19 U.S. patents. Dr. Qian's primary research interests include high-efficiency power converter topologies, portable power supply designs and battery power management ICs and their applications. Dr. Qian was an Associate Editor of the IEEE Transactions on Power Electronics from 1999 to 2001 and a senior member of IEEE in 1999. He earned a Bachelors of Science degree in Electrical Engineering from Zhejiang University in 1985 and a Ph. D. from Virginia Polytechnic Institute and State University in 1997. Dr. Qian can be reached at email@example.com.
Sihua Wen (Simon) is an applications engineer for Portable Power Battery Management group at Texas Instruments. He has published more than 20 papers in international conferences and journals, and has one U.S. patent. His research interests are battery power management and low-power DC-DC conversion IC for portable devices. He received a B.S. degree in Materials Science from Tsinghua University in 1997, China, and a M.S. degree and a Ph.D. degree in Power Electronics Engineering from Virginia Tech in 1999, and 2002, respectively. Dr. Wen can be reached at firstname.lastname@example.org.