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Enhance system safety with battery management electronics

Posted: 19 Nov 2007  Print Version  Bookmark and Share

Keywords:battery management  system safety  power electronics 

By Jinrong Qian and Sihua Wen
Texas Instruments Inc.

It is critical for Li-ion battery pack manufacturers to build safe and reliable products for battery-powered systems. Battery management electronics in battery packs monitor Li-ion battery operating conditions, including battery impedance, temperature, cell voltages, charge and discharge current, and state of charge to provide detailed remaining runtime and battery health information to the system to ensure the right system decisions can be made. Additionally, to enhance battery safety, whenever at least one of the fault conditions, such as overcurrent, shortcircuit, cell and pack overvoltage, overtemperature occurs, the battery cells are disconnected from the system by turning off two back-to-back protection MOSFETs that are in series with the Li-ion cells in the battery pack. Impedance Track technology-based battery management unit (BMU) monitors battery cell impedance over the entire battery life cycle as well as cell voltage imbalance, potentially capable of detecting the cell micro-short and preventing the cell from fire hazard or even explosion.

Li-ion battery safety
Excessive high level operating temperatures accelerate cell degradation and causes thermal run-away and explosion in Li-ion batteries. This is a specific concern with this type of battery because of its highly aggressive active material. Rapid temperature increase can occur if a battery is overcharged at high current or shorted. During overcharge of a Li-ion battery, active metallic lithium is deposited on anode. This material dramatically increases the danger of explosion, because it can explosively react with a variety of materials including electrolyte and cathode material. For example, Li/carbon intercalated compound reacts with water and the released hydrogen can be ignited by the heat of the reaction. Cathode material, such as LiCoO2, starts reacting with electrolyte when the temperature exceeds its thermal run-away threshold of 175°C with 4.3V cell voltage.

Li-ion cells use thin, micro-porous films such as polyolefin to electrically isolate the positive and negative electrodes as they provide excellent mechanical properties, chemical stability, and are of acceptable cost. The low melting point of polyolefin, ranging from 135°C to 165°C, makes it suitable to be used as a thermal fuse. As the temperature approaches the melting point of the polymer, porosity is lost. This is intentional so it will shutdown the cell because lithium ions can no longer flow between electrodes. Also, there is a PTC device and a safety vent to provide additional protection in the Li-ion cells. The case, commonly used as the negative terminal, is typically Ni-plated steel. When the case is sealed, it is possible for the metal particles to contaminate the interior of the cells. Over time, the particles can migrate into the separator, degrading the insulating barrier placed between the anode and cathode sides of the cell. That creates a micro- short between anode and cathode, allowing electrons to flow freely, ultimately failing the battery. Most of the time, this type of failure leads to little more than the battery powering down and ceasing to function properly. In rare instances, however, the battery can overheat, melt, catch fire or even explode. This was reported as the main root cause of some recent battery failures that resulted in mass recall by different manufacturers.

BMU, battery protection
Cell material developments are ongoing to increase thermal runaway temperature. On the other hand, although the battery must pass stringent UL safety tests such as UL1642, it is always going to be a responsibility of the system designer to provide correct charging conditions and be well-prepared for possibility of multiple failures of electronic components. The system should not cause battery catastrophic failures due to overvoltage, overcurrent, shortcircuit, overtemperature conditions and external discrete component failures. This means redundant protection should be implemented—having at least two independent protection circuits or mechanisms in the same battery pack. It is also desirable to have the electronics circuit to detect battery internal micro-short to prevent battery failures.

Figure 1 shows the BMU block diagram in the battery pack, which consists of gas gauge IC, analog front end (AFE) circuit and independent second level safety protection circuit.

Figure 1. Battery management unit

The gas gauge circuit is designed to accurately report available capacity of Li-ion batteries. Its unique algorithm allows for real-time tracking of battery capacity change, battery impedance, voltage, current, temperature and other critical information of the battery pack. The gas gauge automatically accounts for charge and discharge rate, self-discharge and cell aging, resulting in excellent gas-gauging accuracy even when the battery ages. For example, a family of patented Impedance Track gas gauges such as bq20z70, bq20z80 and bq20z90 can provide up to one percent gauging accuracy over battery lifetime. A thermistor is used to monitor the Li-ion cell temperature for cell over temperature protection, and for charge and discharge qualification.

For example, the battery is usually not allowed to charge when the cell temperature is below 0 or above 45°C, and is not allowed to discharge when the cell temperature is above 65°C. When overvoltage, overcurrent or overtemperature conditions are detected, the Gas Gauge IC will command the AFE to turn off the charge and discharge MOSFETs Q1 and Q2. When cell undervoltage is detected, it will command the AFE to turn off the discharge MOSFET Q2 while keeping the charge MOSFET on so that battery charging is allowed.

The main task of the AFE is overload, shortcircuit detection and protection of the charge and discharge MOSFETs, cells and any other inline components from excessive current conditions. The overload detection is used to detect excessive overcurrents (OC) in the battery discharge direction, while the shortcircuit (SC) detection is used to detect excessive current in either the charge or discharge direction. The AFE threshold and delay time of overload and shortcircuit can be programmed through the gas gauge data flash settings. When an overload or short-circuit is detected and a programmed delay time has expired, both charge and discharge MOSFETs Q1 and Q2 are turned off and the details of the condition are reported in the status register of AFE so that the gas gauge can read and investigate causes of the failure.

The AFE serves an important role for the gas gauge two-, three-, or four-cell lithium-ion battery pack gas gauge chipset solution. The AFE provides all the high voltage interface needs and hardware current protection features. It offers an I²C-compatible interface to allow the gas gauge to have access to the AFE registers and to configure the AFE's protection features. The AFE also integrates cell balancing control. In many situations, the state-of-charge (SOC) of the individual cells may differ from each other in a multi-cell battery pack, causing voltage difference between cells and cell-imbalance. The AFE incorporates a bypass path for each cell. These bypass paths can be utilized to reduce the charging current into any cell and, thus, allow for an opportunity to balance SOC of the cells during charging. Since the Impedance Track gas gauges can determine the chemical SOC of each cell, a right decision can be made when cell balancing is needed.

Multiple over current protection thresholds with different activated times as shown in Figure 2 make the battery pack protection more robust. The gas gauge has two tiers of charge/discharge over-current protection settings, and the AFE provides a third level of discharge over-current protection. In case of short-circuit conditions when the MOSFETs and the battery can be damaged within seconds, the gas gauge chipset entirely depends on the AFE to autonomously shut off the MOSFETs before such damage occurs.

Figure 2. Multilevel battery overcurrent protection

While the gas gauge IC and its associated AFE provide overvoltage protection, the sampled nature of the voltage monitoring limits the response time of this protection system. Most applications will require a fast-response, real-time, independent overvoltage monitor that operates in conjunction with the gas gauge and the AFE. It monitors individual cell voltages independent of the gas gauge and AFE, and provides a logic level output which toggles if any of the cells reaches a hard-coded overvoltage limit. The response time of the overvoltage protection is determined by the value of an external delay capacitor. In a typical application, the output of the second level protector would trigger a chemical fuse or other fail-safe protection device to permanently disconnect the Li-ion cell from the system.

Failure protection
It is critical for the BMU 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 overvoltage 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 BMU is designed to detect if the charge and discharge MOSFETs Q1 and 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 battery recalls starting 2006. 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.

Summary
The BMU is crucial for the end user's safety. The robust multilevel protections of overvoltage, overcurrent, overtemperature, 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.

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. He 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 and a Ph.D. from Virginia Polytechnic Institute and State University. He can be reached at jqian@ti.com.

Sihua Wen is an applications engineer for portable power battery management group at TI. He received a B.S. degree in Materials Science from Tsinghua University and M.S. and Ph.D. degrees in Power Electronics Engineering from Virginia Tech. He can be reached at swen@ti.com.




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