How to enhance battery safety

For lithium-ion battery pack manufacturers, it is critical to build safe and reliable products for battery-powered systems. The battery management circuit in the battery pack can monitor the operating status of the lithium-ion battery, including battery impedance, temperature, cell voltage , charge and discharge current , and charge status, to provide the system with detailed information about the remaining operating time and battery health status to ensure that the system makes the right decisions. In addition, to improve the safety performance of the battery, even if only one fault occurs, such as overcurrent, short circuit, excessive voltage of the cell and battery pack, and excessive temperature, the system will turn off the two back-to-back protection MOSFETs in series with the lithium-ion battery to disconnect the battery cell. The battery management unit (BMU) based on impedance tracking technology monitors cell impedance and voltage imbalance throughout the battery life cycle, and may detect micro-shorts in the battery to prevent the battery cell from causing fire or even explosion.

Lithium-ion battery safety

Excessive operating temperatures will accelerate battery aging and may cause thermal runaway and explosion of lithium-ion battery packs. This is of particular concern for the highly active energetic materials in lithium-ion batteries. Overcharging with high currents and short circuits can cause rapid increases in battery temperature. During overcharging of lithium-ion batteries, active metallic lithium is deposited at the positive electrode of the battery, which greatly increases the risk of explosion because the lithium will react with a variety of materials, including the electrolyte and cathode materials. For example, the lithium/carbon intercalated compound reacts with water and releases hydrogen, which can ignite due to the heat of the reaction. Cathode materials, such as LiCoO2, will also begin to react with the electrolyte when the temperature exceeds the thermal runaway temperature limit of 175°C (4.3V cell voltage).

Lithium-ion batteries use very thin microporous film materials, such as polyolefins, to separate the positive and negative electrodes of the battery because of their excellent mechanical properties, chemical stability and acceptable price. The low melting point range of polyolefins, from 135°C to 165°C, makes polyolefins suitable as thermal fuse materials. As the temperature rises and reaches the melting point of the polymer, the porosity of the material will fail, which is intended to prevent lithium ions from flowing between the electrodes, thereby shutting down the battery. At the same time, thermal ceramic (PCT) devices and safety vents provide additional protection for lithium-ion batteries. The battery casing, which generally serves as the negative terminal, is usually a typical nickel-plated metal plate. In the case of a sealed casing, metal particles may contaminate the interior of the battery. Over time, particles may migrate to the separator and cause the insulation between the anode and cathode of the battery to degrade. A small short between the anode and cathode will allow electrons to flow freely and eventually cause the battery to fail. In most cases, such failure is equivalent to the battery being unable to provide power and completely ceasing to function. In rare cases, batteries can overheat, melt, catch fire, or even explode. This has been the main cause of recent reported battery failures and has led to numerous manufacturer recalls.

Battery Management Unit (BMU) and Battery Protection

The continuous development of battery materials has raised the upper temperature limit of thermal runaway. On the other hand, although the battery must pass strict UL safety tests, such as UL16-2, it is still the responsibility of the system designer to provide the correct charging state and deal with a variety of possible electronic component failures. Overvoltage, overcurrent, short circuit, overheating and failure of external discrete components may cause sudden failure of the battery. This means that multiple protections are needed-at least two independent protection circuits or mechanisms in the same battery pack. At the same time, it is also hoped to have electronic circuits for detecting small short circuits inside the battery to avoid battery failure.

Figure 1 shows a block diagram of the battery management unit in the battery pack, which consists of a fuel gauge integrated circuit (IC), an analog front-end circuit (AFE), and an independent secondary safety protection circuit.

Figure 1. Battery management unit.

The fuel gauge circuit is designed to accurately indicate the available lithium-ion battery charge. The circuit's unique algorithm allows real-time tracking of the battery pack's charge evolution, battery impedance, voltage, current, temperature, and other circuit information. The fuel gauge automatically calculates the charge and discharge rate, self-discharge, and cell aging to achieve high-precision fuel measurement over the battery's life. For example, a series of patented impedance tracking fuel gauges, including the bq20z70, bq20z80, and bq20z90, can provide up to 1% accuracy over the battery's life. A single thermistor is used to monitor the temperature of the lithium-ion battery to implement overheat protection of the battery cell and for charge and discharge limits. For example, the battery cell is generally not allowed to charge below 0°C or above 45°C, and is not allowed to discharge when the battery cell temperature is above 65°C. If an overvoltage, overcurrent, or overtemperature condition is detected, the fuel gauge IC will command the control AFE to shut down the charge and discharge MOSFETs Q1 and Q2. When a battery under-voltage condition is detected, the AFE is commanded to turn off the discharge MOSFET Q2 while keeping the charge MOSFET turned on to allow the battery to charge.

The main task of the AFE is to detect overload and short circuit, and protect the charge and discharge MOSFET , battery cells and other components on the line to avoid overcurrent . Overload detection is used to detect overcurrent (OC) of the battery discharge current, while short circuit (SC) detection is used to detect overcurrent of the charge and discharge current. The overload and short circuit limits and delay times of the AFE circuit can be programmed through the fuel gauge data flash. When an overload or short circuit condition is detected and the programmed delay time is reached, the charge and discharge MOSFET Q1 and Q2 will be turned off, and detailed status information will be stored in the AFE status register, so that the fuel gauge can read and investigate the cause of the fault.

The AFE plays an important role in the fuel gauge chipset solution for measuring 2, 3 or 4 Li-ion battery packs . The AFE provides all the required high voltage interfaces as well as hardware current protection features. The provided I2C compatible interface allows the fuel gauge to access the AFE registers and configure the AFE protection features. The AFE also integrates the battery cell balancing control. In most cases, in a multi-cell battery pack, the state of charge (SOC) of each individual battery cell is different from each other, resulting in a voltage difference between unbalanced cells. The AFE integrates bypass paths for each battery cell. Such bypass paths can be used to reduce the charging current to each cell, thereby providing conditions for SOC balancing during battery cell charging. The determination of the chemical charge state of each battery cell by the impedance tracking fuel gauge can make the correct decision when cell balancing is required.

Multi-polar over-current protection with different activation times (as shown in Figure 2) makes the battery pack protection more robust. The fuel gauge has two levels of charge/discharge over-current protection settings, and the AFE provides a third level of discharge over-current protection. In a short-circuit state, the MOSFET and battery may be destroyed within seconds. The fuel gauge chipset relies entirely on the AFE to automatically shut down the MOSFET to avoid damage.

Figure 2. Multi-level battery overcurrent protection

While fuel gauge ICs and their associated AFEs provide overvoltage protection, the sampling nature of voltage monitoring limits the response time of such protection systems. Most applications require a fast-response, real-time, independent overvoltage monitor that works in conjunction with the fuel gauge and AFE. This monitor, independent of the fuel gauge and AFE, monitors the voltage of each battery cell and provides a logic-level output for each battery cell that reaches a hardware-encoded overvoltage limit. The response time of the overvoltage protection depends on the size of the external delay capacitor . In a typical application, the output of the second-level protector will trigger a chemical fuse or other fail-safe device to permanently disconnect the lithium-ion battery from the system.

Permanent failure protection of battery pack

It is important for the BMU to provide conservative shutdown of the battery pack under abnormal conditions. Permanent failure protection includes safety under overcurrent discharge and charge fault conditions, safety under overtemperature discharge and charge conditions, safety under overvoltage fault conditions (peak voltage), and safety under cell balance fault, shorted discharge FET fault, and charge MOSFET fault conditions. The manufacturer may choose any combination of the above permanent failure protections. When any of these faults are detected, the protection device will blow the chemical fuse to permanently disable the battery pack. As an external failure verification for electronic component failure, the BMU is designed to detect failure of the charge and discharge MOSFETs Q1 and Q2. If any of the charge or discharge MOSFETs are shorted, the chemical fuse will also blow.

According to reports, micro short circuits inside the battery are also the main reason for many recent battery recalls. How to detect micro short circuits inside the battery and prevent the battery from catching fire or even exploding? During the shell sealing process, metal particles and other impurities may contaminate the inside of the battery, causing micro short circuits inside the battery. Micro short circuits inside the battery will greatly increase the self-discharge rate of the battery, causing the open circuit voltage to be lower than the battery cell in the normal state. The impedance tracking fuel gauge monitors the open circuit voltage and thus detects the imbalance of the battery cells - when the difference in the open circuit voltage of different battery cells exceeds the preset limit. When such a failure occurs, a permanent failure alarm will be generated and the MOSFET will be disconnected. The chemical fuse can also be configured to melt. The above behavior will make the battery pack unable to be used as a power source and thus shield the micro short-circuited battery cells inside the battery pack, thereby preventing the occurrence of disasters.

summary

The battery management unit is critical to ensure the safety of end users. Robust multi-polar protection - overvoltage, overcurrent, overheating, battery cell imbalance and MOSFET failure monitoring greatly improves the safety of the battery pack. By monitoring the open-loop voltage of the battery cell, impedance tracking technology can detect tiny short circuits inside the battery and permanently fail the battery, ensuring the safety of the end user.