lithium ion battery cells 18650

　　How do lithium ion battery cells 18650management electronics enhance lithium ion battery cells 18650safety?

　　It is critical for lithium-ion lithium ion battery cells 18650pack manufacturers to build safe and reliable products for battery-powered systems. The lithium ion battery cells 18650management circuit in the lithium ion battery cells 18650pack can monitor the operating status of the lithium-ion battery, including lithium ion battery cells 18650impedance, temperature, cell voltage, charging and discharging current, and charging status, etc., to provide the system with detailed remaining operating time and lithium ion battery cells 18650health information to ensure The system makes the right decisions. In addition, in order to improve the safety performance of the battery, even if only one fault occurs, such as overcurrent, short circuit, excessive voltage of the unit and lithium ion battery cells 18650pack, excessive temperature, etc., the system will shut down two back-to-back (back-to-back) lithium-ion batteries connected in series. -to-back) protects the MOSFET and disconnects the lithium ion battery cells 18650unit. The lithium ion battery cells 18650management unit (BMU) based on impedance tracking technology monitors unit impedance and voltage imbalance throughout the lithium ion battery cells 18650life cycle, and may detect micro-shorts in the lithium ion battery cells 18650to prevent lithium ion battery cells 18650cells from causing fires or even explosions.

　　Lithium-Ion lithium ion battery cells 18650Safety

　　Excessively high operating temperature will accelerate the aging of the lithium ion battery cells 18650and may cause thermal run-away and explosion of the lithium-ion lithium ion battery cells 18650pack. This is of great concern for highly activated energetic materials in lithium-ion batteries. Overcharging and short circuiting with high current may cause the lithium ion battery cells 18650temperature to rise rapidly. During overcharging of a lithium-ion battery, active metallic lithium is deposited on the positive electrode of the battery. Its materials greatly increase the risk of explosion, because lithium may react with a variety of materials and explode, including the electrolyte and cathode materials. . For example, a lithium/carbon intercalated compound reacts with water and releases hydrogen gas, which may be ignited by the reaction heat. 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 micro-porous film materials, such as polyolefin, to electronically isolate the positive and negative electrodes of the lithium ion battery cells 18650because such materials have excellent mechanical properties, chemical stability, and acceptable prices. Polyolefin has a lower melting point range of 135°C to 165°C, making polyolefin suitable as a thermal fuse material. As the temperature rises and reaches the melting point of the polymer, the material's porosity fails, preventing lithium ions from flowing between the electrodes, thus shutting down the battery. At the same time, thermal ceramic (pCT) devices and safety vents provide additional protection for lithium-ion batteries. The lithium ion battery cells 18650casing, which generally serves as the negative terminal, is typically a nickel-plated metal plate. When the case is sealed, metal particles may contaminate the interior of the battery. Over time, particles have the potential to migrate into the separator and degrade the insulation between the battery's anode and cathode. A tiny short circuit between the anode and cathode will allow electrons to flow freely and eventually cause the lithium ion battery cells 18650to fail. In most cases, such failure equates to the loss of lithium ion battery cells 18650power and complete cessation of functionality. In rare cases, batteries may overheat, fuse, catch fire, or even explode. This is the main source of recently reported lithium ion battery cells 18650failures and has forced numerous manufacturers to recall their products.

　　lithium ion battery cells 18650management unit (BMU) and lithium ion battery cells 18650protection

　　The continuous development of lithium ion battery cells 18650materials has increased the upper limit temperature of thermal runaway. On the other hand, although the lithium ion battery cells 18650must pass strict UL safety tests, such as UL1642, it is still the responsibility of the system designer to provide the correct charging state and well cope with a variety of possible electronic component failures. Overvoltage, overcurrent, short circuit, overheating, and failure of external discrete components may cause sudden lithium ion battery cells 18650failure. This means multiple protections are needed - at least two independent protection circuits or mechanisms within the same lithium ion battery cells 18650pack. At the same time, it is also desirable to have electronic circuits for detecting tiny short circuits inside the lithium ion battery cells 18650to avoid lithium ion battery cells 18650failure.

　　Figure 1 shows the block diagram of the lithium ion battery cells 18650management unit in the lithium ion battery cells 18650pack, which consists of a fuel gauge integrated circuit (IC), an analog front-end circuit (AFE), and an independent secondary safety protection circuit.

　　Figure 1. lithium ion battery cells 18650Management Unit

　　The fuel gauge circuit is designed to accurately indicate the available lithium-ion lithium ion battery cells 18650power. The circuit's unique algorithm allows real-time tracking of changes in lithium ion battery cells 18650pack capacity, lithium ion battery cells 18650impedance, voltage, current, temperature and other circuit information. The fuel gauge automatically calculates charging and discharging rates, self-discharge, and lithium ion battery cells 18650cell aging, achieving high-precision fuel metering over the battery's service life. For example, a series of patented impedance tracking fuel gauges, including the bq20z70, bq20z80 and bq20z90, provide metering accuracy up to 1% over the life of the battery. A single thermistor is used to monitor the temperature of lithium-ion batteries for overheating protection of the lithium ion battery cells 18650cells and for charge and discharge limits. For example, lithium ion battery cells 18650cells are generally not allowed to be charged in a temperature range below 0°C or above 45°C, and are not allowed to be discharged when the lithium ion battery cells 18650unit temperature is higher than 65°C. If overvoltage, overcurrent or overheating is detected, the fuel gauge IC will instruct the AFE to turn off charging and discharging MOSFETQ1 and Q2. When an under-voltage state of the lithium ion battery cells 18650is detected, the command controls the AFE to turn off the discharge MOSFET Q2, and at the same time keep the charging MOSFET on to allow the lithium ion battery cells 18650to charge.

　　The main task of AFE is to detect overload and short circuit, and to protect charging and discharging MOSFETs, lithium ion battery cells 18650cells and other circuit components to avoid overcurrent conditions. Overload detection is used to detect overcurrent (OC) when the lithium ion battery cells 18650discharge current increases, while short circuit (SC) detection is used to detect overcurrent when the charging and discharging current increases. The AFE circuit's overload and short-circuit limits and delay times are programmable through the fuel gauge data flash memory. When an overload or short circuit condition is detected and the programmed delay time is reached, the charging and discharging MOSFETs Q1 and Q2 will be turned off, and detailed status information will be stored in the status register of the AFE, so that the fuel gauge can read and investigate the cause. Cause of failure.

　　AFE plays an important role in fuel gauge chipset solutions for metering 2, 3 or 4 lithium-ion lithium ion battery cells 18650packs. AFE provides all the required high voltage interfaces as well as hardware current protection features. An I2C-compatible interface is provided that allows the fuel gauge to access the AFE registers and configure the AFE's protection features. AFE also integrates cell balancing control. In most cases, in multi-cell lithium ion battery cells 18650packs, the state of charge (SOC) of each individual lithium ion battery cells 18650cell is different from each other, resulting in unbalanced voltage differences between cells. AFE integrates bypass paths for each lithium ion battery cells 18650cell. Such a bypass path can be used to reduce the charging current to each cell, thereby providing for SOC balancing during lithium ion battery cells 18650cell charging. Determination of the chemical state of charge of each lithium ion battery cells 18650cell based on impedance tracking fuel gauges enables correct decisions to be made when cell balancing is required.

　　Multi-pole overcurrent protection limits with different activation times (shown in Figure 2) make lithium ion battery cells 18650pack protection more robust. The fuel gauge has two levels of charge/discharge overcurrent protection settings, while the AFE provides a third level of discharge overcurrent protection. In a short circuit state, the MOSFET and lithium ion battery cells 18650may be destroyed within seconds. The fuel gauge chipset completely relies on the AFE to automatically turn off the MOSFET to avoid damage.

　　Figure 2. Multi-level lithium ion battery cells 18650overcurrent protection

　　While the fuel gauge IC and its associated AFE provide overvoltage protection, the sampling nature of voltage monitoring limits the response time of such protection systems. Most applications require fast response, real-time, independent overvoltage monitors that work in conjunction with fuel gauges and AFEs. Independent of the fuel gauge and AFE, this monitor monitors the voltage of each cell and provides a logic level output for each cell that reaches a hardware-coded overvoltage limit. The response time of 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 separate the lithium-ion lithium ion battery cells 18650from the system.

　　lithium ion battery cells 18650pack permanent failure protection

　　For the lithium ion battery cells 18650management unit, it is very important to provide a conservative shutdown for the lithium ion battery cells 18650pack in abnormal conditions. Permanent failure protection includes safety under overcurrent discharge and charging fault conditions, safety under overheating discharge and charging conditions, overvoltage fault conditions (peak voltage) and lithium ion battery cells 18650balance faults, short-circuit discharge FET faults, charging Safety under MOSFET fault conditions. The manufacturer may choose any combination of the above permanent failsafes. When any such fault is detected, the protection device will blow the chemical fuse, causing the lithium ion battery cells 18650pack to permanently fail. As an external failure verification of electronic component failure, the lithium ion battery cells 18650management unit is designed to detect the failure of charging and discharging MOSFETs Q1 and Q2. If any of the charge or discharge MOSFETs short circuit, the chemical fuse will also blow.

　　According to reports, tiny short circuits inside the lithium ion battery cells 18650are also the main reason for many recent lithium ion battery cells 18650recalls. How to detect tiny short circuits inside the lithium ion battery cells 18650and prevent it from catching fire or even exploding? During the shell sealing process, metal particles and other impurities may contaminate the inside of the battery, causing tiny short circuits inside the battery. A small internal short circuit will greatly increase the self-discharge rate of the battery, causing the open circuit voltage to be lower than that of the lithium ion battery cells 18650cell in a normal state. The impedance tracking fuel gauge monitors the open circuit voltage and thereby detects cell imbalance - when the difference in open circuit voltage between cells exceeds a preset limit. When such a failure occurs, a permanent failure alarm is generated and the MOSFET is disconnected. The chemical fuse can also be configured to blow. The above behavior will make the lithium ion battery cells 18650pack unable to serve as a power source and thus shield the tiny short-circuited lithium ion battery cells 18650cells inside the lithium ion battery cells 18650pack, thereby preventing disasters.

　　summary

　　The lithium ion battery cells 18650management unit is critical to ensuring end-user safety. Robust multi-pole protection - overvoltage, overcurrent, overheating, cell imbalance and MOSFET failure monitoring greatly improves lithium ion battery cells 18650pack safety. By monitoring the open-loop voltage of the lithium ion battery cells 18650cell, impedance tracking technology can detect tiny short circuits inside the battery, thereby permanently failing the lithium ion battery cells 18650and ensuring the safety of end users.

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