How to enhance the safety of electronic equipment

2022-08-14
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How to enhance the safety of electronic equipment batteries

for lithium-ion battery pack manufacturers, it is crucial to build safe and reliable products for battery power supply system. The battery management circuit in the battery pack can monitor the operation state of lithium-ion battery, including battery impedance, temperature, unit voltage, charging and discharging current, and charging state, so as to provide the system with detailed remaining operation time and battery health information to ensure that the system makes correct decisions. In addition, in order to improve the safety performance of the battery, even if only one fault occurs, such as overcurrent, short circuit, high voltage and temperature of the unit and battery pack, the system will turn off two back-to-back protection MOSFETs in series with the lithium-ion battery and disconnect the battery unit. The battery management unit (BMU) based on impedance tracking technology will monitor the impedance and voltage imbalance of the unit throughout the battery life cycle, and it is possible to detect the micro short of the battery, so as to prevent the battery unit from causing fire and even explosion

lithium ion battery safety

excessive operating temperature will accelerate the aging of the battery and may cause thermal run away and explosion of the lithium ion battery pack. For the highly active energetic materials of lithium-ion batteries, this is of great concern. Overcharge and short circuit of high current may cause the rapid rise of battery temperature. During the overcharge of lithium-ion battery, active metal lithium is deposited on the positive pole of the battery, and its material greatly increases the risk of explosion, because lithium may react with a variety of materials and explode, including electrolyte and cathode materials. For example, lithium/carbon intercalated compound reacts with water and releases hydrogen, which may be ignited by the exothermic reaction. Cathode materials, such as LiCoO2, will also begin to react with electrolyte when the temperature exceeds the thermal runaway temperature limit of 175 ℃ (4.3v unit voltage)

lithium ion batteries use very thin micro porous film materials, such as polyolefin, for the electronic isolation of the positive and negative electrodes of the battery, because such materials have excellent mechanical properties, chemical stability and acceptable price. The melting point range of polyolefin is relatively low, ranging from 135 ℃ to 165 ℃, making polyolefin suitable for use as a thermal fuse material. As the temperature rises and reaches the melting point of the polymer, the porosity of the material will fail. The purpose is to make lithium ions unable to flow between electrodes, so as to turn off the battery. At the same time, thermal ceramic (PCT) equipment and safety vent provide additional protection for lithium-ion batteries. The shell of the battery, generally used as the negative terminal, is usually a typical nickel plated metal plate. When the housing is sealed, metal particles may contaminate the interior of the battery. Over time, particles may migrate to the isolator and aging the insulation between the anode and cathode of the battery. The tiny short circuit between anode and cathode will allow electrons to flow freely, and eventually make the battery fail. In most cases, such failure is equivalent to the failure of the battery to supply power and the complete termination of its function. In a few cases, the battery may overheat, fuse, catch fire or even explode. This is the main source of battery failure reported recently, and many manufacturers have to recall their products

battery management unit (BMU) and battery protection

battery materials have more than 1600 enterprises and institutions across the country engaged in the research, development and production of sensors and tension machine components. Continuous development has raised the upper limit temperature of thermal runaway. On the other hand, although the battery must pass the strict UL safety test, such as ul1642, it is still the responsibility of the system designer to provide the correct state of charge and deal with a variety of possible electronic component failures. Overvoltage, overcurrent, short circuit, overheating and the failure of external discrete components may cause sudden failure of the battery. This means that multiple protections are required - there are at least two independent protection circuits or mechanisms in the same battery pack. At the same time, it is also hoped to have an electronic circuit for detecting the small short circuit inside the battery to avoid battery failure

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

Figure 1 The battery management unit

coulometer circuit is designed to accurately indicate the available lithium-ion battery power. The unique algorithm of the circuit allows real-time tracking of battery pack storage changes, battery impedance, voltage, current, temperature and other circuit information. The electricity meter automatically calculates the rate of charging and discharging, self discharging and battery cell aging, and realizes high-precision electricity measurement within the service life of the battery. For example, a series of patented impedance tracking coulometers, including bq20z70, bq20z80 and bq20z90, can provide measurement accuracy of up to 1% during the battery life. A single thermistor is used to monitor the temperature of the lithium-ion battery to realize the overheat protection of the battery unit, and to limit the charging and discharging. For example, battery cells are generally not tested. Instruments are divided into eight categories according to the object of action and function. Products are allowed to charge within the temperature range below 0 ℃ or above 45 ℃, and are not allowed to discharge when the temperature of battery cells is higher than 65 ℃. If overvoltage, overcurrent or overheating is detected, the coulometer IC will command and control the AFE to turn off the charging and discharging MOSFET Q1 and Q2. When the under voltage state of the battery is detected, the AFE will be commanded to turn off the discharging MOSFET Q2 while keeping the charging MOSFET on to allow the battery to charge

afe's main task is to detect overload and short circuit, and protect charging and discharging MOSFET, battery unit and other components on the circuit to avoid overcurrent. Overload detection is used to detect the over-current (OC) with upward discharge current of the battery, and short circuit (SC) detection is used to detect the over-current with upward charging and discharge current. The overload and short-circuit limits and delay time of AFE circuit can be programmed by flash memory of ammeter data. When the overload or short-circuit state is detected and the delay time set by the program is reached, the charging and discharging MOSFET Q1 and Q2 will be turned off, and the detailed state information will be stored in the AFE status register, so that the coulometer can read and investigate the cause of the fault

AFE plays an important role in the coulometer chipset solution for measuring 2, 3 or 4 lithium-ion battery packs. AFE provides all required high voltage interfaces and hardware current protection features. The I2C compatible interface provided allows the ammeter to access the AFE register and configure the protection characteristics of AFE. AFE also integrates battery cell balance control. In most cases, in a multi cell battery pack, the state of charge (SOC) of each independent battery cell is different from each other, resulting in voltage differences between unbalanced cells. 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 balance during battery cell charging. The determination of the chemical charge state of each cell based on the impedance tracking coulometer can make a correct decision when the cell balance is needed

multipole overcurrent protection limits with different activation times (as shown in Figure 2) make battery pack protection more robust. The coulometer has two layers of charge/discharge overcurrent protection settings, while AFE provides the third layer of discharge overcurrent protection. In the short-circuit state, the MOSFET and battery may be damaged in a few seconds. The ammeter chipset completely relies on AFE to automatically turn off the MOSFET to avoid damage

Figure 2 Multi stage battery overcurrent protection

when the ammeter IC and its associated AFE provide overvoltage protection, the sampling characteristics of voltage monitoring limit the response time of such protection systems. Most applications require a real-time, independent overvoltage monitor that can respond quickly, and work together with the ammeter and AFE. The monitor is independent of the coulometer and AFE, monitors the voltage of each battery unit, and provides logic level output for each battery unit that reaches the hardware coded overvoltage limit. The response time of overvoltage protection depends on the size of external delay capacitance. In typical applications, the output of the second level protector will trigger chemical fuses or other failure protection devices to permanently separate the lithium-ion battery from the system

permanent failure protection of battery pack

for battery management unit, it is very important to provide conservative shutdown for battery pack in abnormal state. Permanent failure protection includes safety under overcurrent discharge and charging fault state, safety under overheating discharge and charging state, overvoltage fault state (peak voltage), battery balance fault, short circuit discharge FET fault, and safety under charging MOSFET fault state. The manufacturer can choose any combination of the above permanent failure protection. When any such fault is detected, the protective equipment will blow the chemical fuse to make the battery pack permanently invalid. As an external failure verification of electronic component failure, the battery management unit is designed to detect the failure of charging and discharging MOSFET Q1 and Q2. If any charge or discharge MOSFET is short circuited, the chemical fuse will also blow

it is reported that the small short circuit of the electric cored preheating device is adopted in the extrusion unit inside the battery, which is also the main reason why the acceptance expert group organized by the High Tech Department of the Ministry of science and technology listened to the report of the project leader on the implementation of the project, resulting in many recent battery recalls. How to detect the small short circuit inside the battery and prevent the battery from catching fire or even explosion? During the enclosure sealing process, metal particles and other impurities may pollute the interior of the battery, causing a small short circuit inside the battery. A small internal short circuit will greatly increase the self discharge rate of the battery, making the open circuit voltage lower than that of the battery cell under normal conditions. The impedance tracking coulometer monitors the open circuit voltage and thus detects the imbalance of battery cells - when the open circuit voltage difference of different battery cells exceeds the preset limit value. When such failure occurs, a permanent failure alarm will be generated and the MOSFET will be disconnected, and the chemical fuse can also be configured to blow. The above behavior will make the battery pack unable to be used as the power supply, and therefore shield the micro short circuit battery unit inside the battery pack, thus preventing the occurrence of disasters

summary

battery management unit is crucial to ensure the safety of end users. Robust multipole protection - overvoltage, overcurrent, overheating, battery cell imbalance and MOSFET failure monitoring greatly improve the safety of the battery pack. By monitoring the open-loop voltage of the battery unit, impedance tracking technology can detect the small short circuit inside the battery, and then permanently disable the battery, ensuring the safety of end users

jingrong Qian, Sihua Wen, Ti

references

[1] Spencer chin, "battery recall could cost Sony over $170 million" EE times, August 25, 2006 (end)

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