For the lithium-ion battery pack manufacturers, building secure and reliable products for battery-powered system is vital. Battery pack battery-management circuits can monitor the operational status of lithium-ion batteries, including the impedance of the battery, temperature, voltage, charge and discharge currents, charge status, system provides details of the remaining running time and the battery health information to ensure that the system made the right decision. In addition, in order to improve the safety performance of the battery, even if only a failure occurs, such as overcurrent, short circuit, modules and battery packs such as high JVC BN-VF808U Battery Charger voltage, high temperature, system will also shut down two and lithium-ion battery back in series (back-to-back) protection MOSFET, disconnect the battery unit. Tracking technology based on impedance of battery management unit (BMU) throughout the cell cycle monitoring unit of impedance and voltage imbalance, and has the potential to detect tiny battery short circuit (micro-short) to prevent battery units cause fires and explosions.

Safety of lithium-ion batteries

Too high a temperature will accelerate the aging of the battery, and can lead to thermal runaway of the lithium-ion battery pack (thermal run-away) and explosions. For lithium-ion batteries for high activity of energetic materials, this is an area of concern. May cause excessive charging current and short circuit the battery temperature rising fast. Lithium-ion battery during overcharge, active metallic lithium deposited on the positive side of the battery, the material greatly increases the risk of explosion, because lithium is likely to react with a variety of materials, explosion, including electrolyte and cathode materials. For example, lithium/carbon intercalation compounds (intercalated compound) react with water and releasing hydrogen, hydrogen has the potential to be ignited by the reaction exothermic. Cathode material, such as LiCoO2, at temperatures above 175 ° c temperature limit of thermal runaway (4.3V unit voltage), it will start to react with the electrolyte.

Lithium-ion batteries using a thin microporous membranes (micro-porous film) material, such as polyolefin, battery positive and negative electron isolation, because such material with excellent mechanical properties, chemical stability and acceptable price. Low melting range of polyolefins, for 135 ° c to 165 ° c, makes the polyolefin thermal insurance applies as (fuse) material. As temperatures rise and reach the polymer's melting point and porosity of the material will fail, the purpose of which is to make lithium-ion does not flow between the electrodes, so as to cut off the battery. At the same time, thermal ceramics (PCT) equipment and safe discharge (safety vent)-lithium-ion battery provides extra protection. Battery enclosure, generally as the negative terminal, usually a typical nickel plating of a metal plate. Shell seal case, metal particle contamination inside the battery. With the passage of time, particles may migrate to the isolation and insulation aging of the battery between anode and cathode. Micro short circuit between the anode and cathode will allow the electronic flow of wanton, and eventually the battery failure. The vast majority of cases, this type of failure is equivalent to the power supply and batteries do not function fully terminated. In a few cases, batteries could overheat, melting, fire and explosion. This is the main cause of recent reports of battery failure, and forcing many manufacturers to recall it.

Battery management unit (BMU), and battery protection

Battery materials development increase the upper limit temperature of thermal runaway. Meanwhile, though the battery must pass strict UL safety test, such as UL1642, but provide the right charge State and very good deal with a variety of possible electronic fault is still the original duties of system designers. Over voltage, over current and short circuit, thermal State and the external discrete component failures have the potential to cause cell mutations fail. This means that the need for multiple protection – within the same battery pack protection circuit or with at least two independent mechanisms. At the same time, also hopes for testing battery internal short circuit of micro electronic circuits to avoid battery failure.

Figure 1 shows the block diagram of cells within the battery pack battery management, its composition includes the electricity meter integrated circuit (IC), analog front-end circuits (AFE), independent secondary protection circuits.

Figure 1. Battery management unit

Circuit design for accurate power meter indicates that the lithium-ion battery power available. Unique algorithm allows real-time tracking of the circuit the battery pack changes, battery charge capacity of circuit of resistance, voltage, current, temperature, and other information. Power meter automatic calculation of rates of charge and discharge, self-discharge and aging of cells, battery life power measurement of high accuracy within the period. For example, a series of patented impedance track power meter, including bq20z70,bq20z80 and bq20z90, are provided within the period of battery life up to 1% accuracy of measurement. Individual thermistor is used to monitor temperature of lithium-ion batteries, to achieve battery cell overheating protection, and for charging and discharging qualified. For example, cells are typically not allowed below 0 ° c or higher than 45 ° c in the temperature range of the charge, and does not allow thePanasonic Lumix DMC-LX5 Battery Charger battery discharge when the temperature is above 65 degrees centigrade. Over voltage, over current is detected or overheating, AFE IC power meter instructions control off the charge and discharge MOSFET Q1 and Q2. When the battery voltage is detected (under-voltage) State, then the command AFE closed discharge MOSFET Q2, and at the same time maintaining the charge MOSFET open, to allow a battery charge.

AFE's main task is to overload and short circuit detection and protection MOSFET, battery charge and discharge unit, as well as on the other line components, avoid the over current status. Overload test for detection of battery discharge flow of current (OC), and short circuit (SC) test is used to detect on a flow of charge and discharge currents. AFE circuit overload and short circuit qualified and delay time can be set by electricity meter data Flash programming. When it detects an overload or short circuit, and reached the program to set the delay time, the charge and discharge MOSFET Q1 and Q2 will be close, detailed status information is stored in a status register, AFE, voltmeter to read and investigate the reasons caused the failure.

For measure 2, 3 or 4 lithium ion battery pack power meter chip set solutions, AFE play a very important role. AFE offers all the high voltage required interfaces as well as the hardware overcurrent protection characteristics. The I2C-compatible interface provides access allows power meter AFE AFE registers and configuration characteristics of protection. AFE integrated battery unit balance control. In most cases, in a multiple-cell battery pack, each independent unit of charge State of the battery (SOC) different from each other, resulting in unbalanced voltage difference between the units. Next to the battery unit integrated AFE for each channel. This road can be used to reduce the charging current to each unit, enabling units during the charging of the battery SOC balance provided the conditions. Track based on impedance voltmeter on each determine the chemical state of charge of battery cell, to make the right decision when the need to balance.

Multipolar overcurrent protection with different activation time limit (as shown in Figure 2) making batteries more robust protection. Power meter with two layers of charge/discharge current protection setting, AFE offers third level discharge over-current protection. In short circuit condition, MOSFET and batteries may be destroyed in seconds, power meters rely exclusively on AFE chip set to automatically shutdown MOSFET to avoid damage.

Figure 2. Multistage battery overcurrent protection

When the power gauge IC and their associated with the AFE with overvoltage protection, voltage monitoring sampling features limit the response time for this kind of protection system. Most applications require rapid response and real-time, independent voltage monitors, and operate in conjunction with power gauges, AFE. The monitor is independent of the electricity meter and AFE, monitor the voltage of each battery cell, and for each to achieve hardware encoding batteries provide voltage limit of logic level output. Over voltage protection of response time depends on the size of external delay capacitor. In a typical application, second level protection output will trigger a chemical fuse failure or other protective equipment, permanent separation of lithium-ion batteries and systems.

Permanent failure of the battery pack protection

For battery management unit, very important is to for an abnormal state of battery pack provided tended to be conservative in cut off. Permanent failure of discharging and charging failure of protection includes the current state of discharge and charge State of safety, thermal safety, overvoltage fault status (peak voltage) and battery-balancing fault, FET shorted discharge power, MOSFET failures under State security. Manufacturers may choose any combination of the above permanent failure protection. When any such failure is detected, chemical fuse fuse protection device, so that permanent failure of the battery pack. As a failure of electronic components failure of external validation, charge and discharge MOSFET battery management unit design is used to detect whether Q1 and Q2 's failure. If any charge or discharge MOSFET circuit, the chemical fuses fuse.

It was reported that micro short circuit inside the battery is also causing major causes of recent battery recalls. How to detect micro-short circuit and prevent the battery inside the battery is on fire and the explosion? Shell closed during processing, metal particles and other impurities is possible pollution of battery internal, thus giving rise to small short circuit inside the battery. Internal micro-short circuit will greatly increase battery self-discharge rate, open circuit voltage battery unit than under normal conditions can be mitigated. Impedance track electricity meter monitor open-circuit voltage, cells and thus detection of non-equilibrium – when the different Nikon EN-EL19 Battery battery cell circuit voltage differences exceed pre-set limits. When this type of failure occurs, will have a permanent failure alarm and disconnect the MOSFET, chemical fuse can also be configured as a fuse. Such behavior makes battery packs for power and thus shielding the battery pack cannot be micro short circuit the batteries inside the unit, so as to prevent the occurrence of a disaster.[code][/code]