I. Introduction
In recent years, with the development of various hybrid vehicles and electric vehicles, the performance requirements for on-vehicle batteries have become higher and higher. In particular, plug-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs) are more like this: compared with gasoline-type hybrid vehicles, the battery capacity is higher, and the charge and discharge loss and self-discharge requirements are as small as possible. Therefore, the status of lithium ion batteries is becoming more and more important.
According to a research report by the Japanese market research organization Fuji Economic Group, in 2013, the market for lithium-ion batteries in the world was 567 billion yen. By 2018, its size has increased by 163.8% to reach 928.2 billion yen.
In addition to its small size and light weight, lithium-ion batteries have a Nominal Voltage of up to 3.6 volts and a high energy density (meaning that the same output voltage can be obtained with fewer battery cells). However, from the viewpoint of safety and deterioration of battery performance in order to prevent overcharge and discharge, it is necessary to provide a subsystem (IC) that monitors voltage and temperature of each battery cell in the battery pack. At the same time, considering that such a subsystem is also likely to fail, an independent parallel system for detecting the operating state of the system is also required.
Second, the inherent problems of the series battery pack
When the number of battery cells in the series battery pack is increased to several tens to hundreds, one problem of the series battery pack becomes prominent, which is the battery cell balancing problem.
Although lithium ion batteries are industrially mass-produced products, all battery cells cannot have the same quality in the existing production environment. For example, during the manufacturing process, the change in tension at the time of winding the electrode of the battery unit affects the deterioration rate of the battery unit. On the other hand, it is not required that all battery packs be used in the same environment when in use. During use, the battery cells that are close to the heat source deteriorate faster, whereas the battery cells that are farther away from the heat source deteriorate more slowly.
The problem that arises from this is that the units in the battery pack have different speeds of deterioration with the change of the use time, resulting in a deviation in the capacity of the battery unit.
The overall performance of the battery pack also follows the "barrel principle (short board principle)", that is, the capacity of the barrel depends on the shortest of all the boards that make up the barrel, and the capacity of the battery pack depends on the battery with the smallest capacity. unit. During the charging process, once one of the battery cells reaches a fully charged state, the charger stops charging. The same is true for the discharge process of the battery pack: when a battery unit is discharged, the entire battery pack will also stop discharging. As a result, the entire battery pack has a reduced charging capacity, and the battery capacity cannot be fully utilized.
Let's take a battery pack consisting of three battery cells as an example: if one of the battery cells deteriorates faster. When this battery pack is discharged, the battery unit that deteriorates faster will end the discharge first than the other two battery units. If the discharge continues, the battery unit is in an over-discharged state. Lithium-ion batteries can generate smoke and fire when they are in an excessively discharged state. In order to prevent accidents, only discharge can be stopped at this time, that is, the remaining energy in the remaining two battery cells cannot be used.
Conversely, when the battery pack starts to be charged, the two battery cells that deteriorated slowly are fully charged first; and the battery cells that deteriorate faster are not fully charged at this time. At this time, if the charging is continued with the battery unit that deteriorates faster, the two battery cells that have been fully charged are in an overcharged state. Overcharging can also cause the burning and explosion of the battery. Similarly, in order to prevent the occurrence of an accident, the battery pack ends the charging in a state where the battery unit that deteriorates rapidly is not fully charged.
Studies have shown that for lithium-ion batteries, when the battery is fully charged, the material composition of the positive electrode is delithiated lithium cobaltate (Li0.5CoO2), and the negative electrode is lithium intercalated carbon (LiC6). Lithium cobaltate undergoes a decomposition reaction at high temperature to release oxygen, and the chemical reactivity of lithium intercalated carbon is substantially similar to that of metallic lithium. So if combustion occurs, it is basically equivalent to the burning of metallic lithium in an oxygen-rich environment! This is a terrible thing.
In summary, when the deterioration state of the battery unit is deviated, the maximum capacity of the battery pack cannot be exerted during charging and discharging, and even an accident is caused. From a small place, I often see news about the explosion of a mobile phone while charging. From a big place, the Boeing 787, known as the "dream passenger aircraft", has been continually failing for a long time on the factory route, and some of them are faulty. It may be because the battery cells of the lithium-ion battery used in the aircraft have a problem with the balance of the battery cells. According to reports in early May 2015, the Boeing 787 may have a defect in power supply, and the US Federal Aviation Administration issued an interim directive requiring airline operators to perform "repetitive maintenance tasks" on the Boeing 787. The specific reasons are still unclear, but from the history of problems with the Boeing 787 lithium-ion battery, I am afraid this time is also from the battery.
Therefore, it is necessary to monitor the operating state of each battery cell in the series battery pack at any time by the battery monitoring IC.
Third, the requirements for the vehicle lithium ion battery monitoring system
At present, the safety mechanism required by the foreign vehicle for lithium ion battery monitoring system has the following structure:
Figure 1 Example of the configuration of the drive section and battery monitoring system of hybrid and electric vehicles
The general vehicle power supply system is shown in Figure 1.
A battery pack is formed by connecting dozens to hundreds of battery cells in series to supply power to the inverter and the motor. Since the voltage of the series battery pack is as high as tens to hundreds of volts, it is not possible to monitor all of the battery cells using a separate battery monitoring system. Therefore, typically each battery monitoring system (IC) monitors 8-16 battery cells simultaneously. The battery monitoring IC mainly monitors the voltage, temperature, and cell balance of a battery unit.
In the in-vehicle battery monitoring system, the battery monitoring IC does not judge the measurement result of the voltage or the like of each battery cell, but merely submits the measurement information to the MCU (microcomputer unit).
Each battery monitoring IC and MCU constitute a battery monitoring unit. The unit integrates battery voltage, current and temperature information, and derives the state of charge of the battery and transmits it to the onboard computer system to control the charging and discharging of the battery pack at this level.
Figure 2 Example of three methods for evaluating the accuracy of battery monitoring IC measurement
Therefore, voltage measurement of the battery unit is an important function of the battery monitoring IC. Accordingly, the evaluation of the measurement accuracy of the battery monitoring IC is also very important. Figure 2 shows three typical circuits for evaluating the accuracy of battery monitoring IC measurements.
Among them, A) circuit uses two sets of ICs to perform redundancy monitoring on the same battery pack; B) provides a standard voltage source 2 from the outside to confirm the measurement accuracy of the IC. C) to generate the standard voltage source internally.
Here, the A) method can increase the redundancy, but it also increases the complexity of the system; B) and C) use two standard voltage sources independent of the standard voltage source 1 of the A/D converter. This voltage was subjected to A/D conversion to evaluate the measurement accuracy of the IC.
However, for this independent standard voltage source 2, it is also necessary to consider the possibility of failure due to the same cause. For example, if the standard voltage source 1 of the A/D converter and the standard voltage source 2 use the same circuit, the same power supply and the same duty ratio, each voltage source is more likely to exhibit the same output voltage. . As a result, failure cannot be detected using this method. To solve this problem, the best way is to use B) to provide an independent standard voltage source 2 from outside the battery monitoring IC, but this may increase the cost. Therefore, how to maintain the independence of the standard voltage source 2 with respect to the standard voltage source 1 of the A/D converter while using the C) method is an important problem. For example, as a means of maintaining independence, measures such as different circuits are employed. This aspect involves the internal secrets of various battery manufacturers, and this article is here to cut love.
Fourth, use the battery monitoring IC to play the biggest role of the battery unit
In summary, the main task of the battery monitoring IC is
1. Determine the voltage of the battery unit
2. A/D conversion
3. Communicate with the MCU
The purpose of performing these three tasks is to complete the most important tasks of the battery monitoring IC:
4. Keep the battery unit balanced
The battery monitoring IC monitors the terminal voltages assigned to each of its own cells at any time and transmits the measurement results to the MCU. The MCU analyzes the voltage of each battery unit and analyzes whether the storage capacity between the battery units is the deviation of the battery unit balance. If a deviation occurs, the MCU gives an indication to the battery monitoring IC to ensure the balance of the battery unit.
At present, there are two ways to ensure battery cell balance: Passive balance and AcTIve balance.
The passive equalization method uses a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) built in a battery monitoring IC, or an externally added MOSFET to discharge thermally.
The advantage of establishing battery cell balance by passive means that the whole system is very simple, but the disadvantages are also great: forced discharge of residual energy will cause the energy efficiency of the whole system to be low, and the main purpose of the battery to save energy as much as possible runs counter to it.
The active equalization method is to transfer the remaining electrical energy in one battery unit to another battery unit, thereby maintaining the equalization of each unit. The disadvantage is that the whole system is more complicated, but at the same time it can improve the energy utilization of this system.
Nowadays, many lithium-ion batteries have begun to add protection circuits to the battery. For example, the 18650 lithium-ion battery commonly used in the market (this type of battery is often used in notebook computers), it should be 65mm long/18mm in diameter from the numbering method. In fact, the most recent battery of this type is added in the middle. The protection circuit and various protective measures, so the length is extended to about 68mm.
The battery monitoring ICs introduced abroad now include:
Linear Technology's LTC3300-1 High Efficiency Bidirectional Battery Monitoring IC
Freescale's battery monitoring IC for industrial and automotive control of 14 battery cells - MC33771
Battery Management Unit (BMU) and Power Gauge Chips from O2Micro InternaTIonal Limited
In addition, ROHM Semiconducto has developed a new electric double layer capacitor (EDLC, Electric DoubleLayer Capacitor) and its associated monitoring IC - BD14000EFV-C.
V. Research on foreign battery monitoring ICs
Now manufacturers are working hard to increase energy density and output density on the basis of cost reduction. At the same time, according to the different ways of using the battery, try to highlight its characteristics. For example, the main development direction of vehicle battery is miniaturization, high energy density and ability to withstand high-speed charge and discharge; battery for home life emphasizes large capacity, low cost and good durability; battery for medical institutions focuses on safety and stability. Sex, but not cost.
In Japan, the unit cost of lithium-ion batteries in 2010 was 200,000-300,000 yen/kWh. In 2015, the cost was reduced to around 30,000 yen, and the target for 2020 was around 10,000 yen. This value is equivalent to the unit cost of using a lead storage battery or a pumping power generation system. Once this goal is achieved, it will be possible to change the power storage structure of the entire society.
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