毕业设计外文翻译----燃料电池

附件2：外文原文

Battery Fuel Gauges: Accurately Measuring Charge Level

Abstract: Battery fuel gauges determine the amount of charge remaining in a secondary battery and how much longer (under specific operating conditions) the battery can continue providing power. This application note discusses the challenges presented in measuring the charge remaining in a lithium-ion battery and the different methods of implementing a fuel gauge to address these challenges.

Introduction

Since the advent of the mobile phone, chargeable batteries and their associated fuel-gauge indicators have become an integral part of our information and communication society. They are just as important to us now as automotive fuel gauges have been for the past 100 years. Yet, while drivers do not tolerate inaccurate fuel gauges, mobile-phone users are often expected to live with highly inaccurate, low-resolution indicators. This article discusses the various impediments to accurately measuring charge levels and describes how designers can implement accurate fuel gauging in their battery-powered applications.

Lithium-Ion Batteries

Lithium-ion batteries have only been in mass production since about 1997, following the resolution of various technical problems during their development. Because they offer the highest energy density with respect to volume and weight (Figure 1), they are used in systems ranging from mobile phones to electric cars.

Figure 1. The energy densities of various battery types.

Lithium cells also have specific characteristics that are important for determining their charge level. A lithium battery pack must include various safety mechanisms to prevent the battery from being overcharged, deeply discharged, or reverse connected. Because the highly reactive lithium can pose an explosion hazard, lithium batteries must not be exposed to high temperatures.

The anode of an Li-ion battery is made from a graphite compound, and the cathode is made of metal oxides with lithium added in a way that minimizes disruption of the lattice structure. This process is called intercalation. Because lithium reacts strongly with water, lithium batteries are constructed with non-liquid electrolytes of organic lithium salts. When charging a lithium battery, the lithium atoms are ionized in the cathode and transported through the electrolyte to the anode.

Battery Capacity

The most important characteristic of a battery (apart from its voltage) is its capacity (C), specified in mA-hours and defined as the maximum amount of charge the battery can deliver. Capacity is specified by the manufacturer for a particular set of conditions, but it changes constantly after the battery is manufactured.

Figure 2. The influence of temperature on battery capacity.

As Figure 2 illustrates, capacity is proportional to battery temperature. The upper curve shows an Li-ion battery charged with a constant-I, constant-V process at different temperatures. Note that the battery can take approximately 20% more charge at high temperatures than it can at -20°C.

As shown by the lower curves in Figure 2, temperature has an even greater influence on the available charge while a battery is being discharged. The graph shows a fully charged battery discharged with two different currents down to a cut-off point of 2.5V. Both curves show a strong dependence on temperature as well as discharge current. At a given temperature and discharge rate, the capacity of a lithium cell is given by the difference between the upper and lower curves. Thus, Li-cell capacity is greatly reduced at low temperatures or by a large discharge current or by both. After discharge at high current and low temperature, a battery still has significant residual charge, which can then be discharged at a low current at the same temperature.

Self-Discharge

Batteries lose their charge through unwanted chemical reactions as well as impurities in the electrolyte. Typical self-discharge rates at room temperature for common battery types are shown in Table 1.

Table 1. The Self-Discharge Rates of Common Battery Types

Chemical reactions are thermally driven, so self-discharge is highly

temperature-dependent (Figure 3). Self-discharge can be modelled for different battery types using a parallel resistance for leakage currents.

Figure 3. Self-discharge of Li-ion batteries.

Aging

Battery capacity declines as the number of charge and discharge cycles increases (Figure 4). This decline is quantified by the term service life, defined as the number of charge/discharge cycles a battery can provide before its capacity falls to 80% of the initial value. The service life of a typical lithium battery is between 300 and 500 charge/discharge cycles.

Lithium batteries also suffer from time-related aging, which causes their capacity to fall from the moment the battery leaves the factory, regardless of usage. This effect can cause a fully charged Li-ion battery to lose 20% of its capacity per year at 25°C, and 35% at 40°C. For partially charged batteries the aging process is more gradual: for a battery with a 40% residual charge, the loss is about 4% of its capacity per year at 25°C.

Figure 4. Battery aging.

Discharge Curves

The characteristic discharge curve for a battery is specified in its data sheet for specific conditions. One factor affecting battery voltage is the load current (Figure 5). Load current cannot, unfortunately, be simulated in the model by a simple source resistance, because that resistance depends on other parameters such as the battery's age and charge level.

Figure 5. Battery-discharge curve.

Secondary lithium cells exhibit relatively flat discharge curves in comparison with primary cells. System developers like this behaviour because the available voltage is relatively constant. However, gradual discharge makes the battery voltage independent of the battery's residual-charge level.

Accurately Measuring Charge Level

To determine the available charge in a battery, simple monitoring methods are preferred. They should consume little energy and should (ideally) allow one to deduce the charge level from battery voltage. Such a voltage-only method can produce unreliable outcomes, however, because no clear correlation exists between voltage and the available charge (Figure 5). Battery voltage also depends on temperature, and dynamic relaxation effects can cause a slow increase in the terminal voltage after a reduction in load current. Thus, purely voltage-based monitoring is unlikely to provide charge-level accuracies better than 25%.

The relative charge level, often called the state of charge (SOC), is defined as the ratio of residual charge to the battery's charge capacity. Hence charge flow must be measured and monitored through a procedure called "coulomb counting." In practice, coulomb counting is accomplished by integrating the currents flowing into and out of the cell. To measure these currents with a high-resolution ADC, one typically connects a small resistor in series with the anode.

Fuel-Gauge Learning

The functional relationship between battery SOC and the parameters mentioned above cannot be related analytically, so cell capacity and charge must be determined empirically. No extensive analytical models are available for calculating (with sufficient accuracy) the capacity of a battery under practical operating conditions such as temperature, number of charge cycles, current, age, etc. Theoretical models apply only to certain "local" conditions. For determining relative charge levels, they are applied locally and calibrated globally.

To achieve sufficient accuracy while a battery is in use, the model parameters must be calibrated constantly through a process called fuel-gauge "learning." In conjunction with coulomb counting, that approach yields fuel gauges accurate to within a few percent.

Fuel-Gauge Selection

Modern integrated circuits can determine the SOC for all types of secondary cells, cell configurations, and applications. Despite their low supply current (about 60μA in active mode and 1μA in sleep mode), these ICs achieve a high degree of accuracy. Fuel-gauge ICs fall into three categories (Table 2). Because lithium-based batteries are preferred for many applications, the examples shown are based on Li-ion and Li-polymer batteries.

Table 2. Fuel-Gauge Circuits.

Coulomb counters, sometimes known as battery monitors, are ICs that measure, count, and convert the battery's parameters mentioned above, including charge, temperature, voltage, load cycles, and time. Because coulomb counters do not process the measured variables, they are not intelligent. One such device, the DS2762, already includes an integrated, highly accurate 25m resistor for measuring current. It monitors temperature, battery voltage, and current, and it features a 1-Wire? bus that allows all readings to be read by a microcontroller residing in the battery pack or host system. It also offers the requisite safety circuit essential for secondary Li cells. The result is a flexible,

cost-effective system that requires considerable knowledge and development effort (although costs are offset by the software, models, and support provided by the IC vendor).

An alternative approach to the coulomb counter is provided by fuel gauges. These

all-in-one devices perform fuel-gauging routines with a learning algorithm, and they perform all necessary measurements on their own. Fuel gauges are typically deployed in intelligent, autonomous batteries called smart batteries. Because development effort is considerably less with integrated fuel gauges, this approach is well suited for applications that demand a quick time to market. One such fuel gauge, the DS2780, allows the host to read the SOC using the 1-Wire bus.

Another option is provided by programmable fuel gauges, which include integrated microcontrollers that provide considerable flexibility. The MAX1781, for example, includes an integrated RISC core, EEPROM, and RAM. This device enables developers to implement battery models, fuel-gauging routines, and measurements as required. Integrated LED drivers support simple but accurate SOC indication.

Summary

Fuel gauging of chargeable battery cells is a complex task due to the many interdependent parameters that influence cell capacity. Simple methods of measurement, therefore, deliver inaccurate results that are adequate only for non-critical applications. By utilizing off-the-shelf fuel-gauge ICs, however, one can implement highly accurate and reliable fuel gauges.

燃料电池,准确测量负载水平

文摘:电池燃料仪检测电荷残留在废旧电池和多少时间(在特定的操作条件)电池可以继续提供电力。应用笔记进行了讨论,提出在测量其电荷残留在锂离子电池和不同的方法来实现燃料表克服这些挑战。

介绍

自问世以来,充电手机电池及其相关fuel-gauge指标都已成为信息和通讯社会不可分割的一部分。现在它们的重要性就像汽车燃油压力表在已经在过去的100年那样。然而,司机不容忍不准确的燃料压力表,手机用户期望生活在高度准确、低解析度的指标下。本文论述了如何准确测量电荷水平和设计者怎样准确实施燃料电池应用的各种障碍。

锂离子电池,

锂电池在1997年左右开始大规模生产,并在接下来的发展中解决了各种技术问题。因为他们提供最高的能量密度体积、重量(图1),它们被用于从手机到电动汽车各种的系统。

图1。各型电池的能量密度。

锂电池也有特定的特点,确定他们的负载是非常重要的。一个锂电池包必须包括多种安全机制,以防止电池被过度充电,分解,或反向连接。因为高活性锂会引起爆炸的危险,所以锂电池不得暴露于高温。

锂离子电池的阳极由一个石墨组成、阴极是由锂金属或锂合金组成。这个过程被称为夹层。因为锂强烈地与水起化学反应，锂电池用有机锂盐的非液体电解物

来构造。当锂电池充电时, 锂离子原子在阴极和阳极通过电解液的运送。

电池容量

一个电池组（除了它的电压之外）的最重要的特性是它的容量（C）指明毫安小时并作为充电电池可提供最高电荷界定。容量为一特定组条件被制造厂商指定，但是在电池是制成的之后，它不变地改变。

图2。温度对电池的容量的影响。

如图2所示，容量与电池温度成正比。上面的曲线显示了在不同温度下锂离子电池用恒流，恒压的过程中的被控。注意该电池在高温时会比它可在-20℃时多消耗大约20％的电荷。

正如图2中的较低的曲线显示, 当电池正在放电时温度就可以具有更大的影响力。图显示了电池在两个不同的电流降到2.5V时的放电情况。两条曲线表现出强烈依赖于温度以及放电电流。在一个给定的温度和放电率,锂电池的容量是由上下曲线之间的差额决定。因此,锂电池的容量较低的温度下或被一个大电流放电或两者兼而有之是被大大削弱。经过放电电流高低温后，电池仍具有显着的剩余电荷，然后可以在在同一温度低电流放电

自放电

电池通过不必要的化学反应以及杂质的电解质失去他们的电量。在室温下,普通电池类型的典型自放电率都显示在表1。

表1 普通电池类型的自放电率

化学自放电率/月

铅酸 4%至6%

NiCd 15%至30%

镍氢 30%。

锂 2%到3%

化学反应的热反应,自放对温度有很高的依赖性(图3)。自放电通过利用并联电阻可以模拟不同类型电池的泄露电流。

图3 锂离子电池自放电

老化

电池容量下降作为充电和放电周期的增加(图4)。这种下降被使用寿命所量化,被定义为一个电池充放电循环的次数, 电池可提供的容量下降至初始值的80％。一个锂电池的寿命是充放电300至500次。

锂电池的老化受时间影响, 这会导致它们的容量从出厂的那刻起开始下降，无论是否使用。这种效应可能会导致完全充电的锂离子电池在25℃诗每年失去其20％的能力，在40℃时每年35％。对于部分充电电池的老化过程是渐进的: 对于一个有40％的剩余充电电池，在25℃时,每年损失约4％的电量。

图4。电池老化。

放电曲线

对于电池的放电特性曲线是在其指定的数据表的具体条件。一个影响电池电压的因素是负载电流(图5)。不幸的是,负载电流不能对模型进行数值模拟,因为一个简单的电源电阻,取决于等参数的电池的年龄和电荷的水平。

图5。电池放电曲线。

二次锂电池放电曲线展现相比于普通的电池相对平坦。系统开发商像这样的行为是因为现有的电压是相对稳定的。然而,逐步放电使电池电压独立于的电池的剩

余充电水平。

准确地测量电荷水平

确要确定一个可充电电池，简单的监测方法是首要条件。他们应该耗能少, 应该（最好）允许从电池电压推断充电水平。这种电压唯一的方法会产生不可靠的结果,然而,由于电压和可用的电荷之间没有明显的相关性(图5)。电池电压也取决于温度、动态松弛效果可以减少电流在负载端电压在缓慢增长。因此，纯粹以电压为基础的监督是不可能提供超过25％的电荷水平精度。

这个电荷密度的相对水平, 通常称为充电状态（SOC），是作为剩余电量的比例，电池的充电定义的能力。于是电荷流量测量和监测必须通过一个被称为“库仑计数”的程序进行。在实践中, 库仑计数是通过集成的电流流入和流出电荷表示。为了用一个高分辨率ADC衡量这些电流，通常使用一系列与阳极连接小电阻。

电量计学习

由于电池SOC及以上提到的不能解析相关参数功能之间的关系，导致电池容量和充电必须通过实践确定。没有大量的分析模型可用于计算（有足够的准确度）实际经营状况的电池容量如温度，充电周期，电流，年龄等。理论模型只适用于某些“局部”的条件。确定相对电荷水平，将它们适用于本地和全球范围内校准。

为了达到足够的精度，而电池在使用中，该模型参数必须不断校准,这个过程称为电量计“学习”。结合库仑计数，这种做法产生的燃料计精确到百分之几。

电量计选择

现代的集成电路可以决定所有类型的二次电池的SOC，电池配置和应用。尽管他们的低电源电流（约60μA的工作模式和睡眠模式仅为1μA），这些IC仍能实现高精确度。电量计芯片分为三类（见表2）。由于锂基电池是许多应用的首选，这些例子所示是基于锂离子和锂聚合物电池。

库仑计数器，有时被称为电池显示器也称为集成电路，用于测量，计数，并将上述转换成电池的参数，包括收费，温度，电压，负载周期和时间。由于库仑计数器不处理标准变量，它们也不聪明。一个这样的设备，DS2762，包括一个集成的，测量电流的高度准确的25m电阻。它监视温度，电池电压和电流，它具有一个1 - Wire ?总线，允许所有数据通过一个电池组或主机系统上微控制器读取。它还提供了锂电池中必需的安全电路。其结果是一个灵活，高性价比，需要相当的知识和发展成就（虽然成本由软件，模型和IC供应商所提供）的系统。

另一种库伦计算的方法与燃料计提供。这些一体化设备进行电量及程序与学习算法,并执行所有必要的测量。燃料传感器通常部署在智能的，自主研发的智能电池中。由于开发工作相当少运用的集成燃油计，这种做法是很好的并能快速适应市场需求。一个这样的电量计，DS2780电量计，允许主机读取SOC的使用1 - Wire 总线。

另一种选择是可编程电量表，其中包括可提供相当大灵活性的集成微控制器。例如MAX1781，包括一个集成的RISC核心，EEPROM和RAM。该器件使开发人员能够实现电池型号，燃料测量例程，并根据需要测量。集成LED驱动器支持简单而准确的SOC的迹象。

摘要

燃料充电电池测量是一项复杂的任务，因为许多相互依存的参数影响电池的能力。因此，如果没有适当的非关键应用，简单的方法测量可以提供准确的结果。通过利用现成的电量计芯片，每个电池都可以实现高度精确和可靠的燃油压力表。