附件2：外文原文Battery Fuel Gauges: Accurately Measuring Charge LevelAbstract: 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. IntroductionSince 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 BatteriesLithium-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 CapacityThe 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 -20C. 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-DischargeBatteries 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 Chemistry Self-Discharge/MonthLead-acid4% to 6%NiCd15% to 30%NiMH30%Lithium2% to 3%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. AgingBattery 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, regardles