蓄电池充电

CHAPTER 4 BAITERY CHARGING We tested a lead-acid battery to detennine its lifetime by discharging it com- pletely and then recharging it to a fully charged state in each charge -discharge cycle. At the end of 50 charge-discharge cycles we had a dead battery Sophisticated new battery-charging technology is key to the efficient and economic use of batteries for vehicle propulsion. In the sections that follow we (I) quantify for each battery the unique profiles in voltage, current, and depth of discharge that give long battery life and (2) discuss new automatie battery- charging controls that make long life possible. 4.1 HISTORY OF BATTERY-CHARGING TECHNOLOGY Gaston Plante invented the lead-acid battery in 1859. By 1900 there were more electric cars than engine-powered cars in the United States. However, owners soon leamed that buying a Model T Ford was much eheaper than replacing the failed battery in an electric vehicle. Then in 1912 Charles F. Kettering invented the lead-acid battery-powered electric starter for automobile engines. The starter was powered by a lead-acid battery. Subsequently brush-and-commutator gen- erators on cars replaced the magnetos on cars. Automobile designers did not understand the requirements for long battery life, so even today lead-acid bat- teries for cars carry a life guarantee of around 60 months . The Cold War brought requirements for postattack power in missile silos and command posts. Then, after serious analysis, and even in testing in which one lead-acid battery exploded, we leamed the limits of lead-acid battery technol- ogy. Then came the need to power Earth-orbiting satellites during their frequently occurring transits through the Earth's shadow when sunlight from solar cell arrays was not available. Years of intensive analysis and testing revealed that nickel-cadmium batteries could last for many years if not more than 30 per- cent of the battery's stored energy was consumed in each eclipse . The cost of launehing these oversized nickel-cadmium batteries motivated intense battery development. Nickel-metal hydride batteries can now power Earth satellites for decades in Earth orbit. Electric Bicycles : A Guide 10 Design and Use, by William C. Morehin and Henry Oman Copyright © 2006 The Institute of Electrical and Electronics Engineers, Inc. 85 86 CHAPTER 4 BATTERY CHARGING 4.2 BASIC FUNCTIONS OF BATTERY CHARGERS To understand the requirements of battery chargers, we review the performance characteristics of batteries that need charge control for achieving long life. The basic functions of battery charge control are : • When supplied with public ac power, the bicycle's battery recharges to a full-charge condition after it has propelled the electric bicycle. • Perform this recharge in a manner that preserves the battery's usable life- time in propulsion service . A voltmeter cannot show a true battery charge status because the terminal voltage in most batteries varies with load and battery temperature. Therefore, a charge control must integrate the current flow, during both charge and discharge, to track the battery's charge status for indicating discharge state. Charge con- trollers that use specially designed integrated circuits to perform these functions have been developed. These controllers are commercially available at reason- able prices. The bicyclist needs to be aware of the state of charge of the battery on his bicycle so that he can avoid completely discharging the battery, which reduces battery life. For example, the Honda EV Plus carries a "management electronic unit" that displays to the driver the distance of travel available with the state of charge of the battery (Fig. 4.1). It also reports a "capacity reduction" factor that indicates aging of the battery. An ancillary function of battery charging would be to report to the cyclist, during his travel, the charge status of the battery, and warn hirn when the battery's completely discharged condition is approaching. 4.3 BATTERY CHARACTERISTICS PERTINENT IN CHARGING Important charging features of batteries are: • The charging voltage is always higher than the discharging voltage at a given state of charge. The voltage difference, which depends on current density, represents a loss. Power Control Unit Pack Vollag e L j--------' Serial Comm. Contactor I Current Pressure TemperatureI----~ Management ECU 1 ..I.. T ..I..-,- ..,. I • • • • • •!.. qp..' Battery Box Figure 4.1 The Honda EV Plus carries a "management electronics unit" that supplies the driver a display of his distance of travel available with the state of charge of the battery, and other data. 4.4 LEAD-ACID BATTERY CHARGING 87 • The life of a battery in terms of number of charge-discharge cycles gen- erally varies with depth of discharge. Deep discharges shorten cycling life. The different characteristics of candidate batteries for electric bicycle propulsion are summarized in Table 4.1. The terminal voltage of any battery type varies with the current ftow, during both charge and discharge activity. The values shown in the table are nominal values. This voltage at different current levels also varies with temperature in a unique manner for each battery type. Lithium batteries had a limited life in charge/service at the time that this chart was created. Subsequently, the causes of their limited life have been discovered, and lifetimes of over 25,000 charge-discharge cycles have been demonstrated. 4.4 LEAD-ACID BATTERY CHARGING A battery cell discharges when the negative plate can deliver electrons through a conductor to the positive plate (cathode). The lead-acid negative battery plate (anode), during discharge, absorbs sulfate ions from the electrolyte, forming lead sulfate and releasing hydrogen ions that drift to the positive plate to form lead sulfate and water. The battery is fully discharged when its plates are covered with lead sulfate. TABtE 4.1 BaUery Characteristics Pertinent to Recharging a Nominal Nominal Deeply Depth of Battery Cell Voltage Charge Voltage Discharged Discharge Life Storage Type (V) (V) State (V) (%) Cycles Capacity Loss Lead-acid 2 2.4 1.5 20 1500 0.27%/day b 80 80 0.174%/day C 0.035%/day d NiCd 1.2 1.4 0.8 20 500 1.56%/day e 20 16500 h 80 625 h NiMH 1.2 1.4 0.8 20 500 1.56%/day e 80 6000 h Lithium 3.4 g 4.2 2.5-2.7 20 1000 0.33%/day/ 80 Zine-air 1.2 Not applicable 0; Na 200 Na a J. S. Enochs and his colleagues reported achieving over 2000 charge-discharge cycles with 80% depth of discharge with lead-calcium-tin grids. (From J. S. Enochs, Nonantimonial Lead-Acid Batteries for Cycling Applications, Proceedings of the 19th Intersociety Energy Conversion Engineering Conference, ANS, 1984, pp. 850-856.) h Standard grid. C Low antimony grid. d Calcium lead grid. e Self-discharge is highest within first 24 h, 60/0 for NiCd and 9% NiMH. f Includes 0.1%/day for self-protection circuits. g At 50% of capacity, 3.0 V at 200/0 of capacity. h Space application NiCd batteries. 'When zinc is consumed, there is no voltage. 88 CHAPTER4 BATTERY CHARGING A simple charger for lead-acid batteries is easy to design and build. It consists of a transformer that reduces the public power supplied voltage and a rectifier that converts the altemating current to direct current at the battery charge voltage. Low-cost chargers are available at stores that seIl computers, automobile repair parts, and amateur radio equipment. The options available for charging a lead-acid bicycle propulsion bat- tery illustrate the range of available characteristics available in battery-charging technology and how their possible benefits can be evaluated. For example, the battery can be recharged with a transformer-rectifier that is plugged into an ac public outlet and designed to deliver to the battery all the current needed to maintain a voltage of 2.4 V per cell until the battery is fully charged and accepts no further current. At this point the cell voltage is maintained at 2.37 V per cell. One common technique for recharging storage batteries is simply connect- ing the battery terminals to a dc voltage source that has a voltage that is greater than the battery voltage. This voltage difference will cause a charging current to ftow through the battery and reverse the chemical reaction that occurred dur- ing discharge. The charging current deereases as the voltage differenee between the eharging voltage and the battery voltage decreases. Typically, the seleeted eharging voltage is greater than the nominal battery voltage in order to cause a slight overeharge of the battery. The battery is deemed to be "eharged" when the battery will aeeept no additional current. Most battery ehargers have an ammeter, and the user is instructed to switeh off the eharger when the indicated current falls to zero. This constant-voltage charging teehnique is relatively safe since as the charging proeess progresses, the eharging eurrent deereases until it is just a triekle. Constant-voltage chargers are designed for overnight restoration of a diseharged battery to a fully charged condition. An alternative for quickly recharging a battery is a constant current charger that varies the voltage that is applied to the battery terminals in order to maintain a eonstant current ftow. The charger automatieally raises its voltage to keep the constant eurrent ftowing. The charger contains a controller that monitors the bat- tery voltage and current ftow. When the battery reaches full charge, the controller stops the eonstant eurrent charging. Otherwise, overeharging would permanently damage the battery and might even boil of the battery eleetrochemicals. Bert- ness [1] uses a controlled eurrent bypass eireuit in a constant current souree. The eharging current is divided between the battery being charged and the bypass eircuit. Both the battery voltage and its temperature are used in controlling the bypass current. . Other charge control methods are similar to those described for lithium and niekel-based batteries. For example "ehopped" current waveforms can change the effective value of charging current. Control eircuitry can also limit charging to a low "trickle" level if the battery had been completely discharged. Otherwise the relatively low battery resistance for this battery condition could overload the charger. As the battery gets charged, this resistance rises so that full charging current can be applied. 4.6 SMARTCHARGERS FOR NEW NI-CD, NI-METAL HYDRIDE, AND LITHIUM HATTERIES 89 4.5 CHARGER DESIGN FOR LONG BATTERY LIFE Another problem with battery charging is that the temperature of the battery typically rises during the recharging cycle. As the temperature of the battery increases, its chemical reactivity increases. The reactivity approximately doubles for every 10°C temperature rise in lead-acid batteries. Furthermore, as the tem- perature of the battery increases, its internal resistance decreases so the battery will accept a higher charging current at a given charging voltage. Bertness [1], uses the function v = 14.32 - O.024°C (4.1) as the voltage to be applied to an automobile lead-acid battery. The term °C is the battery temperature in degrees Celsius (equal to or greater than 0 for the voltage function). The decreasing voltage function is used to decrease an other- wise constant charging current with increasing battery temperature. Other types of lead-acid batteries can be charged with different linear functions by changing val- ues of resistors within the Bertness charger. If it were not regulated, the increased current flow would generate additional heating in the battery, further reducing its internal resistance. This battery heating, followed by an increase in battery charging current, could result in a runaway condition that can damage the battery. The direct current for the charger is obtained from apower conditioner. The power conditioner that rectifies the source current is connected directly to the public ac power supply. It then converts the power to a high-frequency (typ- ically in the order of 25 kHz) pulsed current that goes through a step-down transformer into a rectifier that produces direct current for charging the battery. This approach decreases the size of the transformer because it transforms higher frequency power. Discharging less than the full capacity of a battery during each use, plus sophisticated charge control, can make a lead-acid battery that is designed for bicycle propulsion have a lifetime of many years. For example, the battery carried on a bicycle needs to be a sealed type to avoid the damage that the leaking sulfuric acid electrolyte could cause if the bicycle were laid on its side or suddenly accelerated on bumpy roads or in maneuvers. Also, the bicycle might be ridden in cold weather. Therefore, the charger would need to recognize that the battery is cold and modify the profile of its voltage-current output appropriately. By using integrated circuit technology that is available today, a lightweight charger can be programmed to make a battery have its longest possible life. 4.6 SMART CHARGERS FOR NEW NICKEL-CADMIUM, NICKEL-METAL HYDRIDE, AND LITHIUM BATTERIES There is a growing desire by users to charge batteries quickly. However, batter- ies do not react rapidly to either charging or discharging. The faster batteries are 90 CHAPTER 4 BATTERY CHARGING charged, the less total energy they will accept before reaching voltage or temper- ature limits. Surpassing these limits either causes damage or reduces battery life. There are three regimes of time for battery charging: slow, quick, and rapid. The slow rate is generally at a current rate of C/IO or less, where C is the battery's rating in ampere-hours. The quick rate is generally at a current rate of around C/3 and the rapid rate is C/I.5 or higher [2]. These rates correspond to time spans of 10 h or more for "slow," around 3 h for "quiek," and 1 h or less for "rapid." There is also a maintenance mode for whieh the battery is connected to the charger when it is not being used for a long period, Battery-charging systems have ranged from a simple transformer-rectifier type to complex systems that monitor and control the charging function. To reliably and efficiently charge NiCd, NiMH, and lithium batteries at high rates requires careful control of the charging operation to avoid damage to the cells, partieularly under extreme ambient temperature conditions. 4.6.1 Problems 10 Overcome A characteristic of the nickel-cadmium and nickel-metal hydride battery eells was their fall-off in aceepting charge during the eharging process. Not all of the current supplied to the battery is recoverable or utilized in the ehemieal reaetions by whieh the battery is charged. The pereentage of incremental input eurrent, or charge, that is recoverable at any given point in the charge cycle is referred to as the eharge aeceptance of the battery. The cumulative charge accepted by the battery determines the battery state of charge in terms of the percentage of full charge [3]. To fully recharge the ceIls, as much as 160 percent of their rated energy capaeity needs to be replaced [4]. This extra charging energy is dissipated as heat. The added heat can raise the eell temperature to the point of cell damage beeause of the relatively high thermal resistanee of batteries. Generally, the input eharging eurrent supplied to a battery must be limited in order to prevent an overtemperature condition. It has been shown that eharging a NiMH battery at a safe C/IO rate only 50 percent of the charge was aecepted within 10 h [4]. The battery required 16-h to reach full charge at the C/IO rate. Higher current eharging is required to shorten the charge time, but higher currents also raise the battery temperature. For this reason, the battery temperature is sometimes measured and used to trigger a reduction in the charging current. Also as the battery temperature rises, the charge acceptance is degraded, often resulting in an ineapability of the battery to reaeh its fully eharged state. The most critical factors in determining the maximum allowable charge eurrent that can be safely delivered to these batteries are temperature and state of charge. At low temperatures the oxygen recombination rate is significantly redueed. This limits the allowable overcharge current that may be applied with- out venting the cells if they are fully charged. At high temperatures the heat released by the oxygen recombination reaction may cause exeessive cell temper- ature leading to premature failure of the plate separator material and possibly a subsequent short circuit. 4.6 SMART CHARGERS FOR NEW NI-CD, NI-METAL HYDRIDE, AND LITHIUM BAlTERIES 91 A nickel-metal hydride battery, in particular, has a temperature rise gra- dient that varies greatly with charging current and the already charged capacity. Furthermore, the nickel-metal hydride battery is less resistant to overcharge than is the nickel-cadmium battery. If overcharged, the nickel-metal hydride battery's life is shortened [5]. Figure 4.2 shows an example region of acceptable and unacceptable rates of charge as a function of battery temperature. If overcharging occurs, the current will cause generation of oxygen gas with only a relatively small amount of charge actually being stored in the cell. If the charge rate is too high, the rate of oxygen recombination that occurs within the cell may be insufficient to prevent excessive internal pressure and cell venting, which drastically reduces the useful life of the cell. If the battery is fully discharged, minimal oxygen generation will occur until the battery nears the fully charged condition. And when the battery is nearly fully charged, it can quickly enter the overcharge condition and begin oxygen generation. The difficulty lies in accurate determination of the preceding state of charge to avoid damage to the battery. Repetitive shallow discharging of the cells causes another problem with NiCd cells. Repetitive shallow discharge progressively reduces the capacity of the cell. Figure 4.3 illustrates this. CIS Q) 7ü Cl6.7a:: Q) ~ 0'10ca s: 0 0'20 15 35 55 75 Temp. (OF) 95 115 Figure 4.2 Example regions for nickel-cadmium and nickel-metal hydride cells of acceptable overcharging rate in terms of battery capacity as a function of battery temperature [7]. 1.3 1.2 S(5 1.1 > 1.0 .9 Time (h) Figure 4.3 Example that shows reduced NiCd cell capacity after repetitive cycles of shallow discharging [7]. 92 CHAPTER4 BATTERY CHARGING Lithium-based batteries can be dangerous if overcharged, in contrast to either lead-acid or nickel-based batteries. An overcharged lead-acid battery will electrolyze easily replaced water, and nickel-cadmium or meta1 hydride batteries have voltages that stop rising at full charge. However, the voltage of a lithium-polymer battery cell continues to rise even while being overcharged, and lithium ion cells carry a risk of generating excess gas due to overcharge or overdischarge. Lithium metal is explosive in water and will, in varying degrees, react with the moisture in the atmosphere. Lithium-containing batteries have been known to explode or catch fire, although more recent safety designs have reduced the chances of this occurrence. In the lithium-polymer battery cell, wherein lithium ions are contained in a solid polymer, there is no liquid to vaporize. Overcharging depletes the lithium ions off of the plate, thus breaking its electrical connection. The life cycle of the cell is decreased even if a catastrophe does not happen. The avoidance of overcharge voltage and overcharge current during charging of a lithium-based cell is therefore an important objective in the use of such batteries. Another problem is the equality between cells in a multicell battery. There can be many causes for differences between cells and, consequently, their state of charge.