Comparison of lead-acid batteries and lithium batteries as power batteries

　　Power batteries are one of the key technologies of electric vehicles. When Gustave Trouve built the world's first electric tricycle in 1881, he used lead-acid batteries. At present, many hybrid vehicles and pure electric vehicles still use the new generation of lead-acid batteries. In the past decade, lithium-ion power batteries have been used in the production of electric vehicles, and their superiority has been increasingly demonstrated.

　　American scholar Max JAMas proposed the battery charging acceptable current theorem through a large number of experiments: 1) For any given discharge current, the battery's charge acceptance current is proportional to the square root of the discharged capacity; 2) For any discharge depth, a battery's charge acceptance ratio is proportional to the logarithm of the discharge current, and the charge acceptance ratio can be increased by increasing the discharge current; 3) When a battery is discharged at several discharge rates, its acceptance current is the sum of the acceptance currents at each discharge rate. In other words, the battery 's charge acceptance current can be increased by discharging. When the battery's charge acceptance capacity decreases, discharge can be added during the charging process to increase the acceptance capacity.

　　The performance and life of automotive power batteries are related to many factors, in addition to their own parameters, such as the quality of the battery plates, the concentration of the electrolyte, etc.; there are also external factors, such as the battery charging and discharging parameters, including charging method, charging end voltage, charging and discharging current, discharge depth, etc. This brings a lot of difficulties to the battery management system BMS to estimate the actual capacity and SOC of the battery, and many variables need to be considered. The battery management system of the WG6120HD hybrid electric vehicle is based on the management of SOC values. SOC (state of charge) refers to the change state of the charge parameters participating in the reaction inside the battery, reflecting the remaining capacity of the battery. This has formed a unified understanding both at home and abroad.

　　1 Lead-acid battery

　　Lead-acid batteries are a complex chemical reaction system. External factors such as the size of the charge and discharge current and its operating temperature will affect the performance of the battery. Calculating the SOC value of the battery and determining the operating mode of the vehicle based on the vehicle's operating status and other parameters is a key technology for electric vehicles.

　　Lead-acid batteries have the longest application history and are the most mature and cheapest batteries, which have been mass-produced. However, they have low specific energy, high self-discharge rate and low cycle life. The main problem at present is that they can travel a short distance on a single charge. The recently developed third-generation cylindrical sealed lead-acid batteries and fourth-generation TMF (foil rolled electrode) sealed lead-acid batteries have been used in EV and HEV electric vehicles. In particular, the low impedance advantage of the third-generation VRLA battery can control the ohmic heat during fast charging and extend the battery life.

　　The pulse phased constant current fast charging method can well adapt to the hybrid electric vehicle lead-acid battery in the variable current discharge state, with a short charging time, so that the battery state of charge SOC is always kept in the range of 50%-80%. The test shows that it only takes 196 seconds to charge the battery from 50%C to 80%C. This charging method basically meets the acceptance curve of the battery, the battery temperature rise is small, less gas is generated, the pressure effect is not large, and the charging time is short.

　　The best charging method is that the charging current always follows the inherent charge acceptance curve. During the charging process, the charge acceptance rate remains unchanged. As time goes by, the charging current decreases according to the inherent charge acceptance curve (exponential curve decreases), so the charging time is the shortest. The pulse depolarization charging method can achieve fast and efficient charging, but the equipment is expensive and is not applicable to some batteries.

　　The new VRLA battery for electric vehicles developed by a Japanese company has a voltage specification of 2V and 4V for single cells, and adopts a lean electrolyte type and a horizontal plate design. The spacing between the plates is very small, so there will be no electrolyte stratification. The falling materials are blocked by the plates when they move downward, and there is no accumulation of falling materials at the bottom of the battery.

　　The 12V 112A·h horizontal battery for electric vehicles of Ectreosorce Company has a mass specific energy of 50W·11/kg at a 3-hour rate discharge and a cycle life of more than 900 times at 80% Ⅸ)D (depth of discharge).

　　The lead-acid battery for electric vehicles produced by German Sunshine Company adopts colloidal electrolyte design. After testing, the expected life of its 6V, 160A·h battery can reach 4 years. It has the advantages of large heat capacity and small temperature rise.

　　In 1994, Arias Company of the United States launched a bipolar lead-acid battery for electric vehicles, which has a unique structural technology. The working current of this battery only passes through the thin double electrodes perpendicular to the electrode plane, so it has extremely small ohmic resistance. The technical parameters of the bipolar lead-acid battery for electric vehicles developed by BPC Company of the United States are: combined voltage of 180V, battery capacity of 60A·h, discharge rate specific energy of 50W·h/kg, and cycle life of up to 1000 times.

　　The Swedish company OPTLMA has launched a roll-type lead-acid battery for electric vehicles with a capacity of 56A·h and a starting power of 95kW, which is greater than the starting power of an ordinary 195A·h VRLA battery, but one-fourth smaller in 　　size .

　　The characteristics and price of lithium-ion batteries are closely related to its positive electrode materials. Generally speaking, positive electrode materials should meet the following requirements: ⑴ Electrochemical compatibility with electrolyte solutions within the required charge and discharge potential range; ⑵ Mild electrode process kinetics; ⑶ High reversibility; ⑷ Good stability in air in the full lithium state. With the development of lithium-ion batteries, research on high-performance and low-cost positive electrode materials is constantly underway. At present, research is mainly focused on lithium transition metal oxides such as lithium cobalt oxide, lithium nickel oxide and lithium manganese oxide. Lithium cobalt oxide (LiCoO2) belongs to the -NaFeO2 type structure, has a two-dimensional layered structure, and is suitable for the deintercalation of lithium ions. Its preparation process is relatively simple, with stable performance, high specific capacity and good cycle performance. Its synthesis methods mainly include high-temperature solid-phase synthesis and low-temperature solid-phase synthesis, as well as soft chemical methods such as oxalic acid precipitation, sol-gel method, hot and cold method, and organic mixing method. Lithium manganese oxide is a modified product of traditional positive electrode materials. The most widely used one is spinel-type LixMn2O4, which has a three-dimensional tunnel structure and is more suitable for the deintercalation of lithium ions. Lithium manganese oxide has abundant raw materials, low cost, no pollution, better overcharge resistance and thermal safety, and relatively low requirements for battery safety protection devices. It is considered to be the most promising lithium-ion battery positive electrode material.

　　In the 1990s, Sony Corporation of Japan first successfully developed lithium batteries for electric vehicles. At that time, it used lithium cobalt oxide materials, which had the disadvantages of being flammable and explosive. At present, China Xin Guoan Mengguli Power Supply and other companies have developed 100Ah power lithium batteries with lithium manganese oxide as the positive electrode material, which solves the shortcomings of lithium cobalt oxide batteries.

　　As of October 2006, more than 20 automobile companies around the world have been conducting research and development of lithium-ion batteries. For example, Fuji Heavy Industries and NEC have cooperated to develop cheap single-cell manganese-based lithium-ion batteries (i.e., lithium manganate batteries), which have a lifespan of up to 12 years and 100,000 kilometers in a vehicle-mounted environment, which is equivalent to the lifespan of a pure electric vehicle. The fast-charging lithium-ion battery pack developed by Toshiba, in addition to its small size and large capacity, uses a technology that can uniformly fix nano-particles, which can evenly adsorb lithium ions on the negative electrode of the battery , and can be charged to 80% of its capacity within one minute, and can be fully charged in another 6 minutes. Johnson Controls, a major battery manufacturer in the United States, established a research and development site in Milwaukee, Wisconsin in September 2005 for lithium-ion batteries that meet the needs of electric vehicles. In January 2006, it invested another 50% to jointly establish Johnson Controls-Saft Advanced Power Solution (JCS) with the French battery manufacturer Saft. In August 2006, JCS took over the two-year USABC (United States Advanced Battery Consortium) pure electric vehicle lithium-ion battery R&D program contract led by the U.S. Department of Energy (DOE) to provide high-power lithium-ion batteries. my country's research level in lithium-ion batteries has exceeded the goals set by the USABC's 2010 long-term indicators in many indicators. Suzhou Xingheng, which started industrialization trials in 1997, is a national lithium-ion power battery industrialization demonstration project base. The power battery packs it developed have passed the testing and certification of the U.S. UL and the EU independent organization Extra Energy. It has also built the first power lithium-ion battery production line in Suzhou and successfully trial-produced it, and has now achieved mass production.

　　During the 2008 Beijing Olympic Games, 50 12-meter-long lithium-ion electric buses served in the Olympic Center. This was the first large-scale use of lithium-ion battery electric buses in the world. The charging time of electric buses is long, and this is how the operation of electric vehicles is ensured to be connected: when the electric vehicle enters the charging station, two manipulators take out the battery pack in the chassis of the vehicle and put it into the charging channel. Then, they take out the fully charged battery pack from the charging channel and replace it in the chassis of the electric vehicle. The whole process only takes about 8 minutes.

　　French Citroën, Renault, and Peugeot Automobile Company have completed user test operation of electric commercial vehicles using lithium-ion power batteries. Bordeaux is one of the cities in France where electric vehicles are demonstrated for application. There are 500 electric vehicles of various types, mainly used for municipal vehicles and electric minibuses. There are also 20 parking lots with supporting charging facilities for electric vehicles, 16 of which are equipped with fast charging devices. The charging process of lithium batteries is different from that of lead-acid batteries. The integrated block of lithium polymer (Lipo) charger has very few external components. Since the integrated block itself is very small (2mm×3mm), the entire charger is also very small. The charging process of Lipo batteries is: when the battery voltage is very low (0.5V), it is charged with a small current. The typical value of this current is less than 0.1C (here C is the nominal battery capacity). If the voltage is high enough but lower than 4.2V, the battery is charged with a constant current. Most manufacturers will specify a current of 1C in this process. The voltage on the battery will not exceed 4.2V. During the constant voltage period, the current passing through the battery will slowly decrease, and the battery charging continues. When the battery voltage reaches 4.2V and the charging current drops to 0.1C, the battery is about 80~90% charged, and then it turns to trickle charging. There are two parameters that can be adjusted in the charger, namely the normal charging current and the trickle charging current (when the battery is "fully charged"). It should be noted that the charging current should be selected carefully and should be kept below the maximum value recommended by the manufacturer.

　　The current power batteries used in French electric vehicles are mainly lead-acid batteries, and the second generation of lithium-ion electric vehicles have been put into test operation. Its electric vehicle charging device adopts conductive charging. The conductive charging method includes two categories: conventional charging devices and fast charging devices. Conventional charging is provided by charging facilities with standard civilian AC power interfaces, with simple leakage protection functions. It takes 6 to 7 hours to complete charging for electric vehicles with on-board chargers, and is widely used. Fast charging is provided by the charger with DC output for fast charging of electric vehicles.

　　An electric car with 25% residual power can be fully charged in 25 minutes. Fast charging is rarely used and is mainly used by industrial users and street emergencies.

　　Charging facilities have a unified charging interface, and the standard AC power interface is one of the important technical directions. Using an ordinary household socket plus a charging cable with a special plug can provide AC power for electric vehicles equipped with an on-board charger.

　　Lithium-ion power battery technology still needs further development. (1) Currently, most of the pure electric vehicle lithium-ion battery data released by various companies are laboratory test data, such as acceleration performance, charging time, and continuous mileage. Their reliability and mass production quality control need to be further verified under actual operation in a complex external environment. (2) There has been no substantial breakthrough in the diaphragm material required for lithium-ion batteries, and it is expensive, accounting for more than 30% of the cost of power batteries. If large-scale production technology is achieved on this material, the cost can be greatly reduced.