When Gustave Trouve built the world's first electric tricycle in 1881, it used lead-acid batteries. Currently, many hybrid and pure electric vehicles still use next-generation lead-acid batteries. In the past decade or so, lithium-ion batteries have been increasingly used in electric vehicle production, demonstrating their superior performance.
American scholar J.A. Mas proposed the battery charging acceptance current theorem through numerous experiments: 1) For any given discharge current, the battery's charging acceptance current is proportional to the square root of the discharged capacity; 2) For any depth of discharge, 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) For a battery discharged at several discharge rates, its acceptable current is the sum of the acceptable currents at each discharge rate. In other words, the battery's charging acceptance current can be increased by discharging. When a battery's charging acceptance capacity decreases, discharging can be incorporated into the charging process to improve its acceptance capacity.
The performance and lifespan of automotive power batteries are related to many factors. Besides their own parameters, such as the quality of the battery plates and the concentration of the electrolyte, there are also external factors, such as the battery's charging and discharging parameters, including charging method, charging termination voltage, charging and discharging current, and depth of discharge. This presents many challenges for the Battery Management System (BMS) in estimating the actual capacity and State of Charge (SOC) of the battery, requiring consideration of numerous variables. 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 in the charge parameters participating in the reaction within the battery, reflecting the remaining capacity of the battery. This is a consensus both domestically and internationally.
1. Lead-acid battery
Lead-acid batteries are complex chemical reaction systems. External factors such as the magnitude of the charging and discharging current and its operating temperature all affect battery performance. Calculating the battery's state of charge (SOC) and determining the vehicle's operating mode based on its operating conditions and other parameters is a key technology for electric vehicles.
Lead-acid batteries have the longest history of application and are the most mature and lowest-cost type of battery, already in mass production. However, they have low energy density, high self-discharge rate, and short cycle life. The main current problem is their short range on a single charge. Recently developed third-generation cylindrical sealed lead-acid batteries and fourth-generation TMF (tissue-coated metal foil) sealed lead-acid batteries have been applied to EV and HEV electric vehicles. In particular, the low impedance of the third-generation VRLA battery can control ohmic heat during fast charging, extending battery life.
The pulsed, staged constant-current fast charging method is well-suited for hybrid electric vehicle lead-acid batteries operating under variable current discharge conditions. It requires short charging times and maintains the battery's State of Charge (SOC) consistently within the 50%-80% range. Tests show that the battery can be charged from 50%C to 80%C in just 196 seconds. This charging method largely meets the battery's acceptance curve, resulting in minimal temperature rise, less gas generation, minimal pressure effects, and a short charging time.
The optimal charging method is one where the charging current always follows the inherent charge acceptance curve. During charging, the charge acceptance rate remains constant, and as time increases, the charging current decreases according to the inherent charge acceptance curve (exponential decrease), thus minimizing charging time. Pulse depolarization charging can achieve fast and efficient charging, but the equipment is expensive and not suitable for some batteries.
A Japanese company has developed a new type of VRLA battery for electric vehicles, which is available in 2V and 4V single-cell voltage specifications. It adopts a lean electrolyte and horizontal plate design. The spacing between the plates is very small, which prevents electrolyte stratification. The plates also prevent detached material from moving downwards, and there is no accumulation of detached material at the bottom of the battery.
Ectreosorce's 12V112A·h horizontal battery for electric vehicles has a specific energy of 50W·L/kg at a 3-hour rate discharge and a cycle life of more than 900 cycles at 80% depth of discharge (D).
The lead-acid batteries for electric vehicles from the German company Sonnenschein use a gel electrolyte design. Tests have shown that the expected lifespan of its 6V, 160Ah batteries can reach 4 years, and they have advantages such as large heat capacity and low temperature rise.
In 1994, Arias, an American company, launched a bipolar lead-acid battery for electric vehicles, featuring a unique structural technology. The operating current of this battery flows only perpendicularly to the electrode plane through the thin dual electrodes, resulting in extremely low ohmic resistance. The technical parameters of the bipolar lead-acid battery for electric vehicles developed by BPC, an American company, are: a combined voltage of 180V, a battery capacity of 60Ah, a discharge rate specific energy of 50Wh/kg, and a cycle life of up to 1000 cycles.
The Swedish company OPTLMA has launched a roll-type lead-acid battery for electric vehicles with a capacity of 56Ah and a starting power of 95kW, which is greater than the starting power of a typical 195Ah VRLA battery, while being a quarter smaller in size.
2. Lithium-ion batteries
The characteristics and price of lithium-ion batteries are closely related to their cathode materials. Generally speaking, cathode materials should meet the following requirements: (1) electrochemical compatibility with the electrolyte solution within the required charge/discharge potential range; (2) mild electrode process kinetics; (3) high reversibility; and (4) good stability in air in the all-lithium state. With the development of lithium-ion batteries, research on high-performance, low-cost cathode materials is ongoing. Currently, research mainly focuses 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, possessing a two-dimensional layered structure suitable for lithium-ion insertion/extraction. Its preparation process is relatively simple, with stable performance, high specific capacity, and good cycle performance. Its synthesis methods mainly include high-temperature solid-state synthesis and low-temperature solid-state synthesis, as well as soft chemical methods such as oxalic acid precipitation, sol-gel method, cryo-thermal method, and organic mixing method. Lithium manganese oxide is a modified form of traditional cathode material, with spinel-type LixMn2O4 being the most widely used. It possesses a three-dimensional tunnel structure, making it more suitable for lithium-ion insertion and extraction. Lithium manganese oxide has abundant raw materials, low cost, no pollution, better overcharge resistance and thermal safety, and relatively lower requirements for battery safety protection devices, making it considered the most promising cathode material for lithium-ion batteries.
In the 1990s, Sony Corporation of Japan was the first to successfully develop lithium batteries for electric vehicles. At that time, they used lithium cobalt oxide, which had the disadvantage of being flammable and explosive. Currently, companies such as China's Xinguo Anmeng Guli Power have developed 100Ah power lithium batteries using lithium manganese oxide as the positive electrode material, overcoming the shortcomings of lithium cobalt oxide batteries.
As of October 2006, more than 20 automotive companies worldwide were engaged in lithium-ion battery research and development. For example, Fuji Heavy Industries, in collaboration with NEC, developed a low-cost single-cell manganese-based lithium-ion battery (i.e., lithium manganese oxide battery) with a lifespan of up to 12 years and 100,000 kilometers in automotive environments, comparable to the lifespan of a pure electric vehicle. Toshiba's fast-charging lithium-ion battery pack, in addition to its small size and large capacity, employs a nano-particle uniformity fixation technology, allowing lithium ions to be evenly adsorbed on the negative electrode of the battery. It can be charged to 80% capacity in one minute and fully charged in another 6 minutes. Johnson Controls, a major US battery manufacturer, established a research and development site in Milwaukee, Wisconsin in September 2005 to address the specific needs of electric vehicles. In January 2006, it further invested 50% to jointly establish Johnson Controls-Saft Advanced Power Solution (JCS) with the French battery manufacturer Saft. In August 2006, JCS undertook a two-year USABC (United States Advanced Battery Consortium) contract led by the US Department of Energy (DOE) to provide high-power lithium-ion batteries for pure electric vehicles. my country's research level in lithium-ion batteries has exceeded the targets set by USABC's long-term goals for 2010 in several indicators. Suzhou Xingheng, a national demonstration project base for the industrialization of lithium-ion power batteries, which began industrialization trials in 1997, has developed power battery packs that have passed testing and certification by UL in the United States and ExtraEnergy, an independent organization in the European Union. It has also built the first power lithium-ion battery production line in Suzhou and successfully completed trial production; mass production has now been achieved.
During the 2008 Beijing Olympics, 50 12-meter-long lithium-ion electric buses served in the Olympic Center area, marking the first large-scale use of lithium-ion battery electric buses internationally. The long charging time of these electric buses was mitigated by the following system to ensure uninterrupted operation: When an electric bus entered a charging station, two robotic arms removed the battery pack from the chassis and placed it in the charging channel. Then, a fully charged battery pack was removed from the already charged channel and installed back into the electric bus's chassis. The entire process took only about 8 minutes.
French automakers Citroën, Renault, and Peugeot have completed user testing of their electric commercial vehicles powered by lithium-ion batteries. Bordeaux, one of France's demonstration cities for electric vehicles, has 500 electric vehicles of various types, mainly used for municipal vehicles and electric minibuses, and has built 20 parking lots with charging facilities for electric vehicles, 16 of which are equipped with fast charging devices. The charging process for lithium batteries differs from that of lead-acid batteries. Lithium polymer (Lipo) chargers have very few external components, and because the integrated circuit itself is extremely small (2mm × 3mm), the entire charger is also very small. The charging process for Lipo batteries is as follows: when the battery voltage is very low (0.5V), it is charged with a small current, typically less than 0.1C (where C is the nominal battery capacity). If the voltage is high enough but below 4.2V, a constant current is used to charge the battery. Most manufacturers specify a 1C current during this process, and the voltage on the battery will not exceed 4.2V. During the constant voltage period, the current through the battery slowly decreases, while the charging process continues. When the battery voltage reaches 4.2V and the charging current drops to 0.1C, the battery is charged to approximately 80-90%, after which it switches to trickle charging. Two parameters can be adjusted in the charger: the normal charging current and the trickle charging current (when the battery is fully charged). It is important to choose the charging current carefully and keep it below the manufacturer's recommended maximum value.
Currently, lead-acid batteries are the primary power source for electric vehicles in France, while second-generation lithium-ion electric vehicles are already undergoing testing. Their electric vehicle charging systems employ conductive charging. Conductive charging includes two main categories: conventional charging devices and fast charging devices. Conventional charging uses a standard household AC power interface provided by the charging facility, has basic leakage protection, and takes 6-7 hours to complete charging for electric vehicles with onboard chargers; it is widely used. Fast charging uses a charger to provide DC output for rapid charging of electric vehicles; a car with 25% remaining battery power can be fully charged in 25 minutes. Fast charging is less common, mainly used by industrial users and for emergency street use.
Charging facilities feature standardized charging interfaces, with standard AC power interfaces being a key technological direction. A standard household socket combined with a dedicated charging cable with a special plug can provide AC power to electric vehicles equipped with onboard chargers.
Lithium-ion power battery technology still needs further development. (1) Currently, most of the lithium-ion batteries for pure electric vehicles published by various companies are based on laboratory test data, such as acceleration performance, charging time, and continuous mileage. Their reliability and mass production quality control still need to be further verified under complex external environmental conditions. (2) There has been no substantial breakthrough in the separator material required for lithium-ion batteries, and it is expensive, accounting for more than 30% of the cost of power batteries. If mass production technology for this material can be achieved, the cost can be significantly reduced.
