Power Source Of New Energy Vehicles

From the perspective of the development of new energy vehicles around the world, their power sources mainly include lithium-ion batteries, nickel-metal hydride batteries, lead-acid batteries, and supercapacitors, among which supercapacitors mostly appear in the form of auxiliary power sources. The main reason is that these battery technologies are not yet fully mature or have obvious shortcomings.Compared with traditional vehicles, there are many gaps in terms of cost, power, and cruising range, which is also an important reason restricting the development of new energy vehicles.

Lead-acid battery
Among all battery technologies, lead-acid batteries have the longest development history. The battery uses metallic lead as the negative electrode and lead oxide as the positive electrode. During the discharge process of the battery, lead sulfate will be generated at both the positive and negative electrodes. Sulfuric acid is both a reactant and a product of the reaction process in the electrolyte solution. In the past ten years, research and development on lead-acid batteries have mainly focused on the application of hybrid electric vehicles.

Nickel-metal hydride battery
The work of nickel-metal hydride batteries is based on the release and absorption of OH- by nickel oxide anodes and hydrogen metal cathodes. In the past, nickel-metal hydride batteries were regarded as a good temporary choice for electric vehicles, given the serious safety issues of lithium-ion batteries. However, its energy density of 50~70Wh/kg cannot meet the energy density requirement of 150~200Wh/kg for electric vehicles. At the same time, the large proportion of nickel in nickel-metal hydride batteries limits its future price reduction. Therefore, nickel-metal hydride batteries are not a reliable choice.

Lithium-ion batteries
Lithium-ion batteries are the most commonly used power battery technology in electric vehicles today, thanks to their high energy density and increased power in single cells, which allows such batteries to develop smaller mass and density at a competitive price. Currently, these power batteries can power electric vehicles for about 150km. Lithium is inserted into the electrodes of lithium-ion batteries, that is, the electrode material is a carrier of lithium ions. Studies have shown that the power (800~2000W/kg) and energy density (100~250Wh/kg) of lithium-ion batteries used in electric vehicles have increased. Lithium batteries only need to be charged to "seven points full". Lithium batteries do not need to be fully charged, and not fully charged will not affect the battery life.

Supercapacitors
If the battery needs to provide both long-term energy storage and short-term pulse power for engine starting or vehicle start, the battery design needs to adopt a compromise solution. A thicker electrode is used in each battery cell to increase the total surface area. The increased current is distributed over a larger electrode area, which can keep the battery voltage drop to meet the system requirements. If the power demand can be provided by other devices, the battery can use thicker electrodes to achieve better durability while meeting the energy storage requirements at a lower rate. A more ideal method is to have the supercapacitor provide the pulse power and the battery only provide energy storage. The supercapacitor can be recharged at a lower rate to prepare for the next power output or to charge using brake energy recovery. After charging with the supercapacitor, the battery can operate over a wider battery state of charge (SOC) range because the power required for starting is already stored in the supercapacitor. Combining batteries and supercapacitors inevitably requires a more complex charging system because the charging and discharging characteristics of batteries and supercapacitors are significantly different, so their charging cut-off voltages are quite different. Therefore, some kind of DC/DC converter or switching device may be required to control two devices on the same DC bus.