1. High energy conversion efficiency. Motor is an isothermal electric drive system of execution unit. Its energy conversion efficiency generally can reach 75% ~ 75%. It was much higher than the efficiency of heat engine. Stirling engine is the current advanced high efficiency heat engine. Its efficiency is only 30%, but general engine has a lower energy conversion rate. Therefore, the energy efficiency of the electric car is higher than ordinary gasoline engine car around 40% when driving in the urban area. Therefore, hybrid electric vehicles are the favorite of all countries in the world.

2. Pollution-free, zero-emission and environment-friendly. The development of automobile industry has greatly polluted the living environment of human beings, especially in big cities. The exhaust gas from various motor vehicles seriously endangers human health, especially in the industrialized countries. As a result of the work of the electric drive system, no matter will be discharged. Therefore, the gradual replacement of gasoline cars with electric vehicles is the world's efforts.

3. Flexible and convenient control of working status. As long as the driver gives the electric signal instruction, the output power of the system and the working state of the control system can be immediately changed, without the distance limit, and without the intermediate transmission device, that is, the electric drive system is responsive.

4. The working state of the system will not be affected by the external environment. The use of a heat engine as a power source requires an oxidizer (or air) to keep the system in working order, so it cannot be used as an underwater propulsive power source.

5. The total weight remains unchanged. The electric drive system does not discharge any waste, so its total weight is unchanged. This feature is very important for submarines and torpedoes working underwater, and it does not affect the overall rise and fall of submarines and torpedoes.

6. No noise and no impact on the environment. With no mechanical transmission, the electric drive system works without noise, which is especially suitable for electric vehicles in cities and some special equipment.

7. Good security. Whether it is on land or under water, the safety of the propulsion system is the key to its adoption. Some batteries have high energy density, but they are not safe enough to be phased out. The power of the ac electric power drive system comes from the grid, so the vehicles using the ac electric power drive system can only drive by certain routes. The electric vehicles used in urban traffic adopt the dc electric power drive system, and the power source comes from the battery or fuel cell. This book is limited to discussing power batteries in dc power drive systems.