Abstract: This article systematically explains how to better design the safety of power battery systems from six aspects: power connection structure, sealing and insulation structure, fixing and shock absorption structure, high-voltage risk control, thermal management design, and battery management system.

The power battery system is the core component of new energy vehicles. On July 26, the 2017 China New Energy Vehicle Testing and Evaluation Technology Development Summit Forum was jointly hosted by Chongqing Vehicle Testing Research Institute Co., Ltd. and Electric Vehicle Resource Network. The forum started from the power The six aspects of connection structure, sealing and insulation structure, fixing and shock absorption structure, high-voltage risk control, thermal management design, and battery management system explain how to make the safety design of the power battery system better.

Power connection structure

Power connection refers to the connection technology in which single cells are connected in series and parallel to form a battery module, and battery modules are connected in series and parallel to form a battery pack. The connection structure is required to have the characteristics of high strength, high reliability and low impedance. At present, the main power connection method is mechanical connection or welding connection.

The most common mechanical connection is bolt connection, which is simple, convenient, low-cost, easy to assemble and disassemble, and convenient for maintenance. However, the disadvantage of this connection method is that it requires high assembly operations, and the assembly pre-tightening force and surface quality will cause unstable contact resistance; at the same time, the easy looseness of the bolts will also increase the contact resistance. Regarding how to ensure the reliability of bolted connections, the forum puts forward the following points: 1. Choose bolts and nuts of sufficient strength to withstand sufficient locking force and mechanical impact; 2. Choose locknuts to ensure long-term use The connection can resist mechanical shock and thermal shock; 3. Maintain the cleanliness and flatness of the contact surface of the connecting conductor to ensure that the contact area is large enough; 4. The contact surface needs to be coated to prevent surface corrosion; 5. The bolts and nuts ensure locking The force reaches the design value to ensure that the contact resistance is as small as possible.

The most commonly used welding connections are resistance welding and laser welding. This connection method has a high degree of automation, good consistency, and high production efficiency, but the biggest problem is relatively high maintenance costs.

Sealing and insulating structure

Regarding the sealing and insulation structure of the power battery system, it is worth noting the sealing design. The protection level of high-voltage connectors, MSD, and the limits of elastic components are often ignored when designing seals. In fact, the tighter the sealing ring is, the better. There should be some limits to keep the elastic range from deforming.

The insulation structure is mainly divided into the insulation of the cell, the insulation of the module and the insulation of the battery assembly. The insulation design of the cell is mainly the insulation between the positive electrode and the negative electrode collector. It mainly relies on the cell diaphragm to realize the insulation design. Good mechanical properties and insulation properties ensure the insulation between positive and negative electrodes. The insulation between the battery core (the assembly composed of positive and negative current collectors and diaphragms) and the battery case is mainly achieved through the diaphragm. The positive and negative current collectors are stacked or wound, and then wound for 2~ 3 layers of separator to ensure the insulation between the battery core and the case. The insulation between the positive and negative tabs of the battery and the shell is mainly through adding a layer of insulating material between the tabs and the shell.

The insulation design of the assembly requires insulation protection when the module is installed and fixed, and the connection harness between the modules, the output pole of the battery box, and the manual maintenance switch also need insulation protection.

Fixed and shock-absorbing structure

The fixed structure must have sufficient strength to support the heavy battery pack under high acceleration conditions. The bottom of the box is designed with longitudinal and horizontal beams. After installation and fixing, it can be connected with the body to increase the load-bearing strength of the body. The grade and thickness of the steel plate on the car body are selected as the basis, and the battery pack is effectively fixed. Through force analysis and calculation, the specific structure, material type and thickness of the longitudinal beam are determined. For the fixed connection between the longitudinal beam and the body, the strength should be ensured by measures such as adding strong plates or improving the structure, so that the battery box will not be deformed even under severe working conditions. If the thickness of the body cannot meet the relevant welding standards, the method of partially thickening the body can be used to ensure the welding quality, so that the fixed structure of the battery pack has a sufficient safety factor.

The bolt fastening method is selected for connection, and the bolt specifications and models are determined by comprehensively considering the mass of the battery pack, the collision acceleration and the friction coefficient of the joint surface. The principle of selection of fixed points is to arrange them as evenly and symmetrically as possible, so that each bolt can bear the load more evenly. It is recommended to install 6~10 fixed points.

High Pressure Risk Control

In terms of high-voltage control, the internal high-voltage circuit is generally divided. The battery pack is equipped with a relay. In the non-working state, because the relay contacts are disconnected, the battery pack has no external voltage, which is relatively safe for battery transportation, testing, and installation personnel. A manual maintenance switch is installed in the middle of the battery to protect the safety of technicians who repair electric vehicles in a high-voltage environment or to deal with emergencies. It can quickly disconnect the connection of the high-voltage circuit, making maintenance and other work in a relatively safe state, such as external Short circuit protection, high voltage needs to be disconnected for maintenance. The currently designed high-voltage division is a modular design scheme. Considering the factors of mass production and cost reduction in the future, the BMS will be put in from the battery cell to the group, the connection will be more reliable, and the number of man-hours required for mass production will be small. .

In terms of collision protection, the relay can be integrated into the system security strategy, usually with the collision sensor power off as an active safety strategy. The front, rear and side of the car are equipped with collision sensors. When the car is hit by a collision, the collision sensor response is read and processed by the airbag controller, and the collision information is fed back to the vehicle controller and battery management system, and the relay is controlled in the battery pack. Disconnected, the high voltage of the battery pack is disconnected by the relay to ensure driving safety.

In terms of insulation detection, the higher power supply voltage puts forward higher requirements for the electrical safety of the vehicle, especially for the insulation performance of the high-voltage system. Insulation resistance is an important parameter to characterize the electrical safety of electric vehicles. Relevant electric vehicle safety standards have been clearly stipulated in order to eliminate the potential threat of high voltage to the vehicle and the personal safety of drivers and passengers, and ensure the safety of electric vehicle electrical systems. .

In terms of overcurrent protection, in order to ensure the safety of vehicle electrical appliances and passengers, and prevent short circuit and overload phenomena, fuses are generally used for protection. The selection of fuses involves the following factors: current characteristics applied to the fuse, voltage characteristics, ambient temperature of the fuse, installation size restrictions, application lines, etc. When the applied voltage and installation size are fixed, the selection of the fuse is mainly considered from three factors: current characteristics, ambient temperature and application circuit. For the high current protection area, the selected fuse should have the following properties: ① large capacity, usually tens to hundreds of A; ② able to withstand instantaneous high current and high pulse; ③ high safety and reliability; ④ operating environment temperature is relatively low High; ⑤ good mechanical properties.

The purpose of the high-voltage interlock is to confirm the integrity of the entire high-voltage system. When the high-voltage system circuit is disconnected or the integrity is damaged, safety measures need to be activated. There are several key points here: the existence of HVIL can make the integrity of the entire system known before the high-voltage bus is powered on, that is to say, it can prevent problems before the main and negative relays of the battery system are closed to supply power. The existence of HVIL requires the composition of the entire system. HVIL is mainly completed through connectors. The main object of prevention is the connection of wire harness & The detection circuit is independent.

thermal management design

Power battery thermal management is to be able to accurately measure and monitor the battery temperature; effective heat dissipation and ventilation when the temperature of the battery pack is too high; rapid heating under low temperature conditions, so that the battery pack can work normally; effective ventilation when harmful gases are generated; Uniform distribution of the group temperature field.

Due to the small size of the batteries used in the power battery, when the ventilation channel is reserved, the air channel can be increased to a certain extent to help dissipate heat. When designing thermal management, it is necessary to consider the overall matching of the system, including the power of the motor and the capacity of the battery, and minimize the rate of the battery. If the battery rate design is relatively perfect, normal heat dissipation is sufficient. In terms of heating, it is necessary to reserve a space for heating to realize heating during charging in cold weather.

battery management system

In terms of the battery management system, there is still a lot of room for improvement in the part where the battery management system is integrated with the entire vehicle. The battery management system needs to have a very thorough understanding of the characteristics of the battery such as temperature and working conditions, but in fact this is not easy. At present, the various problems of the battery are mainly due to insufficient understanding of the characteristics of the battery. For the battery at any time and under any temperature environment, how much is the battery’s discharge capacity, how much rate can it provide for the whole vehicle, and how long is the battery life? , in what range interval, this is the work that the battery management system needs to do.