1.1 Lithium-ion power battery pack
1.1.1 Lithium-ion power battery assembly Lithium-ion power battery assembly (PACK) is to connect single cells in series and parallel, and assemble them with the protection control board, charge and discharge ports, and housing. Some products also assemble the charger together. When designing the battery pack structure, various factors must be considered comprehensively, while taking into account the main performance. The following aspects should be considered in battery assembly:
(1) Security. Flame-retardant materials must be used when assembling.
(2) Tightness. The battery pack may cause malfunctions in humid or rainy environments, and even cause safety hazards
(3) Heat dissipation. The battery pack will generate heat during the charging and discharging process, the electronic components in the protection circuit will also generate heat, and the battery pack will also generate heat due to factors such as ambient temperature. The main measures to improve the heat dissipation of the battery pack include: minimizing the heat generation of the heat source and optimizing the structure of the heat dissipation system.
(4) Shock absorption. CR foam can be used, and flame-retardant materials with good mechanical strength can be used, so that safety and shock absorption can be taken into account.
(5) The layout of high current lines should be as short as possible.
1.2 Parameters and characteristics of lithium-ion power battery packs
The lithium-ion power battery pack is composed of multiple lithium-ion battery cells combined in series and in parallel. The group voltage, group power and group watt-hour capacity are the fundamental reasons for the grouping of batteries. Due to the impact of group use, battery packs have additional characteristics compared to single cells. The relationship between the parameters of the lithium-ion power battery pack and the cell parameters includes: the group voltage is equal to the sum of all cell voltages plus the voltage drop generated on the connecting conductor, the latter is often ignored in the analysis; the group internal resistance is equal to all the cells in series The sum of the internal resistance plus the resistance of the connecting conductor; the group watt hour capacity is approximately equal to the product of the minimum unit capacity and the total number of units; the group ampere-hour capacity and group life are the same as the minimum unit; the group power is equal to The sum of the power of all the monomers; other performance indicators of the battery pack also often depend on the poorer relevant indicators in the monomer.
As an energy supply device for new energy vehicles such as pure electric vehicles, hybrid electric vehicles, and extended-range electric vehicles, the most important characteristics of lithium-ion power batteries are high power and high energy. These two performance indicators are actually contradictory. In order to increase the power, it is necessary to increase the charging and discharging current. When designing the battery structure, it is necessary to consider increasing the equivalent reaction area and reducing the contact impedance, which requires an increase in the volume and weight of the battery. Lithium-ion power battery needs to be designed according to the optimization index to make the two main indexes meet the actual demand. From the point of view of use, lithium-ion power battery applications have the following characteristics:
(1) Tandem group application. The voltage of lithium-ion power battery cells is limited. When applied to new energy vehicles, several battery cells need to be connected in series to meet the requirements of load voltage supply, so the external characteristics of the battery pack are also restricted by the characteristics of the battery cells. .
(2) Large power capacity. The working current of the power battery often reaches tens to hundreds of amperes, and special current detection and protection devices should be designed for it.
(3) High charging and discharging intensity. The current in the charging process is controlled by the charging strategy. The state of the power battery is relatively stable and the current is determined by the characteristics of the load when it is used. The more violent the electrochemical power behavior of the battery fluctuates, the more difficult it is to effectively implement the hardware protection method based on voltage changes. When the lithium-ion power battery is applied to the vehicle, the discharge current of the power battery during most of the driving process is 0.3C~1.5C, and occasionally there is inverter charging during deceleration such as braking. The voltage fluctuation range and frequency of the battery pack are very disordered, and the hardware Management protection is easy to be disturbed and make wrong judgments.
(4) Large energy capacity. The capacity of traditional low-power batteries is often limited to less than a few ampere hours, the discharge process is stable, the working time is long, and the capacity (unit: Ah) is relatively easy to measure. Lithium-ion power battery has a large energy capacity, and the calculation of power is more complicated. When the vehicle is fully loaded, the power battery discharges quickly and the current changes drastically. The capacity measurement needs to be more accurate and timely to calculate the remaining power of the battery pack.
(5)The gradient voltage is high. Lithium-ion power battery packs are composed of multiple single cells connected in series to achieve a higher stack voltage, and the voltage value of each connection terminal increases step by step like a staircase, which is called a gradient voltage. Therefore, it is necessary to design an appropriate solution to integrate a single protection circuit.