# 2.What determines the basic parameters of the battery system? The second part

1. The structure of the power supply system
(1) Selection of battery cells.
Single cell monomers can be selected according to the advantages and disadvantages of cylindrical batteries (such as 18650), soft-pack batteries, and square-shell batteries. For example, consider Panasonic’s NCR18650PF as the most basic power battery cell for analysis. The reasons are as follows:
(A) The NCR18650PF battery cell can withstand the maximum discharge rate of 3C, which can meet the demand of peak power to a certain extent. The battery capacity reaches 2700mA.h, the mass is small, and it has a high specific energy;
(B) The 18650 battery is a cylindrical battery with a large specific surface area. The large amount of heat generated during the charge and discharge process can be quickly dissipated, reducing the operating temperature of the battery cell
(C) The 18650 model lithium-ion battery has a large output, has certain applications in the field of electric bicycle power batteries, and its reliability has been verified. And there are many supporting battery manufacturers, which can reduce the cost of productization of electric vehicle power battery modules and use There is no maintenance problem of battery cells during the process.

(2) Power supply system parameter matching.
The matching of power battery parameters must meet the requirements of maximum power, voltage, maximum current and mileage. The nominal voltage of a battery cell is 3.7V. Assuming that the battery system requires m batteries in series and n batteries in parallel, then there is
m×3.7V≥calculated nominal voltage
n×2.7A-h≥ rated capacity
Then there is m≥calculated nominal voltage/3.7, rounded; n≥rated capacity/2.75, rounded; then the entire battery system is composed of n parallel and m strings of battery cells.
The entire power system is connected in series and parallel with m×n battery cells. If a battery system is designed, the size and mass of the single battery is relatively large, which is not conducive to the overall assembly. Therefore, a design scheme of multiple battery systems is required. For example, using multiple small battery systems, in order to facilitate processing and maintenance, small battery systems have the same size and specifications. Each battery system has the same number of batteries in series and parallel, the same nominal voltage, and the same nominal capacity.

1. Battery system frame design
The process of large-capacity battery cells is complicated, and the manufacturing cost and safety are very problematic. Therefore, the power system, especially the battery system of automobiles, needs to be combined in parallel or in series with small-capacity batteries to meet the needs of electric vehicles.
(1) Selection of battery series and parallel connection
The connection method of the battery cells affects the consistency, reliability and service life of the battery.
Figure 9-2 and Figure 9-3 are commonly used Shenlian and parallel models. Assuming that the failure probabilities of the cells in the model in the figure are the same and that the cells are independent of each other, the following mathematical model can be established:

Among them, R, (1) is the reliability of the system; R, = (1, 2, .,, n) is the reliability of the i-th unit; m is the number of batteries connected in parallel; n is the number of batteries connected in series.

To complete the design of the battery system, m×n battery cells are required. Assuming that the reliability of the battery cell is 0.90 (in fact, it should be greater than 0.90), the reliability calculation results of different methods are as follows.
The reliability of the system is as follows:

The reliability of the system is as follows:

Through the above calculation, it can be obtained that the way of connecting in parallel and then in series is better than the way of connecting in series and then in parallel. The damage of a single battery of parallel battery modules will not affect the operation of other battery units, and the capacity of the battery module will decrease. As long as the battery modules are connected in parallel, the reliability of the battery pack can be improved as long as the battery modules are arranged properly.
For example, after running the two series and parallel modes for 500km and standing for 10 hours, the consistency test of the battery pack voltage found that the battery voltage distribution is concentrated and no low-voltage battery appears by using the connection mode of parallel first and then series. The way of connecting in series and then in parallel has a large battery voltage distribution range. If it is continued to be used, it will affect the consistency of the battery voltage and accelerate the loss of the battery. The battery cells connected in parallel can be charged and discharged with each other, which has a certain equalizing effect on the voltage of the battery.
The lithium-ion battery charging method is a constant current and then a constant voltage method, and each voltage value during the charging process corresponds to a SOC value. As long as the lithium-ion battery is overcharged and overdischarged, the battery will be damaged or completely fail, so the BMS must monitor the voltage of each battery cell. From this perspective, when the battery system adopts a parallel connection and then a series connection, the BMS of the battery system only needs to detect the voltage value of each battery module, thereby reducing the design requirements of the BMS.

1. The frame design of the battery system
By comparing two different series and parallel methods, the reliability of the battery pack, the consistency of battery voltage and the calculation cost of BMS have greater advantages in parallel and then series. The way.

The battery system adopts a layered mode. Every n battery cells are connected in parallel to form a battery module, and then m battery modules are connected in series according to the battery system voltage requirements. Calculate the basic parameters of the battery module.