Power battery from monomer to system
Usually, a single battery without any other components is called a cell or a battery cell. The voltage of the battery cell is usually within 5V, the battery cell capacity of electronic consumer products is generally within 10A·h, and the power battery cell capacity is generally within the range of 2~200A·h (individual products exceed this range).
For electronic consumer products, most battery cells are sufficient, such as mobile phones and tablet computers. For electric vehicles, a voltage of several hundred volts is required to meet the high efficiency of the electric drive system (passenger cars generally require 200~400V, passenger cars generally require 500~700V), and a capacity of several hundred ampere hours (or say tens to hundreds of kilowatt-hours of electricity) is required to meet the requirements of driving range. Because the battery cells cannot provide such high voltage and energy, many single cells must be connected in series to meet the voltage requirements, and the cells must be connected in parallel to meet the capacity requirements, or to meet the voltage and power requirements at the same time through series and parallel connections.
In order to understand the battery system in series and parallel, we understand the battery cell as a container, as shown in Figure 1, the height of the container is the nominal voltage, the cross-sectional area of the container is the nominal capacity, and the container volume is the full capacity (electricity equals capacity multiplied by voltage). The percentage of the current installed object volume to the full power is the SOC.
When N cells and batteries are fully connected in parallel or in series, the power of the system is N times that of a single cell. From a practical point of view, if the battery cell capacity is large enough to meet the system requirements, then all of them can be connected in series.
If the capacity of the battery cell is not large enough, a larger power system requires both battery cells in series and in parallel. Practical applications mainly include three grouping modes: first parallel and then serial, first serial and then parallel, and hybrid connection.
Advantages: Parallel cells are used as one cell, the monitoring structure is simple, the BMS management channel is few, and the cost is low. Disadvantages: (1) If the cell is large, the direct parallel process may cause uneven current between cells; (2) If the cell is large, there are many parallel points, the parallel current is large, and the overcurrent capability is not easy to improve. Application: Suitable for slow charging systems with low power requirements.
String before Parallel
Advantages: It is only connected in parallel at both ends, and the system has a strong overcurrent capability; the batteries flow between the two branches. Disadvantages: Each branch cell needs to be independently monitored, BMS management channels are many, and the cost is high. Application: Suitable for systems with fast charging requirements or high power requirements.
The hybrid system is suitable for systems with small battery cell capacity and large battery system capacity requirements.
Power battery system product appearance and installation location
For electric vehicles, the power battery system is an assembly made up of many batteries in series and parallel. They have their own complete structure and appearance. The size and shape of the battery system products are also different due to different uses and models.
Most of the power battery systems of passenger cars are composed of a single battery box (extremely have 2 or 3 battery boxes). The power battery system of large and medium-sized commercial vehicles (passenger cars or trucks) is composed of multiple battery boxes.
In order to adapt to the compact structure of the passenger car, the shape of the power battery system is generally irregular, small, or flat, and the installation position and the connection position of the pipeline are related to the specific vehicle type. Take general Volt, Nissan’s Leaf, and Tesla’s S60 as examples, as shown in Figure 2~Figure 6.
Battery installation location: under the central aisle and the rear row
Battery installation location: under the chassis and the front and rear cabins
Battery installation position: below the bottom plate
There are other passenger cars with different structure types, and their battery system shapes are also different, as shown in Figure 5.
Because of the large space and regularity of passenger cars, the battery box can be made very regular, and it is also convenient for generalization and platformization. Generally, the battery system of a passenger car will be composed of multiple battery boxes, and there may also be independent high-voltage boxes. The composition of the battery box is similar to that of a passenger car. For the independent battery system of the high-voltage box, the main control unit and high-voltage electrical components of the BMS are generally in the high-voltage box, and the battery cell monitoring unit is generally in the battery box. The multi-battery box system of an electric bus is shown in Figure 6.