1 Introduction
The development of clean energy has become one of the core strategies of countries in the world to deal with financial crisis and economic recession. Since electric vehicles can achieve zero pollution in use, it is the most effective way to solve car pollution and energy problems. Hybrid electric vehicles have become a hotspot for the development of new vehicles worldwide in recent years due to the advantages of internal combustion engine vehicles and electric vehicles.
Lithium ion batteries are considered to be ideal energy sources for electric vehicles due to their excellent performance. However, the driving conditions of hybrid electric vehicles are complicated and changeable, and the charging and discharging of batteries are frequent and extremely irregular, which will seriously affect the battery life. Therefore, proposing effective battery control and management methods to extend the service life of batteries has become one of the important topics in hybrid vehicle research and development.
2 The meaning of equilibrium
In order to achieve a certain voltage, power and energy level, batteries for hybrid vehicles need to be used in series in series. Each single battery is not the same in terms of manufacturing, initial capacity, voltage, internal resistance and temperature. During use, a battery with a low charge will reach the limit voltage prematurely, easily causing overcharge or overdischarge, and Even a small difference will become a difference that cannot be ignored after a certain number of cycles. Furthermore, the abuse resistance of lithium batteries is poor, and the application technology is not perfect, which greatly increases the probability of abuse of lithium batteries, which affects the cycle life of the entire battery. Therefore, during the operation of a hybrid vehicle, the consistency between the batteries is a very important factor that determines the battery life. To reduce the impact of this inconsistency on the lithium-ion battery pack, during the charging and discharging process of the battery pack, the increase Equalization circuit.
3 Principle of balanced control
3. 1 Choice of balancing method
The battery balancing process is actually the balancing of the battery state of charge (SOC). For lithium batteries, the voltage of the battery changes rapidly at the high and low ends of the SOC, the curve in the middle part is relatively stable, and the entire curve is not linear. But when the hybrid car is working, the range of SOC is about 30% to 70%. The voltage in this range is basically linear with the SOC in the figure, which means that the battery voltage can reflect the battery SOC well in this interval. The value of indicates that balancing the battery with voltage is an ideal balancing method for hybrid vehicles.
There are many theoretical equalization methods for economically and reliably solving the problem of voltage balance of series battery packs. The current balance control circuit is divided into two types of energy consumption type and non-energy consumption type according to the energy consumption.
The energy consumption type is to connect a controllable resistor in parallel at each end of each single battery. When the battery voltage reaches or exceeds the limited value, the resistance is turned on and bears a part of the charging current, so that the charging current flowing through the battery is reduced to maintain Limit voltage. The electric energy flowing through the resistance is released in the form of heat, so it is called energy consumption type. This method has a simple structure and low cost. The energy consumption of the non-energy-consuming circuit is smaller than that of the energy-consuming circuit, but the circuit structure is relatively complicated and the cost is relatively high. Combined with the application environment of this project, considering that the batteries for hybrid vehicles are often in an irregular charge and discharge mode, taking into account the economy and space in the car, etc., the energy consumption balance is selected here.
3. 2 Control of the balance circuit
The balanced circuit structure is shown in Figure 1.
Figure 1 Structure diagram of resistance discharge equalization circuit.
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