The core functions of the battery management system
2.1 SOC estimation
The SOC is used to describe the remaining battery power and is one of the most important parameters in the battery life. The SOC estimation is the basis for judging the overcharge and overdischarge of the battery. The accurate estimation can avoid the problem of overcharge and discharge of the battery pack to the maximum extent, so that it can operate more reliably.
The estimation of the battery SOC exhibits a very strong nonlinearity under the influence of the internal working environment and the external environment change. There are many internal and external factors that affect the battery capacity, such as battery temperature, battery life, battery internal resistance, etc. It is very difficult to accurately complete the SOC estimation.
The existing SOC estimation methods are as follows:
(1) An hour measurement method. The Anshi measurement method does not consider the changes in the internal structure and state of the battery, and thus has the advantages of simple structure and convenient operation, but the accuracy of the method is not high. If the current measurement accuracy is not high, then as time goes by, the SOC cumulative error will continue to increase, affecting the final result. The method is suitable for measuring the battery SOC on an electric vehicle. If the measurement accuracy can be improved, it is a simple and reliable SOC measurement method.
(2) Open circuit voltage method. The open circuit voltage of the lithium ion battery has an approximately linear relationship with the SOC, which can be used to determine the internal state of the battery. However, due to strict measurement requirements, the battery needs to be at least 1h, which is not suitable for online real-time detection of batteries in electric vehicles. Under normal circumstances, due to the open circuit voltage method, the accuracy of the initial and final estimates of the charging of the charger is relatively high, and the open circuit voltage method is often used in combination with the ammeter measurement method.
(3) Kalman filtering method. The Kalman filter method is especially suitable for hybrid batteries with high current fluctuations due to its excellent correction error capability. The disadvantage of this estimation method is that it requires high processing speed of the system.
(4) Neural network method. Neural networks have features such as distributed parallel processing, nonlinear mapping, and adaptive learning, so they can be used to simulate battery dynamics and estimate SOC. However, this method requires a large amount of reference data for the neural network to learn, and the data and training methods are required to be high, otherwise unacceptable errors will be caused.
2.2 Balance Management
There are many processes in the process of producing a battery, and differentiation can cause inconsistencies. The difference in battery cells is mainly reflected in the change in internal resistance and capacity over time and temperature. Large differences between cells are more likely to cause overcharge or overdischarge, resulting in battery damage. Achieving battery balance maximizes the utility of the power battery, extending battery life and increasing safety. At this stage, the mainstream balance methods at home and abroad are as follows:
(1) Resistance equalization method. This method is the main representative of the energy dissipation type equalization method. The method is simple and the cost is low, but the energy loss is relatively large and the efficiency is low. It is only suitable for systems with small current charge and discharge.
(2) Switched capacitance method. This method is the main representative of the non-energy dissipative type equalization method, which makes up for the shortcomings of resistance equalization. However, its control circuit is complicated, the equalization speed is slow, and it takes a long time, which is not suitable for large current use.
(3) Transformer equalization method. This method is based on a symmetrical multi-winding transformer structure of a series battery pack active equalization control method. Its shortcomings are complicated circuits, many devices, and too large a volume, which is not easy to expand the battery pack. Generally suitable for charging and discharging of large currents.
(4) Centralized equilibrium. The method can quickly transfer the entire battery pack to the battery cells, and the centralized equalization module is smaller in size. However, the balancing operation of multiple batteries cannot be performed in parallel, and a large number of cable connections are required, which is not suitable for a battery pack having a large number of batteries.
2.3 Thermal Management
Temperature has an impact on all aspects of battery performance. Temperature field non-uniformity will exacerbate the inconsistency of the battery pack, so it is necessary to manage it. The purpose of thermal management is to maintain the temperature of the battery system within a certain range by heating or heat dissipation, and to maintain the temperature uniformity within the battery as much as possible.
Temperature management mainly completes the following four functions: (1) rapid heating of the battery pack under low resistance conditions; (2) ensuring uniform distribution of the battery temperature field; (3) accurate measurement and monitoring of the battery temperature; (4) in the battery pack When the temperature is too high, the amount of heat is effectively dissipated. Commonly used cooling methods include natural convection method, forced air convection method, liquid flow method, phase change material method and thermal management method. Common heating methods include battery internal heating method, heating plate method, heating jacket method and heat pump method.