BMS is one of the key elements in the overall architecture of new energy vehicles. An intelligent implementation scheme can not only extend the battery life but also increase the driving distance of the vehicle when adopting a pure electric drive mode.
1. Monitoring of battery voltage, current signals, and detection of battery pack temperature
The voltage and current of battery cells are the foundation of all top-level control logic in BMS, just like tire mechanics is the foundation of vehicle dynamics. When the SOC of the battery is in the middle, the voltage range varies very little, but the SOC range varies greatly, which requires the voltage sensor of the cell to meet certain accuracy requirements. There is also a corresponding voltage sensor for the Pack.
The temperature sensor of the battery pack does not correspond to every cell, but you need to first conduct CFD analysis and find the most suitable point in the Pack to collect temperature information with the minimum number of sensors.
2. SOC Estimate
Undoubtedly, this is the most core and difficult part of the common control modules of BMS. Common SOC estimation methods include Ampere-hour integration method and open circuit voltage calibration method. The biggest problem with Ampere-hour integration is that the error will become larger and larger over time. The problem with open circuit voltage calibration is that the SOC corresponding to the open circuit voltage of the battery after a long period of stagnation is accurate, but the voltage collected by the car while driving is not accurate for SOC calibration.
Balancing is mainly divided into active balancing and passive balancing. Currently, mainstream hybrid cars use passive balancing. There are pros and cons to both active and passive balancing.
4. Power limit
Batteries have capacity and power types, and the two cannot be balanced at the same time. Batteries with high energy capacity do not necessarily have high power. As the energy provider of the motor, the lithium battery pack must meet the power requirements of the motor. Therefore, when selecting a battery, we must consider the capacity and power of the battery comprehensively.
After determining the battery type, the power limit of the battery is affected by the environmental temperature and SOC. To ensure the battery life, different power limits must be set under different temperatures and SOC values to limit the output of the battery.
5. Thermal control
The chemical performance of batteries is greatly affected by the environmental temperature. To ensure the battery life, the battery must operate within a reasonable temperature range. Temperature control involves CFD simulation analysis, the temperature sensor mentioned in part one, how to use the minimum number of sensors to effectively monitor the temperature distribution of the entire battery pack, and feedback the monitoring information to BMS. The BMS controls the temperature based on sensor information. The most common temperature control methods are air cooling and water cooling.