In large energy storage systems, the safety and life of the battery are important parameters to consider. While we look for better energy storage systems, it is important for us to figure out how to make best use of the batteries that exist in today's world. One way is to use a Battery Management System.
In simple words, a Battery Management System, popularly known as BMS, is an embedded system that monitors battery voltage, state of charge (SOC), state of health (SOH), temperature and other critical parameters and also controls charging and discharging of a battery.
In general, the BMS does the following tasks:
- Detects unsafe operating conditions and ensures the safety of the host application and its user.
- Protects the cells of the battery from abuse.
- Enhances the life of the battery.
- Maintains the battery in a state which can fulfill the host application’s requirements.
All these are targeted towards the main motive of making the best use of the battery that is currently available.
A typical block diagram of a battery pack with a BMS is as follows:
All the components within a battery pack are interconnected with the BMS which, as a whole, is connected to the host application controller.
There are five major functionalities of BMS.
- Sensing and High Voltage Control
This includes measurement of voltage, current, temperature, switching devices, thermal management and ground fault detection. All the individual cell voltages are to be measured in a battery pack. This indicates the relative balance of cells as overcharging them may lead to thermal runaway. The cell voltages play an integral role in SOC and SOH estimation algorithms.
Temperature plays a major role when it comes to the battery operating characteristics and degradation rates. Excessive heat can damage the battery and reduce its life. As the battery charges or discharges, the battery heats up due to its internal resistance. It may seem that lower temperatures are not harmful for batteries, but in reality, either temperature extreme adversely affects its chemical behaviour. Thermistors or thermocouples are usually utilized for the purpose of sensing the temperature inside the battery pack.
- Protection against Over Voltage, Under Voltage, Over Current, Short Circuit and High Temperatures
High energy batteries can be extremely dangerous if this energy is released in an uncontrolled way like a short circuit. The protection scheme in this case must be very quick as a large current can develop within microseconds.
The over-voltage protection starts by shorting the cell through a bleed resistor when the threshold level is reached. In the case of under voltage, the protection scheme will prevent the discharge of a cell or the entire battery. Both of these prevent the cell voltages from moving out of a safe range of operation. During short circuit or when the current exceeds the threshold level, the battery is cut off to ensure safe operation. Thermal shutdown occurs when the battery is not in its safe operating temperature.
- Interface with the Host Application
The interface is usually through the Controller Area Network (CAN) bus. In certain cases, I2C protocol is also used. This is mainly for data logging, charger control and as a source of data for range estimation algorithms.
- Performance Management
This includes the state of charge (SOC) estimates and cell balancing / equalization. Cell equalization is simply making the voltage levels of all the individual cells equal. This ensures that there is no stress on any one particular cell. Cell balancing techniques are employed in order to attain cell balance or cell equalization. In passive cell balancing, individual cells are shorted using a bleed resistor to equalize the cell voltages whereas in active cell balancing, there is distribution of charge from overcharged cells to the undercharged cells during charging/discharging cycles. Active cell balancing is more efficient as the energy is distributed instead of the energy just being dissipated. The SOC estimation is done through certain algorithms that allow us to monitor the remaining capacity of the battery.
This includes state of health (SOH) estimation and abuse detection. The BMS detects and logs the external abuse which includes any violations of current, voltage or temperature limits. Certain SOH estimation algorithms in the BMS enable us to monitor battery health due to normal degradation. There is also something called the state of life (SOL) estimation algorithms which approximately predicts the remaining life of the battery.
In addition to this, the BMS can have authentication features to prevent installation of counterfeit packs. Not all the discussed functionalities are required in a BMS and the inclusion or elimination of features usually comes down to cost. When the investment on the battery is greater, we would want a better BMS. It is meaningless to invest in battery management for cheap batteries. A battery is usually considered cheap when we don’t remember when we last replaced it.
In summary - A BMS is like the Brain of the Battery telling it when to eat and when not to eat, keeping it healthy and making it live longer.