What is a Modern Battery Management System and How It Works?
In part 1, the basics of a battery management system and basic BMS known as “Battery Protector” are explained. In part 2, we will dig into modern battery management systems, which employ complex measurement & protection mechanisms to both protect the battery in many advanced ways and provide detailed information.
Modern battery management systems primarily comprise a measurement front end, which measures cell voltages, cell temperatures and battery current. The measurement resolution and accuracy are very high, i.e. at least 12 bits for cell voltages with +/-5 LSB maximum total error. This measurement capability enables the system to use measured values to provide complex protection mechanisms, which we will discuss now.
The primary advantage of accurate cell voltage measurement for BMS is the ability to balance cells. Cell balancing means making each cell’s charge level equal in a general manner. By measuring each cells’ operating conditions, the BMS can estimate cell charges individually and equalize them using cell balancing circuits included in the BMS. We will later discuss balancing techniques in the part 3 of this discussion. If the BMS is able to measure each cell’s voltage accurately, it is possible to keep each cell’s charge level equal. Thus, each cell can be charged and discharged equally, resulting in increase of usable battery capacity. The BMS’s, which does not have balancing capability, will terminate battery usage as soon as any single cell hits a voltage limit. At this point, other cells may not be at the same limit if the cells are not balanced. However, with balancing, cell states are kept close to each other and charging and discharging can occur up to the limits of the whole battery pack.
Is State Of Charge (SOC) Important For BMS?
Another advantage of this measurement capability comes into place for estimating state of charge (SOC) and state of health (SOH) of the battery. First let’s explain these terms. State of charge (SOC) is the percentage of charge available in the battery with respect to the whole capacity of the battery. The state of health (SOH) on the other hand is the percentage of the capacity of the battery with respect to the available capacity of the battery before any use (i.e. as soon as it is manufactured). The batteries naturally degrade over time due to both repeated charge/discharge of the battery and environmental conditions. Therefore, SOH is as important as SOC since both of them determines the usable energy at any given time.
SOC of a battery is strongly dependent on battery temperature and battery current. Most of the time, the cell voltage is indicative of SOC. However, the cell voltage cannot be independently used to estimate SOC and battery temperature, battery current and battery voltage should be combined to have a good estimate of SOC. Theoretically, SOC can be estimated in 2 different ways; namely voltage measurement method and coulomb counting method.
In voltage measurement method, the BMS primarily knows the voltage vs. SOC of the battery for various temperatures and battery currents. At any given time, the BMS combines current temperature, current and voltage of the battery and extracts SOC value by looking up the known values.
The voltage measurement method is useful if the battery voltage changes considerably with varying SOC level. However, some battery chemistries like LiFePO4 has a nearly constant voltage over a wide range of SOC. Therefore, SOC estimation error is high for flat voltage range of the battery.
Another known method for SOC estimation is the coulomb counting method. Coulomb counting method calculates an initial SOC using the voltage measurement method. Then, the battery current measurements are integrated over time to estimate how much charge is in & out of the battery to find out the SOC. Coulomb counting is useful for batteries with flat voltage curves. However, the battery naturally has an internal loss due to internal impedance. Furthermore, the charge calculated by the method may not be immediately available due to temperature of the battery. For using the coulomb counting method, current measurement circuit should have almost zero offset since the measurement is integrated over time. If there is a significant measurement offset, this offset will grow to a huge error in SOC.
Another important feature provided by modern BMS’s is rich communication features. Since all operating parameters are numerically calculated, these values can be sourced for several purposes. In many applications, the BMS parameters are shared to a human – user interface (HMI) to graphically show operating conditions. The communication is widely held on robust industrial interfaces such as MODBus or CANBus. Recent BMS’s also provide Wi-Fi or Bluetooth connection so that wireless communication is possible. Using Wi-Fi, it is possible to monitor the system over a computer network or even on the web.
One other important communication application is interfacing BMS to battery charger so that charger can intelligently manage the charging process and interact with BMS so that the battery is safely charged. Battery charging normally takes 3 – 4 steps and these steps follow each other with respect to battery voltage. However, an intelligent charger can modify these steps in accordance with revealed parameters from BMS’s such as pausing charger while battery is being balanced, or early stepping to constant voltage stages.
In the third part, we will discuss balancing capabilities of modern BMS’s and different balancing techniques.