Fundamental Understanding of Battery Management System

A Battery Management System (BMS) is an electronic system that manages and monitors the charging and discharging of rechargeable batteries. A given BMS has many different objectives such as I/V (current/voltage) monitoring, cell balancing, temperature monitoring, over-current protection, short circuit protection, etc. However, in this series, we will take a closer look at I/V monitoring and balancing functionalities.

Understand the Essentials and Innovations in BMS

A BMS is a system that manages and monitors the performance of rechargeable batteries, such as those used in electric vehicles, solar power systems, PSUs (Power Supply Units), remote data centers, and portable electronics. The growing trend of devices that require recharging, including Electric Vehicles (EVs) and E-scooters, is driving the exponential growth of the global BMS market.

The main objective of BMS is to ensure the safety, longevity, and efficiency of the batteries by regulating their charging and discharging and monitoring the state of charge (SOC) and state of health (SOH) of each individual cell within the pack. The BMS prevents overcharging, over-discharging, and overheating. Additionally, the BMS can provide information about the battery pack's performance and health to the user or system controller, and even the manufacturer.

In this two-part series, we will discuss the basics of battery management systems, main functionalities, and two main objectives of any given battery management system: monitoring and balancing. In part one, we will discuss various common monitoring methods. Part two will focus on different balancing options.

Monitoring

In a BMS, monitoring refers to the process of continuously measuring and analyzing various parameters of the battery pack to ensure its safe and efficient operation. These parameters include voltage, current, temperature, SOC, SOH, and other relevant data. As was mentioned above, the BMS uses this information to make decisions about charging, discharging, and balancing the battery cells to prevent overcharging, over-discharging, overheating, and other potentially dangerous conditions. The monitoring function is critical for maintaining the performance, reliability, efficiency, and safety of the battery cells.

There are three main methods of monitoring any given battery’s SOC:

• Voltage measurement method

  In this method, the voltage across the battery terminal is measured and then it is correlated to the SOC value using the discharge curve (voltage vs. SOC) of the battery which is usually provided by the battery manufacturer or determined by user characterization. The voltage measurement method is simple and easy to implement, but it may not be accurate due to the battery's internal resistance and its performance drift over time. Another disadvantage of this method is that during testing, the system function is interrupted (offline method) contrarily to the coulomb counting method (online method), mentioned below.

• Coulomb counting method

  Coulomb counting is a popular choice, especially for online battery-type applications that cannot be interrupted for testing. This method measures the amount of charge that enters or leaves the battery. The Coulomb counting method is accurate—however, the main drawback of this method is that it requires a current sensor and a complex algorithm to calculate the SoC. In addition, this method is affected by temperature, discharge current, and cycle life, therefore it doesn’t take into account batteries’ aging; thus, it might produce inaccurate results after a few hundred battery cycles.

• Ohmic or impedance measurement

  Ohmic/impedance measurement is a common method used to estimate the SOC of a battery. The method involves measuring the internal resistance of the battery, which changes as the battery discharges and charges. By monitoring the changes in the internal resistance, it is possible to estimate the SOC of the battery. The impedance measurement method is based on the fact that the internal resistance of a battery increases as the battery discharges. This increase in resistance is due to the depletion of the active materials in the battery, which reduces the battery's capacity to store and release energy. As the battery charges, the internal resistance decreases, indicating that the active materials are being replenished.
  
  This impedance measurement can be used to estimate the SOC of the battery, as well as other battery parameters such as the SOH and state of function (SOF). It is important to note that the impedance measurement method is not always accurate and can be affected by factors such as temperature, age, and usage patterns. Therefore, it is often used in combination with other methods, such as voltage and Coulomb counting measurements, to improve the accuracy of the SOC estimation.

Now that we have a good understanding of a Battery Management System’s role in monitoring battery cells and different methods of monitoring, we can apply this understanding to pick the right monitoring method for our BMS applications. In part two, we will continue exploring the different balancing architecture and their tradeoffs.

In addition, make sure to check our low voltage BMS reference design. Microchip Technology offers a low voltage BMS solution for various battery chemistries, including lithium-ion, lead-acid and nickel-metal hydride. Our low voltage BMS evaluation platform demonstrates monitoring a stack of 6 to 8 series 18650 Li-Ion batteries using the PAC1952 analog front end. This battery management solution offers state-of-charge determination using all three methods demonstrated in this post: voltage measurement, coulomb-counting, and impedance measurement to enable accurate monitoring of battery cells. In addition, this demo supports passive cell-balancing using a network of discrete FETs and resistors. It also comes with GUI support showing battery cells’ SOC in real time.