Fundamental Understanding of Battery Management System – Part 2: Balancing
Why is Cell Balancing Crucial for Battery Management Systems (BMSs)?
In part one of this series, we discussed basics of battery management systems, main functionalities and two main objectives of any given battery management system: I/V (current/voltage) monitoring and balancing, which are crucial for safety, longevity and efficiency of the battery cells. In part one, we discussed different I/V (current/voltage) monitoring methods. In part two of this series, we will be talking about different balancing methods and pros and cons for each method.
In Battery Management Systems, balancing is a process that ensures all cells in a battery pack are at the same voltage level. This is important because individual cells can have slightly different capacities and states of charge, which can lead to imbalances over time. Balancing helps to maximize the capacity and lifespan of the battery pack, and it's a critical function of a BMS for several reasons:
Battery life extension: balancing ensures that all cells in a battery pack are charged and discharged equally, which prolongs the overall lifespan of the battery. If cells are not balanced, some cells may become overcharged or over-discharged, which can lead to premature failure.
Battery performance optimization: balancing ensures that all cells in the battery pack are operating at their maximum potential. If some cells are undercharged, they can limit the overall capacity of the battery pack, reducing its performance.
Safety: Unbalanced cells can lead to overcharging or over-discharging, which can cause thermal runaway and potentially lead to a fire or explosion. Balancing helps to prevent these dangerous situations.
Efficiency: Balancing ensures that all cells contribute equally to the battery pack's output, maximizing its efficiency. Without balancing, some cells may end up doing more work than others, which can reduce the overall efficiency of the battery pack.
There are several types of balancing methods used in BMS; however, passive balancing and active balancing methods are two of the most common methods used currently. Passive balancing involves discharging the cells with higher charge through a resistor until they match the cells with lower charge. While this method is simple and cost-effective, it wastes energy in the form of heat and is slow. Active balancing is a method which transfers the charge from higher charged cells to lower charged cells. This can be done in several ways, such as using capacitors, inductors or DC-DC converters. Active balancing is more efficient than passive balancing as it doesn't waste energy. However, it is more complex and expensive.
Passive balancing method
As mentioned above, passive balancing, also known as passive equalization, is a method used to maintain all cells in a battery pack at the same state of charge (SOC) by dissipating excess energy from the higher charged cells in the form of heat to bring the voltage down to the level of the lower charged cells. This is typically done using bleed resistors that are switched on when a cell reaches a certain voltage. While passive balancing is simpler and cheaper to implement than active balancing, it's less efficient because the excess energy is wasted as heat, leading to extra work for thermal dissipation in the design. It's also slower because the balancing only occurs when the cells are fully charged. Passive balancing method is currently the most common balancing methods used in all battery management systems and is often a good choice for smaller battery packs, where the differences in cell voltages are not as large and the energy loss from the balancing process is not as significant.
Active balancing method
Unlike passive balancing, active balancing does not waste energy as heat. Instead, it transfers energy from higher charged cells to those that are lower charged, therefore this balancing method is more efficient than passive balancing. It requires additional components and sophisticated control algorithms, which can increase the design and manufacturing complexity and cost. In addition, active cell balancing systems may require more maintenance and monitoring than passive systems due to their complexity. They also could be larger and heavier than passive systems, which can be a disadvantage in applications where size and weight are critical.
Now that we have a good understanding of a Battery Management System’s role in cell balancing of battery cells and different method of balancing, we can apply this understanding to pick a right balancing method for our BMS applications.
In addition, make sure to check our low voltage BMS reference design. We offer 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.
