98 Chapter 8 So using this example, we see that cell balancing can be done in one of two different man- ners. Either the water is removed from the fuller glasses or the water is transferred from the fuller to the less full glasses. The next section will describe these two actions, passive and active balancing. Active versus Passive Balancing There are two main methods for achieving cell balancing within a large lithium-ion battery pack, either active or passive balancing. The simplest explanation of the difference is based on what is done with the energy in the cell.
In passive balancing, the excess energy of the highest SOC cells is converted into heat energy and dissipated, essentially wasting the excess energy of the highest SOC cells. This can be done through several methods but the most common is to use a resistor to convert the energy into heat. Of course, this means that appropriate-sized resistors must be integrated into the slave boards that monitor the cells. This also means that you can use one resistor to balance multiple cells, although not at the exact same time. The main benefit of passive balancing is that it is less expensive than an active balancing system, which makes it the preferred system for automotive applications.
However, the detriment of this system is that energy is wasted as it is converted into heat energy. This creates a secondary problem as this heat generation must now be managed within the battery pack system. Another potential challenge to passive balancing is that it can be more time consuming than in an active balancing system ( Figure 9 ). Figure 9 Passive cell balancing. In an active balancing system ( Figure 10 ), the excess energy of the higher SOC cells is moved to the lower SOC cells until all of the cells are at the same SOC.
This may actually be done through a repetitive process whereby as more capacity is freed up, the charging will resume until the lowest cell again hits its maximum limit, then the balancing again resumes until all cells are at the same SOC. The benefit of the active balancing system is that the excess energy is not wasted, instead moved into the other cells. However, the detriment of this is that the hardware required for