# Battery Management System Hardware Concepts: An Overview
## 5 Balancing  
Due to several reasons, at a given time SOC, values can differ in a serial connection of cells. Figure 6a shows the causes that can lead to inhomogeneities between the single cells in a battery pack. The cause for the inhomogeneities are production related differences on the one hand and different operational and environmental conditions, e.g., temperature, on the other hand. These causes can lead to different initial conditions, different ageing and different self discharge rates, which then result in deviating SOC, capacity and resistance values [55]. The inhomogeneities regarding the inner resistance of the cells are not covered in this section, but rather the differences of the SOC and capacity. Ref. [23] has reviewed the changes of different cells' capacities in a batch of 18,650 consumer cells with the same initial capacity and that were stressed with the same load profile. Even though all cells were selected to have the same initial capacity, the capacity deviates after some cycles. If a remaining capacity of 80% is chosen as an end of life criterion, the tested 18,650 cells showed a cycle life between about 1000 and 1500 cycles. This spread shows that, at least for some batteries, the selection of equal battery cells at the beginning of life is not sufficient to guarantee a permanently equal capacity after usage. In fact, the capacity of each cell should rather be monitored and, if necessary, handled. Even though the evolution of the self discharge rate of the cells was not in the scope of this work, one can assume that there is also variation in this parameter. Ref. [56] has analysed the self-discharge behaviour of commercial pouch cells by Kokam. An equivalent self-discharge resistance was introduced and four cells were stored at 40 \({}^{\circ}\)C. Even though cell type and storage conditions were the same, the cell self-discharge resistance varied between 10 k\(\Omega\) and 14 k\(\Omega\), which is an increase of 40% from the weakest to the strongest cell. Additionally, the Arrhenius like temperature dependency from 20 \({}^{\circ}\)C onwards is shown. Since there are always temperature inhomogeneities in battery packs, also a deviating self discharge rate for different battery cells in one pack can be observed due to this effect. The difference in SOC, as well as the differences in capacity and internal resistance, can cause the available energy of a battery pack to decrease and can be addressed with balancing circuitry. Figure 6: **(a)** causes for the imbalance of several battery cells, figure based on [57]; **(b)** categorization of different balancing methods, shown names of non-dissipative methods refer to the direction of energy transfer, figure based on [55]. Literature describes numerous hardware implementations for electronics that are capable of equalizing the voltage of several connected batteries. An overview of the possible topologies is given in [55; 58]. Figure 6b shows the possible variants. Other sources categorize different balancing methods by the control methods (e.g., active/passive) or by commercial availability. Especially when it comes to active balancing methods, a wide range of solutions is offered by literature. Nevertheless, most commercially available battery packs use controlled passive balancing systems (see, e.g., the systems shown in Chapter 3.3). These systems consist of a balancing resistor that can be switched in parallel to a battery cell for each battery cell or parallel connection of battery cells. The main advantage of this approach is its simplicity compared to more complex active and non dissipative solutions. Since the cells can only be discharged with this balancing topology, only SOC changes can be addressed. Reverse engineering of the aforementioned battery packs showed that these systems are using small balancing currents in the range of 100 mA (Balancing resistances in the range of 30 \(\Omega\) to 40 \(\Omega\) were observed, which leads to the described balancing current if a cell voltage of 4.2 V is assumed.).