# Battery Management System Hardware Concepts: An Overview
## 3 Topologies of BMS  
The following section shall present an overview of possible battery system structures and the resulting implications for the BMS. In order to achieve the electrical specifications (e.g., stored amount of energy, power, voltage range, maximum current) necessary for an individual system, in many cases, multiple battery cells have to be combined to form a battery pack. In principle, different connection topologies are possible for these kinds of batteries. A schematic representation is shown in Figure 2 and Table 1.  
To realize a specific voltage range on the battery pack level, which reduces the current that is drawn for a given power value, cells have to be connected in series (Figure 2c), while parallel connections increase the capacity (Figure 2b). In today's systems, one possible variant is to use multiple cells with small capacity in parallel to form modules with higher overall capacity, which are then connected in series to increase the voltage (Figure 2e), also see example _Tesla Model S_ below). Another variant is the usage of battery cells with a high capacity, which are connected in series (Figure 2c), also see examples _Mitsubishi i-MiEV_, or _smart fortwo ed_ below). Both variants are the most reasonable ones in terms of BMS complexity. A parallel connection of multiple strings of battery cells (e.g., for special redundancy requirements) would increase the expenditure for cell voltage monitoring, balancing, etc., by a factor of the number of parallel strings, while this way only one voltage measurement channel per parallel connection of \(n\) cells is needed. _Example:_\(m\) cells in series are needed to reach the specified voltage, as well as \(n\) cells in parallel to provide the necessary capacity. Case 1: Using a series connection of \(m\) times \(n\) parallel cells, \(m\) voltage channels would be needed. (see Figure 2e)  
Case 2: Using a parallel connection of \(n\) strings of \(m\) cells in series, \(m\cdot n\) voltage measurement channels would be needed. (see Figure 2d)  
In some special cases, where size, weight and power consumption are very critical, battery modules are even built without single cell monitoring and balancing. Examples are the European Space Agency (ESA)'s Mars Express and Rosetta probes. According to [20], there are three battery strings (composed of Sony 18650 HC (Tokyo, Japan) [21]), each with individual DC/DC converters, which connect them to a main bus. Single cell monitoring for the strings is not mentioned, however. In [21], the authors claim that single cell monitoring is not necessary for the described application, given that a careful selection and testing of the cells are guaranteed. In [22], it is stated that the cells should originate from the same lot, which means an uninterrupted unchanged production run with the same raw materials. Other research, however, shows that, although the cells that were examined seemed equal at the time of selection, their ageing behaviour can differ substantially (see Section 5 and [23]). Thus, it is questionable if single cell monitoring really can be omitted, even when carefully selected cells of the same lot that seem to be totally equal at the time of manufacturing are used. Admittedly, the benefit of carrying battery monitoring electronics would be limited, as there would be no way to exchange damaged cells, once the spacecraft has been launched. Furthermore, a failure of these electronics could lead to the whole system failing. Apart from that, omitting single cell monitoring can be less critical for small systems with only a few cells in series. Especially when for a string the total voltage remains in a region, where theoretically (assuming identical voltages per cell)