# COTS Battery Charge Equalizer for Small Satellite Applications
## 2 Study of the Existing Battery Cell Balancing Methods  
Nowadays, advancements in technology have increased the quantity of equalizer topologies. Many of these new designs are based on FPGAs or microcontrollers, which are used for determining the state of charge (SoC) and state of health (SoH) of battery cells, thereby optimizing the equalization process [12; 13]. Despite this, in this study, it was decided to use topologies with an analog design in order to improve the reliability of the final system. These types of topologies can be found in [14; 15]. In this study of the existing balancing methods, the equalizers were divided into five groups according to their balancing method:  
* Resistor equalization (RE): excess energy is dissipated through resistors. * Switched capacitor equalization (SCE): these circuits transfer energy by charging and discharging capacitors. * Switched inductor equalization (SIE): in this case, inductors are used to supply current to discharge cells from the most charged cells. * Resonant switched capacitor equalization (RSCE): these circuits typically improve SCE performance by incorporating resonance, which reduces switching losses. * Converter-based equalization (CBE): power converters facilitate energy transfer for equalization. Another crucial aspect of each balancing method is the energy transfer path during the equalization process. The literature identifies six distinct types of balancing modes:  
* Cell-to-resistor (C2R) is associated with passive equalizers, surplus energy is dissipated in resistors and not reused by other cells. * Adjacent cell-to-cell (AC2C): energy transfer is limited to adjacent cells, often resulting in slower balancing. * Direct cell-to-cell (DC2C): energy is moved directly between cells. * Pack-to-Cell (P2C): energy flows from the entire battery pack to cells with the lowest voltage. * Cell-to-Pack (C2P): this is the inverse of P2C, where the cells with highest voltage charge the entire battery pack. * Any cell to any cell (AC2 AC): energy can be transferred among all cells simultaneously. Table 1 provides an overview of the main equalizer topologies from the literature. All entries are classified according to the aforementioned categories. Furthermore, the number of components needed for the implementation is indicated. Please note that in certain  
topologies, the number of resistors cannot be determined (**), because it is determined by the final implementation. After studying all the relevant existing equalizer topologies, a thorough selection process was conducted based on the desired application: a CubeSat operating in the harsh environment of a deep space mission. In this context, designing fault-tolerant circuits, also known as single-point failure-free (SPFF) designs as shown in [29], is critical to ensure mission longevity and prevent premature operational failures. Another critical consideration is efficiency. Deep space missions often experience reduced solar radiation, which directly impacts the available power. Therefore, identifying an optimal balance between efficiency and high reliability is crucial. Furthermore, the chosen topologies must be suitable for analog control and the use of COTS components, which is a fundamental principle of the New Space philosophy. In CubeSat missions beyond Earth's orbit, where systems must operate reliably under severe conditions, ensuring fault tolerance using readily available components is critical to the design. Considering these factors, passive balancing topologies were dismissed. Although they are a viable alternative for low-earth-orbit (LEO) CubeSats, their inherently low efficiency makes them unsuitable for deep space applications. Within the SCE-based topologies, it is noteworthy that certain versions were not well suited for the purpose of this work. This was due to the fact that the failure of capacitors was considered. Multiple events can degrade the capacitor properties and precipitate a failure [30].