Active charge balancing for batteries or ultra-capacitors
The latest Lithium-ion batteries and double-layer ultra-capacitors have amazing energy-storage capacities. However, the voltage of a single cell is usually not sufficient for higher power applications, and series connection is then required. Due to differences in individual cells some cells may experience different voltages. To compensate for variation in cell voltages and protect cells from over- or under-charging a charge equalisation circuit can be incorporated in the charge/discharge circuit (usually a DC/DC converter). This project will investigate different technologies and a prototype of the most promising one(s) must be constructed and tested.
Active Charge Balancing for Batteries
Dana Al-Jahor
In many electronic applications, the use of multiple battery cells is a key requirement for their longer operating life time. In many cases battery packs consist of multiple cells in series. However, it is also well known that there can be small differences in the capacity and the output voltage of each cell due to production tolerances and operating conditions such as for example temperature. These differences can be worsened during the charging and discharging process of each cell. Under such conditions, the weaker cells are over-stressed during charging which causes them to be weaker leading to a premature failure of the whole battery.
In order to avoid such problems, two types of balancing mechanisms are readily available which are commonly termed as passive and active balancing. The aim of both methods is to improve to the life of the batteries in order to utilise their optimum performance. Passive balancing mechanisms do not guarantee that all cells will reach their 100% capacity at full charge and are dependent heavily on the changing impedance of the battery thus leading to energy loss. Active cell balancing on the other hand, overcomes the energy loss experienced by passive balancing methods by using capacitive or inductive charge storage. This technique is used for designs requiring optimum efficiency and maximum run time.
In this project, three types of active cell balancing methods will be assessed.
• The first active balancing method known as the Terminal-Voltage pumping compares the individual voltages of the cells and is the simplest most widely used technique.
• The Open-Circuit Voltage pumping is a further advancement of the above technique where the difference in the impedance of individual cells is compared.
• The state-of-the-art predictive balancing is a technique which determines the exact charge that needs to be transferred between the cells in such a way that the cell capacities are balanced at the end of the charge. This technique by far has a superior performance and active-balancing accuracy then the other two methods described above.
The aim of this project is to investigate the three techniques and construct simple prototype circuits to prove the concepts. Preliminary investigations will be focused towards the understanding, analysis and theoretical modelling of each of the techniques mentioned above. Local components shall be sourced in order to construct prototype concepts. Charging and discharging mechanisms will be introduced with the aim of deliberately creating small and large differences amongst individual cells. A dedicated control through a microcontroller shall be achieved. The process will also involve writing source codes and algorithms in order to achieve optimum balancing and to assess the short term and long term performance of the prototype circuits. Finally the performance of each technique implemented will be compared and analysed.