On Optimal Cell Monitoring in a Sensor Minimal Battery Management System with Integrated Direct Active Balancing

Lithium-ion batteries with multiple cells in a serial or parallel connection are used in an increasing number of applications. Especially in regard to the growing demand for electromobility, a monitoring and balancing system is necessary for every cell within a battery system. The presented paper outlines two different concepts for a direct active cell balancing circuit. For both concepts, an energy exchange between each cell in the system is possible. With an active balancing circuit with either inductive or capacitive balancing mode, the energy is temporarily stored and can be transferred between any two cells in the system. Due to the circuit topology, it is also possible to achieve a minimum sensor measurement setup. The individual cell voltages can be measured consecutively via the balancing circuit but simultaneously to the balancing process. An accurate estimation of the cell voltages can be made by a suitable adjustment and parameter determination. These active balancing concepts allow not only to reduce effort for monitoring, but also an energy-based or voltage-based balancing. The results indicate that the quality of measurement can be significantly improved by appropriate means. Therefore, separate cell observer can be substituted by the integrated balancing and monitoring system with adjustable accuracy.