Automobile Engineering

A lithium-ion battery pack consists of a number of cells in a series-parallel arrangement. The series connection, like in any chain, makes the performance limited by the weakest cell. If the cells were identical, this would not be a problem. However, there are cell-to-cell differ  ences, which increase over battery lifetime, that lead to limited electrical performance and uneven temperature distributions. Nowadays, automotive industry overcomes this problem in the design of battery packs for Electric Vehicles partially by adopting the so-called passive balancing systems. These systems carry out selective discharge of cells during battery charging, allowing all the cells to be fully-charged at the end-of-charge. Thus, losses in useful capacity related to differences in initial stateof- charge are compensated. Active balancing systems have also been proposed. They conduct selective dis/charge of cells during dis/charging, allowing all cells to be always at a similar state-of-charge level. Thus, losses in useful capacity related to differences in state-of-charge at any time are minimized. However, they do not get the full endorsement of automotive industry, which usually considers the partial advantages offered to be insufficient to justify the extra costs and complexity. Our research aims to bring aparadigm shift, by improving the design and control of these systems. The goal is to overcome not some, but all the problems related with cell-to-cell variations, by developing a smart balancing system (SBS) that makes the performance of any battery pack virtually ideal. The SBS transfers the energy between cells in such a smart way that the temperatures are minimized and equalized, and the electrical performance is optimized, in terms of losses, power capability and useful capacity. Potential advantages in Electric Vehicle performance include extended driving range, extended operation without de-rating the max. available power and longer battery lifespan.