Interchangeable cells make traction batteries cheaper

December 01, 2015 // By Christoph Hammerschmidt
Fraunhofer researchers have developed a battery for electric vehicles that promises to be significantly more cost-effective than conventional batteries throughout its entire life cycle: In contrast to available batteries, the cells of the new design are exchangeable. But what looks like an obvious design requires a totally different battery monitoring system.

Today, traction batteries for electrical vehicles (EV) and hybrid electrical vehicles (HEV) are designed, built and sold as a monolithic block that contains hundreds of single battery cells. These cells are connected in series, which leads to a significant problem: The whole battery is only as strong as the weakest cell. If one cell fails, the entire battery renders unusable and needs to be replaced as a whole. The principle provides that all cells are identical and can store the same amount of energy, and battery monitoring system is applied to the battery as a whole. The reality however is somewhat different: Due to manufacturing tolerances the capacities of the individual cells vary. The manufacturers try to prevent this effect by thoroughly selecting and matching the cells. During this process however, several cells are sorted out. The result: The entire battery becomes more expensive. Another problem is that if only one cell breaks down, so does the car. In such cases, the whole battery must be replaced.

Researchers from Fraunhofer Institute for Production Technology and Automation (Fraunhofer IPA) have devised an alternative to this unsatisfactory situation. They developed a modular battery system in which each cell can be replaced individually. Therefore however, each cell needs to be equipped with its own battery monitoring system (BMS) – a microcontroller that measures relevant physical parameters of like temperature and charge level. Thus, every cell is aware of its conditions. Across the existing high-current connection between the batteries the cells can exchange data (powerline communications). These microcontrollers also communicate with external devices such as the battery system controller that computes the remaining energy based on the data from the cells.

While with today’s batteries the car ceases to work if only one cell is empty, the Fraunhofer design enables vehicles to continue their ride. This is possible because a faulty of empty cell is simply deactivated and will be isolated; the energy flow is directed around the deactivated cell. “According to cell quality, we can increase the driving range by at least f percent”, explains Fraunhofer IPA team manager Kai Pfeiffer. “During the lifetime of a battery, this effect becomes even more pronounced. If you deactivate the empty cells in older batteries, it is possible that you can stretch the range by as much as 10 percent”.