A battery system with built-in intelligence could improve the range of electric vehicles and also help push the price of batteries down. Developed by the Fraunhofer Centre for Manufacturing Engineering and Automation in Stuttgart, the system allows the individual cells that make up a battery module to communicate with each other, allowing the entire module to use the available charge in the cells in a more efficient way than is currently possible.
Large batteries like those in electric vehicles are composed of a number of cells linked in series. In theory these all have the same capacity, but in practice they vary slightly because of manufacturing variability. The unfortunate consequence of this is that the whole battery is effectively only a strong as its weakest cell; when this is depleted, regardless of the charge state of the other cells in the array, the whole battery has to be recharged. And if a cell stops working altogether, the whole battery becomes useless.
To avoid this problem, battery manufacturers will often test and pre-sort the cells before assembling the batteries. Some cells are rejected, and this pushes the price up.
The Fraunhofer team, led by Kai Pfeiffer, has developed a cell with a built-in microcontroller that records its physical parameters, including temperature and charge state and, using signals sent along the existing connections between cells, can send this information to the other cells and the control system for the car. If an individual cell runs out of charge, it decouples itself from the battery and effectively becomes a current bypass; the system as a whole keeps working. If a cell malfunctions, it can be replaced without having to scrap the whole battery.
This should reduce the need for manufacturers to pre-sort cells, which should reduce their costs and, in turn, the price of the whole battery. Increasing the efficiency of use should also improve battery range, claims Pfeiffer: “Depending on the cell quality, we can therefore increase the range by at least four percent,” he said. “Over time, this effect is amplified: in the case of an old battery, and if the empty cells are replaced, it is conceivable that a range up to ten percent higher can be achieved.” Pfeiffer’s team has developed a prototype and is now working on miniaturising the electronics that control the cell monitoring and inter-cell communication and reducing their cost to about a Euro.