When (or if) we watch a Formula One race on television this year, the commentators will probably talk about the KERS system, and how (or if) it is being used. The KERS units have yet to attain the same impressive level of reliability of the highly stressed internal combustion engines used alongside them. This is hardly surprising; we understand combustion engines pretty well after having developed them for more than 130 years, and modern race engines, in series where regulations are essentially static, have incrementally increased performance and solved any reliability problems as they occur.
There are three main subsystems of the electric KERS systems used in Formula One - the electric motor/generator, the power electronics and the battery. The battery, for both hybrid and pure electric production vehicles, holds the key to producing a light, affordable vehicle. In racing, where the mass of the car is lower and the packaging of major components is critical, the battery represents something that not only has a critical effect on the output of the combined engine/KERS unit but also on the design of the rest of the car. The Red Bull Formula One car this year would not compromise the design of the rest of the car in order to package the KERS system, and this has given the team KERS reliability headaches throughout the season.
The battery is made up of multiple cells, connected in series to give the system voltage. They are thought to be exclusively based on lithium ion technology, with each cell giving about 3.6 V output. With systems operating at hundreds of volts in order to limit maximum currents, there can be hundreds of cells in a typical KERS battery. The current 60 kW discharge limit, for a 100-cell battery, would result in an average current draw of about 160 A, with peak currents being higher than this.
I asked battery expert Nigel Vincent of ABSL Power Solutions, which makes lithium ion cells, about various aspects of the cells and their development. He explained that there is a trade-off between energy density (the amount of energy stored per kilogramme of cells) and power density (the charge/discharge power per kilogramme of cells), with differing chemistries offering different advantages.
Lithium ion cells based on LiMnO2 (lithium manganese oxide) and LiFePO4 (lithium iron phosphate) show high power density, but lack energy density compared to newer 'mixed oxide' cells which have excellent energy densities (>225 Wh/kg). These lithium ion chemistries concern the cathode but changes in anode chemistry, according to Vincent, could yield "further significant increases in specific energy". Moving away from graphite anodes to ones based on silicone or titanate could see energy densities reaching 350-400 Wh/kg in the next five to ten years.
The other important metric against which batteries are judged is the charge/discharge rate, and this is limited, otherwise damage can be caused. This limiting rate is affected by the construction of the cells, with features such as electrode surface areas being important factors, as well as the chemistry being used.
Of great importance in a race application is not only the consideration of energy/power density but also space efficiency. Traditional cylindrical cells are being displaced to a certain extent by 'pouch cells', which are essentially flat and can therefore be packaged more efficiently.
Developments in cell chemistry and manufacturing have also improved the operating life of the cells we might choose to use for race hybrid systems. Other factors affecting cell life, as discussed with Vincent, are said to be the 'depth' of charge/discharge, rate of charge/discharge, charge voltage and ambient temperature.
The advent of KERS in Formula One and hybrids in other forms of racing will, hopefully, help drive forward advances in cell chemistry and construction for the benefit of racing and also for production hybrids in future.
Fig. 1 - The 'pouch' cell offers packaging advantages over traditional cylindrical cells (Courtesy of ABSL Power Solutions)
Fig. 2 - A traditional cylindrical lithium ion cell (Courtesy of ABSL Power Solutions)
Written by Wayne Ward