Engineering solutions to EV range anxiety

4 min read

Electric vehicles on general sale today offer adequate ranges for tasks such as commuting, shopping and urban journeys. Nissan, for example, claims up to 124 miles per full charge for its all electric Leaf, while Tesla suggests up to 310 miles for its Model S when specified with the optional 85kWh battery. Even so, range anxiety – the fear of running out of charge before journey's end – is recognised as a potential barrier to widespread acceptance of electric vehicles.

Electric vehicles on general sale today offer adequate ranges for tasks such as commuting, shopping and urban journeys. Nissan, for example, claims up to 124 miles per full charge for its all electric Leaf, while Tesla suggests up to 310 miles for its Model S when specified with the optional 85kWh battery. Even so, range anxiety – the fear of running out of charge before journey's end – is recognised as a potential barrier to widespread acceptance of electric vehicles. One response to range anxiety can be seen in the new BMW i3, a full electric vehicle (EV) that offers the option of an additional small petrol engine as a range extender. Unlike in a hybrid drive system, this range extender is used only to charge the i3's battery, increasing its maximum range from 190km to a more impressive 340km. Of course, the goal for EV advocates is the elimination of the conventional combustion engine. Improvements in the technologies used throughout electric drives – which comprise the battery and battery management system (BMS), motor controller, inverter and the motor itself – are expected to yield a fivefold increase in maximum range by the end of the coming decade, according to a recent report by IDTechEx. Build a better BMS Whilst car makers like Tesla have shown that increasing the battery capacity can offer a solution to range limitations, better BMS performance can improve energy use. A basic BMS will monitor critical parameters such as voltage and temperature. Individual cells in the battery pack will naturally display variations in voltage and the battery monitor can prevent cells with the lowest voltage becoming damaged due to over discharge by signalling to the vehicle controller when the system should be shut down. Similarly, the BMS can control charge termination when cells with the highest voltage reach full charge. In this way, a BMS helps to prolong the life and optimum performance of the battery. Justin van't Hoff, senior electrical systems engineer at Delta Motorsport, says a BMS that provides charge balancing can have a more significant effect on the use of battery energy and may improve the range of the vehicle by several percent. The BMSs currently used in EVs implement passive balancing, using banks of resistors to ensure that all cells are kept at the same state of charge. The BMS communicates with other vehicle ECUs via the CAN bus to share information about the battery, which is vital for the performance and safety of the vehicle. It is responsible for providing State Of Charge information – the EV's 'fuel gauge' – which the driver needs in order to assess the available range. The BMS can communicate via CAN with the vehicle's user interface software to display status information via the vehicle's instrumentation. Other data collected by the BMS can include input and output current, overall voltage, individual cell voltages, internal resistances and environmental data such as temperature. These enable the vehicle controller to determine its optimum response to the driver's demands. For example, if the accelerator position demands a certain power, the motor controller uses data from the BMS to determine whether the battery can provide the requested power or, if not, to determine the closest possible response. The BMS may also record historical data, such as cumulative operating hours, which can be used to advise when maintenance or replacement is needed. Improved charging infrastructure Today's EVs are typically sold with a home charging kit, which may operate at the standard 13A current or at higher current for faster charging. Ideally, the owner will charge the vehicle while at home and use the car routinely to reach destinations within the vehicle's return travel range. However, an accessible and easy to use public charging infrastructure has a role to play in eliminating range anxiety. Wireless Electric Vehicle Charging (WEVC) offers a secure and easy to use approach, while achieving charging efficiency comparable to that of a plug in charging station. After the car is driven into the charging bay, a secure communication protocol for authentication handles the procedures needed to verify user account details for payment and the commencement and termination of charging. Safety features – such as foreign and living object detection – are built in and known to operate satisfactorily. Qualcomm is currently trailing its wireless EV charging with a small number of users in London, the main objective being to understand the improvement to the user experience that wireless charging brings over plug-in charging. Qualcomm Europe's senior director Joe Barrett said the Qualcomm Halo WEVC system uses cutting edge multi-coil resonant magnetic induction and can be interoperable with older single coil-based systems from other manufacturers. The Qualcomm Halo WEVC charging system comprises multiple coils to ensure optimal field characteristics for high charging efficiency and greater tolerance for any positional misalignment between the transmitting pad installed in the charging bay and the receiving pad on the vehicle. See figure 1. The SAE International Hybrid-EV Committee, responsible for developing the J2954 wireless EV charging standard, recently adopted an operating frequency of 85kHz; earlier proposals from a variety of companies operated at frequencies ranging from 20kHz-145kHz., This standardisation of the operating frequency is a significant step towards a common standard for WEVC. The use of a higher operating frequency (85kHz v 20kHz) for wireless charging confers a number of advantages including allowing a smaller charging pad size for a given output power. The committee selected 85kHz after taking into account factors such as potential interference with transmitted radio frequencies and implantable medical devices for example. Further standardisation efforts are focusing on the communication protocols connecting the vehicle to the grid for charging system management. Qualcomm is working with multiple standards bodies to support the continuing work on communication, interoperability, vehicle pad location. Standardisation will facilitate interoperability between base charging systems and vehicle-mounted charging hardware from various manufacturers. A number of EV types have been involved in the London trial; Delta Motorsport is supplying several examples of its Delta e4 all electric EV to the project. The current trial is static WEVC, although wireless charging of moving vehicles is also possible. The prospect of recharging while on the move could further enhance the usability of EVs for both public service vehicles and general consumer use. Conclusion Now that electric vehicles are reaching consumers, the pace of development should continue to increase. There is room for improvement in many areas, including: battery technology, to improve energy storage and discharge; regenerative braking, to reclaim and store recoverable energy; and electric motors, aimed at reducing weight and increasing the torque produced in relation to input power. The combined effects of improvement in all of these areas, and probably more as EV design continues to advance, could finally deliver on the promise of anxiety free, low carbon mobility. Peter Dallimore is a freelance technology writer.