Ultrafast Charging Compatibility of Electric Vehicles

To increase the appeal of electric vehicles, the trend is to shorten the charging time and to lengthen the autonomy. The new ultrafast charge compatible battery on lithium-titanate gives the best compromise between high power and energy densities. The main objective of the contribution is to demonstrate the state-of-the-art of the fast charged electric vehicles and propose an evaluation tool for determining the possibility of shortened charging times from the traction battery viewpoint. Introduction An evident nuisance for electric vehicle (EV) owners is the limited average speed while driving long distances on highways, as recharging stops, defined by the battery performance, lengthen the total travelling time. Moreover, in the framework of on-board energy storage, one of the main problems is that the charge and discharge processes of a battery are asymmetrical in time, meaning that a sustainable recharge takes usually much more time than discharge. Therefore, substantial progress can be expected in the area of on-board energy storage technologies; this paper shows the compatibility of ultra-fast battery for the existing EV. As for today, the commercially available solutions allow recharging of an EV within 20 min as minimum. High energy density lithium batteries, based mostly on the lithium-manganese spinel (LMO) or lithium-iron phosphate (LFP) electrochemistries are widely used by manufactures in order to reach the highest autonomy possible with a disadvantage long charging time in scale of 6 h from a conventional household socket. Whereas high power density batteries require less time to recharge, their energy density and autonomy show poorer figures. A qualitative comparison between available EV battery chemistries is shown in Fig. 1. It should pointed out, however, that the manufacturers often specify the specific power as absolute maximum during a 10 s pulse, therefore the datasheet values should be interpreted with reservation unless there are more detailed figures available, not to mention different specific power for charging and discharging. Fig. 1. A comparison between main lithium ion battery chemistries The Fig. 1 together with additional market research permits concluding that the optimal EV battery electrochemistry in terms of recharge-discharge symmetry, life span, specific power and energy is that of the lithium titanate (LTO, or chemically expressed Li4Ti5O12). However, a trade-off between charging power density and specific energy exist, meaning an EV must be recharged more frequently than by using existing battery though the average speed is improved (Fig. 2). A nearly similar approach has been recently applied in refilling compressed air propelled vehicles Fig. 2. Charging-driving ratio at existing and proposed battery electrochemistries Materials and Methods The design of an EV ultrafast charging compatibility evaluation tool starts with creating a generic EV model, composed of the main battery and traction subsystem (Fig. 3). In the battery subsystem, the cell modules supply the vehicle with electric energy through the dc bus. A standard halfbridge dc/dc chopper is used to adapt the variable voltage from the battery to the quasi-constant voltage in the dc bus. Braking energy is partially recovered during deceleration. The traction subsystem is composed of an electric machine and a full bridge voltage source inverter (VSI). Transmission gear is used to adapt the speed ratio between wheels and the motor shaft, followed by a mechanical differential changing the speed ratio between the two traction wheels. Fig. 3. Studied EV model The movement of an EV is simulated during the New European Driving Cycle (NEDC), which reflects both urban and extra-urban driving conditions and is set as standard to characterise the energy consumption of small vehicles. The simulation model helps to validate the battery’s ability to supply sufficient power to follow the NEDC load curve. The performance of the upgraded EV is modelled using the Energetic Macroscopic Representation (EMR). EMR is a graphical description, which organises the system into interconnected basic subsystems: accumulators, sources, conversion and distribution. All elements are connected according to the interaction principle. The product of the action (for example, the force) and reaction (for example, the velocity) always leads to the power exchanged by the connected elements. Moreover, all elements are described using the physical causality (i.e. integral causality). These properties enable a systematic deduction of the control scheme. In the first step, the EV main battery is considered as an equivalent electric source, where the voltage as state variable is derived from the current. The LTO has been modelled by its nonlinear open circuit voltage (OCV) connected in serial with two parallel resistances representing the internal resistance values for charge and discharge (Fig. 4).