Research Results by Nassim Rizoug, Teacher-Researcher at ESTACA'Lab
The Li-ion batteries remain the best technology to supply the electric vehicle (EV), because of their high power and energy. The use of this battery in some applications like motor sport can be limited by the thermal behavior of this technology. For that, we must decrease the stresses applied to the battery cells.
Fig. 1: Hybrid storage system (Battery/Supercapacitors) for electric vehicles.
One way to improve the Li-ion battery performances and reduce its masse is to associate this battery with other technology as second power supply (Fig. 1). This strategy (hybridization) allows using a smaller battery for low power and the second powerful technology like supercapacitor to supply the very high power during the accelerations and regenerative brake. Hybrid energy storage systems (HESS) are used to optimize the energy management for electric vehicles. These solutions use separate energy and power sources in order to use their characteristics at their best, what allows a reduction of the size, efficiency or cost of the embedded source. In addition, one of the most important advantages of this novel approach is the improvement of the thermal battery behavior. As a result of this development, significant reductions in the cost and optimizing the performance of electric vehicles can be achieved.
Experimental results show that the RMS power of battery is effectively reduced. In our case, the performances of the battery alone are compared to that of the hybrid storage system. Three different strategies are used to manage this hybrid source (EMS1, EMS2 and EMS3).
|Fig. 2: Weight of the hybrid embedded power supply according to vehicle operating range
||Fig. 3: RMS power stress applied to battery cell according to vehicle operating range.
The obtained sizing results prove the interesting of the hybridization in the electric vehicle applications. As we can see from the Figure 2, the hybrid embedded power supply with EMS3 offers a lower weight over the entire vehicle operating range. We can distinguish two main phases of the weight evolution for all proposed configurations. Firstly, when the weight is constant over vehicle operating range, and the second, when the weight increases rapidly according to vehicle operating range. This can explain the fact that the hybrid embedded power supply is sized in the first phase under the discharge power constraint, which is constant and more stringent than energy and charge ones. However, in the second phase, the energy constraint is heavily dependent on the vehicle operating range. As a result, this constraint of the energy consumption imposes the final size of the hybrid embedded power supply.
Figure 3 shows the RMS power stress applied to one battery cell for all proposed configurations. This test allows us to set up a first evaluation of the battery aging. In our case, we consider that the battery management system BMS prevents the imbalanced cell voltage in a series-connected battery pack. As a result, it is also assumed that the maintenance of the voltage balance at the charging and discharging phases is completely ensured by the BMS. As it can be seen clearly from Figure 3, the hybrid power supply with EMS3 provides a high RMS power stress in the first phase of constant weight when the discharge power constraint is more stringent than others. Thereafter, for high values of the vehicle operating range, this solution became much more interesting against the configuration of the single HP battery. The hybrid embedded power supply with EMS1 and EMS2, offer a low RMS power stress over vehicle operating range. This is owing to the fact that the battery pack contains a large number of cells.
Fig. 4: Multiphysics model of the hybrid energy storage system.
Based on the HESS multiphysics model (Fig. 4), an improved rule-based strategy including the limitation of the battery power has been tested. Simulation tests of the aging cycling were done for a realistic electric vehicle profile in various operation ranges. It was noticed that proposed model can be used to study the influence of driving cycles and energy management strategies on the lifetime of the Li-ion battery and supercapacitor.
Future work is currently undergoing for improving the HESS multiphysics model, developing and designing new energy management strategies.