## 4 Optimizing Power Flow in EV Powertrains
### Continuously Variable Transmission (CVT)
The latest review paper providing further descriptions on the estimation techniques for battery state of health in an EV can be accessed in [66]. Finally, in terms of cost benefits for 11-year of operation, when compared the application of CVT against single speed transmission (both using the hybrid battery technology), a saving of around USD 4541 can be expected for the consumers for the battery cost, resulted from the reduction of the required battery capacity due to the improvement in the powertrain efficiency. In addition, a further saving of USD 1768 can also be gained due to the reduction in the electricity cost. In overall, after reflecting the battery replacement cost as well as the penalty cost for using CVT, the total cost benefit was estimated at USD 3178 for 11-year of operation. More advanced research on CVT application in an EV powertrain was explained in [67] which involved the optimization of an eco-driving strategy. The optimization objective was to minimize the reduction of the battery's SOC during the driving by taking into account not only the motor's efficiency, but also the instantaneous SOC as well as the efficiency of the CVT. Here, the CVT efficiency model was developed using mathematical equations introduced in [68; 69; 70]. NEDC was used as the driving cycle, in which it was divided into three driving conditions, namely, constant driving speed, acceleration condition and deceleration condition. In these conditions, the powertrain efficiency was analyzed for different SOC, CVT ratios and ambient temperature. One interesting aspect of this work is that it analyzed the SOC until the range of below 10%. Such SOC is rarely discussed in other literatures because in the actual application, the cut-off set point for the battery is usually set at around 20% to avoid any risk of damage. The analysis showed that, low SOC (at about 10%) decreased the powertrain efficiency by 33.12% and the acceleration time became longer by 68.8%. Such inefficiency was caused by the degradation of the battery that becomes significant starting from 10% SOC and lower. Because of the degradation, the battery internal resistance increases and its open circuit voltage decreases, which resulting in the increase losses percentage in the current flow from the battery to the EV motor. Moreover, at 10% SOC, the battery also became more sensitive to ambient temperature, which caused the losses in the current flow to increase even further. Above 10% SOC, however, the ambient temperature was insignificant in influencing these parameters, and the battery degradation was negligible, hence powertrain efficiency became more stable. By incorporating this knowledge in the eco-driving strategy, the constant driving speed condition in the NEDC can be increased from 61 to 70%, and the total driving time can be reduced by 12.1%, resulting in a more economical driving. This study highlighted the importance of eco-driving method that can only be implemented if the EV powertrain has the required flexibility in providing diverse driving modes. In addition, the study also served as the starting point to integrating battery's health conditions in the powertrain analysis. To summarize, CVTs can provide better flexibility in ratio selection due to their continuous ratio range as opposed to any multispeed discrete transmission. This flexibility allows the EV's motor to operate optimally for various driving conditions. However, for the actual operation of the CVT in an EV, an appropriate shifting strategy must be formulated, either continuous shifting or discretized shifting. The first strategy leads to better motor efficiency, but requires higher actuation power and advanced shifting logic. The second strategy, on the other hand, compromises the motor's efficiency slightly as compared to the first strategy, but its shifting logic can be made simpler and the actuation power consumption can be reduced. Another area that has to be studied is the actuation system for ratio shifting and belt's clamping in the CVT.