**2.1. Electric drive configurations in various powertrains**

*2.1.1. Electric drives for pure electric powertrain with battery, ultracapacitor, or Hybrid Energy Storage System (HESS)*

During the AFV running, the load profile consists of high current peaks and steep valleys due to the repetitive starting, acceleration, and braking in the urban driving cycle. Thus, the battery alone faces several challenges: 1) huge addition to the battery numbers to meet the peak power demand; 2) severe adverse effect on battery life because of frequent charge or discharge operations; 3) extra difficulty on battery thermal management in high-power-load situation [5].

The ultracapacitor has much higher power density, longer cycle life, lower temperature sensitivity, higher efficiencies, but lower energy density, which makes it an attractive pairing for batteries [6], [7].

**Figure 1.** The configuration of hybrid energy storage system(HESS)-based electric vehicle

The topology of hybrid energy storage system is shown in Figure 1. Ultracapacitor and DC/DC converter are in series connection, with battery parallelly connected to the DC bus [8].

Hori [9], [10] adopted a heuristic control approach based on vehicle speed by taking into account ultracapacitors' energy utilization efficiency. Wang and Deng, et al. [8] adopted an optimization method for power distribution with the objective of minimum battery energy loss as the optimization goal. Ortuzar and Moreno [11] adopted heuristic algorithm based on two main rules and optimized algorithm obtained from a neural network as the energy management algorithms, respectively.
