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3. Battery electric bus 3.1. Overview of electric buses technology The battery electric bus, often described as a pure electric bus, uses an electric motor for pro- pulsion and a battery for energy storage [29]. In most cases the battery is the primary energy source, although for trolley buses power is delivered from overhead cables during operation. The configuration for electric buses is typically fairly straightforward since it is basically a bat- tery driving an electric motor to propel the vehicle [30], as shown in Figure 6. During braking it is also possible to make use of regenerative braking to recharge the battery during braking. The main battery technologies that have been used in transportation are Ni-MH, Zebra (Na-NiCL2) and lithium batteries [31]. The most promising of these are the lithium batteries, where three main categories exist, these being Li-ion, lithium polymer (LiPo) and Lithium-iron-phosphate (LiFePO4) batteries [32]. Most current buses use lithium-based batteries [33] due to their high power and energy densities and fast charging capabilities, although their high cost is still prob- lematic [32]. A problem faced by all battery technologies is their cycle life; typically, these are short Bus type Route Diesel Diesel hybrid Year References Fuel economy (l/100 km) Fuel economy (l/100 km) Single decker (Euro V) 276 44.8 43.5 2010 [26] 360 36.7 34.9 371 34.1 26.7 E8 35.3 42.2 129 47.1 33.6 Double decker (Euro V) 141 60.1 50.4 2010 [26] 328 65.7 54.3 24 49.6 42.2 482 50.4 34.9 16 50.4 39.2 New routemaster (Euro V) 11 60.1 38.2 2014 [27] 24/390 52.3 38.2 9 72.4 45.6 148 56.5 40.9 10 64.2 43.5 159 Not available 24.4 2013 [28] Table 1. Available data for diesel hybrid bus fuel economy in London. The values for l/100 km have been converted from miles per gallon using Litres100 km = (100*4.54609)/(1.609344*mpguk). Hybrid Electric Vehicles40