Electric Bus Basics
Similar to the BEVs discussed above, BEBs and ESBs run on electricity only and require recharging their onboard battery packs from an external power source.1 BEBs are categorized as long-/extended-range or fast-charge depending on the size of their battery packs. Long-/extended-range BEBs have larger battery packs (250– 660 kWh) and are meant to only be charged once or twice per day, whereas fast-charge BEBs have smaller battery packs (50 – 250 kWh) that can receive more frequent high-powered charges; ESBs generally fit into this category, as they tend to have shorter routes with a midday break for charging. The average range for BEBs and ESBs varies based on the battery pack capacity and is significantly impacted by weather, driving behavior of the operators, topography, and ridership load.
There are three types of charging infrastructure for BEBs, all of which can be installed at the maintenance or storage facility (depot) or on-route:
Plug-in charging has both AC and DC options to charge at a low power (40 – 125 kW). The number of buses accommodated will depend on the configuration of chargers and ports, which are often installed in depots as buses are generally charged for multiple hours or overnight. In rural areas where school buses are sometimes parked at drivers’ homes, Level 2 chargers may be installed to allow for overnight charging. Larger batteries typical of BEBs (250 – 660 kWh) require long charging time at low power. There are a number of faster (up to 350 kW) plug-in charging solutions available for transit vehicles. Faster still plug-in charging options are under development, with a 1 MW standard introduced in summer 2022.
Overhead conductive charging
Overhead conductive charging, also known as pantograph charging, requires physical contact with flow of current between an overhead charger and the onboard battery. Transit buses can be recharged in 5 – 20 minutes at a higher power level (165 – 600 kW). Currently, ESBs do not use this method of charging.
Wireless inductive charging
Wireless inductive charging uses floor-mounted charging pads that are charged using a magnetic field passed through two coils and an onboard battery. This system uses a lower power level than conductive charging (50 – 250 kW) , thus requiring a longer charge time.
1For more detailed information on BEBs, ESBs, and their charging infrastructure, see the Transit Cooperative Research Program’s “Guidebook for Deploying Zero-Emission Transit Buses”,” the National Renewable Energy Laboratory’s “Electrifying Transit: A Guidebook for Implementing Battery Electric Buses.”,” and DOE’s “Flipping the Switch on Electric School Buses” series.