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All-electric range (AER) is the driving range of a vehicle using only power from its electric battery pack to traverse a given driving cycle. In the case of a battery electric vehicle, it means the total range per charge. For a plug-in hybrid (PHEV), it means the range of the vehicle in charge-depleting mode. PHEVs can travel considerably further in charge-sustaining mode which uses both on-board fuel and the battery pack.
Calculating AER is made more complicated because of variations in PHEV design. A vehicle like the Fisker Karma that uses a serial hybrid design has a clear AER. Similarly a vehicle like the Chevrolet Volt which disengages the internal combustion engine (ICE) from the drive train while in electric mode has a clear AER, however blended mode PHEVs which use the ICE and electric motor in conjunction do not have a clear AER because they use gasoline and grid provided electricity at the same time. Equivalent AER is the AER of vehicles following this architecture. One example of this calculation can be found in Argonne National Labs report titled "TEST PROCEDURES AND BENCHMARKING Blended-Type and EV-Capable Plug-In Hybrid Electric Vehicles."
This procedure uses the formula below to calculate an equivalent AER for vehicles that operate in blended mode:
Where GPMCD designates efficiency in charge-depleting mode, and GPMCS charge-sustaining mode as designated and dCD is distance in charge depleting mode.
A plug-in hybrid's all-electric range is designated by PHEV-(miles) or PHEV-(kilometers) km representing the distance the vehicle can travel on battery power alone. For example, a PHEV-20 can travel 20 miles without using its internal combustion engine, or about 32 kilometers, so it may also be designated as PHEV32km.