A straightforward improvement to hybrids can allow them to be plugged into the electric grid and run in an all-electric mode for a limited range between recharging. Since most vehicle use is for relatively short trips, such as commuting, which are followed by an extended period of time during which the vehicle is not being driven and could be charged, even a relatively modest all-electric range of 20 or 30 miles could allow these vehicles to replace a substantial portion of gasoline consumption and tailpipe emissions.
If the electricity were from CO2-free sources, then these vehicles would also have dramatically reduced net greenhouse gas emissions. Because they have a gasoline engine, plug-in hybrid electric vehicles (PHEVs) avoid two of the biggest problems of pure electric vehicles. First they are not limited in range by the total amount of battery charge.
If the initial battery charge runs low, the car can run purely on gasoline and on whatever charging is possible from the regenerative breaking. Second, electric vehicles take many hours to charge, so that if for some reason owners were unable to allow the car to charge either because they lacked the time between trips to charge or there was no local charging capability then the pure-electric car could not be driven. In some respects, PHEVs combine the best of both hybrids and pure electric vehicles.
Battery improvement will lead to increased functionality for PHEVs. Improvements in specific energy (Wh/kg) and specific power (W/kg) will reduce weight. Reductions in cost and increases in cycle life (durability) will make PHEVs more affordable. Adequate safety is a requirement. Operating temperature is important, but batteries with unusual operating temperatures may be considered if other benefits are demonstrated. Convenience of recharging is crucial, but the definition of "convenience" varies by users.
A full recharge overnight from an ordinary home outlet is generally considered to be sufficient for a personal PHEV. Larger vehicles might require higher charging voltages, or possibly undergo a battery cartridge replacement rather than recharging.