Plug-in Electric Vehicles

plug-in hybrid electric car being charged
Charging a PHEV at Google


All battery electric and hybrid electric vehicles.


plug-in vehicles, PEV, battery electric vehicles, BEV, hybrid electric vehicles, plug-in hybrid electric vehicles, PHEV, cars, renewable energy sources, range, mpg, energy density, charging stations, power source, electricity

Predecessor Patterns

. . . Using Local Energy Sources reduces dependence on expensive foreign energy sources. Dependence on foreign oil can be radically reduced by exploiting renewable local energy sources, all of which can generate electricity, to power plug-in electric vehicles.

Problem Summary

If an electric vehicle can't be charged from an external electric power source, it has to get most of its energy from an internal combustion engine which most likely burns a fossil fuel.


The phase "electric vehicle" can refer to any vehicle propelled by electric motors, from cars and pickups, to buses and trucks, to street cars and trains, or even electric airplanes, boats and scooters. Here we use the phrase "plug-in electric vehicles" (PEVs) to refer to both battery electric vehicles (BEVs) and plug-in hybrid electric vehicles (PHEVs) that are driven on surface roads, and whose batteries can be charged by connecting them to an electric power outlet.

The major advantage of electric vehicles is that the electricity can come from a number of primary sources, in particular renewable energy sources like wind, hydro, solar and geothermal, at least one of which is available almost everywhere in the world. In addition, electric motors are up to 90% efficient and mechanically simple, they have high torque, and can be so finely controlled that they eliminate the need for transmissions. Environmentally they are quiet and so clean that their direct emissions are nearly zero.

Battery Electric Vehicles

Unless an all-electric vehicle permanently connects to an electric power source, like an electric train or street car, that vehicle is by necessity a battery electric vehicle. Without an internal combustion engine (ICE) for its power like a PHEV, a BEV depends on being plugged into an electric power source to charge its batteries. Because of the typical limited range of such a vehicle, most BEVs are small vehicles operating close to their charging sources, such as golf carts, motorized bicycles, scooters, small utility vehicles such as forklifts, and small local delivery vehicles such as British milk floats. However, with better battery design and other advances in efficiency, all-electric cars are making a comeback after many decades of loosing out to gasoline powered vehicles.

Hybrid Electric Vehicles

Hybrid electric vehicles combine an internal combustion engine and an electric motor to propel the vehicle in one of three different configurations: the series drivetrain, the parallel drivetrain, and the series/parallel drivetrain. Describing these drivetrain configurations is beyond the scope of this design pattern and the problems it solves, but essentially all three achieve higher fuel economy by using a small highly efficient ICE only for the average rather than the peak power requirements of the vehicle, e.g., to maintain speed over flat terrain. The electric motor helps supply the average power, but it draws extra energy from the battery to supply all of the peak power, e.g., for acceleration and going up hill. This way the vehicle doesn't have to lug around the redundant cylinder displacement and weight (both consuming more fuel) of a big engine for peak power requirements. Since most drivers need peak power less than one percent of the time, the liquid fuel savings can be considerable.

In some hybrids, further fuel savings are accomplished by turning off the engine when the vehicle is stopped, and restarting it when the brakes are released. This could be implemented in a conventional ICE vehicle, although not as easily or efficiently as running an electric motor for an instant start.

Whatever drivetrain setup is used, hybrid electric vehicles that cannot be plugged into an electric power source get their electric energy from two sources:

  1. They utilize regenerative braking, turning the electric motor as a generator to charge the batteries and using its resistance to slow the vehicle.
  2. They use the internal combustion engine to turn a generator to either charge the batteries or to directly power the electric motor.
While achieving higher fuel economy than conventional ICE vehicles, this second method is inefficient, because the energy used by the engine will always be more, due to losses from heat production and friction, than the energy it delivers to the electric motor, either directly or via the battery. (This is ruled by the first law of thermodynamics or the law of the conservation of energy.) Furthermore, although an electric motor propels the vehicle, most of the energy is ultimately obtained from the engine's fuel, most likely a fossil fuel like gasoline, diesel or natural gas, with only a small part derived from the regenerative braking system.

If kicking an addiction to (foreign) oil is an objective, your hybrid electric vehicle has to plug in and drop out.

Plug-in Hybrid Electric Vehicles

A PHEV is both a plug-in electric vehicle like an all-electric BEV and a hybrid electric vehicle, with the advantages of both. Greater liquid fuel economy is achieved over a conventional hybrid because its batteries can be charged from an external electric power source, either from the electric grid or a standalone source. The internal combustion engine of a PHEV is a kind of backup or insurance policy, eliminating the anxiety of being stranded with a dead battery.

The economy of a PHEV over a regular hybrid electric vehicle is demonstrated by the results of Google's RechargeIT driving experiment. In this experiment, the Ford Escape Hybrid got an average of 32.2 mpg for all trips, and the Toyota Prius Hybrid got an average of 48.4 mpg. Mileage for the plug-in hybrid versions of those two vehicles was much higher: 49.1 mpg for the Ford Escape Plug-in Hybrid and 93.5 mpg for the Toyota Prius Plug-in Hybrid.

But that's just the beginning of the potential of plug-in electric vehicles. PHEVs still require a liquid fuel engine, usually burning fossil fuel, to power the vehicle when the battery is low. Ultimately a PHEV should only be a transitional solution by so reducing its dependence on liquid fuel, especially from foreign petroleum sources, that it essentially becomes a full-time BEV. Consider the following.

1000 Miles Per Gallon!

Former CIA Director, James Woolsey, is a strong advocate of energy independence and plug-in electric vehicles, calling for the complete elimination of petroleum as a strategic commodity. He has a scenario whereby light-weight vehicles could get 1000 miles per gallon of gasoline. It goes like this:

Plug-in Vehicle Range

The only downside to plug-in electric vehicles is range because the energy density of batteries is low compared to liquid fuel, making them heavy and bulky, and the recharging time is slow compared to filling a tank. Adding more batteries to increase range between charges adds to the weight of the vehicle, offsetting the weight reduction achieved by the down-sized internal combustion engine. Therefore, better solutions to increasing range entail batteries with greater efficiency and more opportunities to recharge them.

The first problem involves the energy density or power to weight ratio in battery design. Heavy lead-acid batteries have been replaced by Nickel-Metal Hydride (NiMH) and Lithium-ion (Li-ion, like those in a laptop) which have much higher energy densities, and whose costs are rapidly falling. There is as much energy in 1 gallon of gasoline weighing 6.15 lbs as there is in a bank of lead-acid batteries weighing 1,979 to over 2800 lbs. However, state-of-the art Li-ion batteries with the same energy only weigh about 400 to 530 lbs, and prototype Li-ion nanowire batteries reduce that weight about 3.5 to 5 times. In addition, since electric motors are more efficient than internal combustion engines, the amount of energy in those batteries will get you down the road further than that gallon of gasoline. For example, the all-electric Tesla roadster has an Li-ion battery pack weighing about 990 lbs, hence an energy density of roughly 2 gallons of gasoline, with a mixed city/highway range of more than 200 miles. This is an energy use roughly equivalent to 100 mpg with a cost greatly cheaper than going that 100 miles on fossil fuel.

The second problem is slow recharging time. Since quick-charging batteries tends to harm them due to heat, you have to trickle-charge them. Generally this means that PEV batteries are most conveniently charged when they are parked at home during the night, fortunately an off-peak (thus cheaper) period for electricity.

Charging Stations Everywhere

Both of these problems can be ameliorated with an infrastructure of ubiquitous trickle-charging stations, for example in parking lots and garages. If battery packs could be charged wherever BEVs and PHEVs are parked, they could then be kept relatively small, staying fully charged with frequent short boosts instead of one long one.

A 2009 PHEV typically goes 40 to 50 miles on electricity alone with a fully charged battery pack. Since the average US trip is within 40 miles of the home, at least 1/2 of the round trip can be powered by electricity, but with enough charging stations everywhere that trip's range under electric power could be increased substantially. Since the average commute to work is about 25 miles in the US, the average round trip to work would require going a maximum of 10 miles on liquid fuel in a PHEV, unless the vehicle could be charged during the workday. If your workplace is within the range of your PEV's battery, and if it could be fully charged while you are working, for example under a solar carport with an electrical outlet, you would never have to burn liquid fuel (namely gasoline) to commute round trip to work.

Woolsey's 1000 mpg scenario is based on the potential of PHEVs, assuming today's scarcity of electric charging stations. At most, only a tiny liquid fuel engine should be required to recharge a PEV's batteries in an emergency, rather than to drive it down the road. (This would employ the more simple series rather than the parallel drivetrain configuration, which doesn't use the ICE to directly propel the vehicle, but just to run the generator.) With plug-in charging stations almost everywhere we could eliminate the need for an engine except for emergency backup charging, one similar to the generators in RVs to supply power when an electric outlet isn't available, something on the order of 2 to 6 horsepower.

Now we're talking something like 5000 mpg on gasoline. If this small engine burned, say, pure ethanol, the vehicle itself is off petroleum as a fuel entirely.

A lot of electricity is generated from non-renewable sources like coal, diesel and natural gas, so we're still using fossil fuels when we plug our PEVs into the electric grid. However, there are renewable energy sources everywhere on Earth, such as solar and wind, that can generate enough electricity to charge plug-in vehicles without using any fossil fuel whatsoever.

Solution Summary


Provide hybrid electric vehicles with an on-board battery charger that can draw energy from an electric power outlet, whether supplied by the electric grid or by standalone renewable sources such as photovoltaic solar panels and wind generators. Further reduce dependence on fossil fuels with engines in PHEVs that run on flex fuels derived from biomass.

Successor Patterns

Increase the electric range of PEVs by building charging stations, such as Solar Carports, everywhere they are parked for a significant time. For long range trips, create Battery Exchange Stations where low batteries can be quickly swapped for fully charged ones. . . .


  1. Battery electric vehicle at Wikipedia.
  2. Hybrid electric vehicle at Wikipedia.
  3. Plug-in hybrid electric vehicle at Wikipedia.
  4. How Hybrid Cars Work at, a project of the Union of Concerned Scientists. (In particular, see the excellent interactive animations of series, parallel and series/parallel drivetrains in part 2).
  5. RechargeIT Driving Experiment at Google.
  6. Jim Woolsey, at the Google/Brookings Plug-In Electric Vehicles 2008 conference in Washington DC, 6/11/08, on
  7. Energy density at Wikipedia.


Gary Swift, 10 February 2009.
Fixed broken link, 20 October 2009.
Clarified 1000 mpg scenario, 03 August 2013.
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