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Reader Question: Why do Electric Cars Need to be Plugged in to Recharge?


I have a question about the power supply for electric cars. I was reading recently that the plug-in hybrid could have exceptional gas mileage, up to 600 if memory serves, but would have to be plugged in overnight to recharge the battery. I don't understand why an electric car needs to be plugged in to recharge. We have had alternators and voltage regulators in vehicles for years. Why can't someone develop one capable of recharging the battery pack in an electric car so that it doesn't have to be plugged in? Perhaps I'm looking at it too simply. If so, please advise why. Thanks in advance. Bill

Dear Bill,

You’re not looking at it too simply at all--it’s a good question. We love readers with inquisitive minds, and the answer to your wonderment is basic physics. In regular cars, and even in normal hybrids--to a lesser, but similar, degree--the internal combustion engine drives the alternator/generator to keep the battery charged. This works well for two reasons: the engine is already running to keep the car moving, so it’s also available to power the alternator, plus these relatively small starter batteries don’t require much power to keep them charged, so the alternator load is light. However, in pure electric cars, the battery is the only power supply to the electric traction motor--there is no engine to drive an alternator, and using the electric traction motor to drive an alternator, or act as a generator, would be counter productive. Since an electric car’s batteries are its only source of power, they are large and require copious amounts of electricity to recharge and must be plugged in. There is no engine to do that work (though they do employ the assistance of regenerative braking, it is only thus, assistance, and it cannot keep up with demand) and asking the electric motor to charge the battery as well as drive the car would consume most or all of its power.

See the conundrum? In fact, if that actually could be done efficiently, the elusive perpetual energy machine would be a reality. (i.e. the car could produce its own operating energy, and that would be in direct violation of the Law of Conservation of Energy in physics). To paraphrase this law, it states simply that energy can neither be created nor destroyed--it can only change form. One of those forms is friction. Overcoming the friction of wheels rolling on the road, bearings slipping over journals, and wind resistance (to name just a few) already “changes the form” of some of the car’s energy. It simply cannot magically produce that “lost” power.

Plug-in hybrids are a different animal. In these cars, there is a good size battery pack, traction motor, and a small onboard engine that runs a generator/alternator. Under most folk’s average driving situations, daily commutes would only use up the energy stored in the battery, and then they would be plugged-in again at night for a full recharge for the next day’s tasks. However, under extended use (say, a long distance family vacation), the onboard gasoline powered generator would produce power to directly power the electric traction motors as well as charge the battery. This would allow the car to travel as long as the fuel supply (gasoline) lasted. That’s nice insurance, but it does defeat the purpose of electric drive trains--powering a vehicle without a fuel-burning engine.

All things considered across the board, it is cheaper, more efficient, and less polluting to plug-in an electric car for a few hours for a battery recharge, than to try and overcome the (nearly, if not completely, impossible) burden of forcing it to produce its own juice.

Hope that makes sense, Bill, and thanks for a great question.
Christine and Scott

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