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Location: Pantego, Texas, United States

Tuesday, August 25, 2009

Yesterday when I wrote about the economics of electric vehicles (EV) I didn't discuss re-charging the car battery. People often think that the EV would be plugged into an electrical outlet at home. That could be done, but would require some special equipment. The Tesla battery weighs 1000 lb., has a capacity of 53 kw-h, and operates at 375 V. The electrical outlets in homes have voltages of 110 V or 220 V. Thus it would appear that some sort of transformer would be required to increase the voltage for charging the car battery. But, ignore the losses in the transformer for the moment. The typical 110 V outlet usually has a 15 A breaker. To provide 53 kw-h from such an outlet would require more than 32 hours (53,000 kw-h/[110V X 15A = 32.12 hours]). Charging from a 220 V outlet with a 30A breaker would require over 8 hours. Note that charging at work or at a hotel or shopping center would require special equipment in addition to the electrical outlet. The car could have the transformer installed in it, but that would entail carrying a lot of extra weight around in the car, something that would be undesirable considering that the car already carries a 1000 lb. battery. Tesla Motors says that they will have a 440 V, 70 A charging system that will recharge the Tesla battery in 3.5 hours. That sounds plausible even allowing for losses in efficiency.

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