why can’t electric cars be charged by their own movement?
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We use the word “generator” to describe anything that uses movement to create electricity.
An electrical load on the generator’s electrical parts *creates* a mechanical load on the mechanical parts.
In other words, when you have an electrical current that can do something like charging a battery, it *acts as a brake on the generator*.
This is good when you’re in a situation where you want the generator to stop. For example the user is indicating they want the car to stop by putting their foot on the brake pedal. A lot of electric cars actually do charge the battery in this situation, it’s called *regenerative braking*.
However you can’t get more energy out than you put in. If you start with the battery at say 60% charge, then discharge the battery to 59% charge to make the car go, then use the car’s motion to charge the battery, you’ll discover that the car comes to a complete stop when the battery’s only recharged to 59.7% or so, considerably less than its original level.
As other posters have noted, you can sort-of get around this by driving downhill. That’s because you’re adding an additional source of energy, the car’s elevation above sea level. If you’re up somewhere in the mountains, and go downhill while engaging the regenerative braking, you could charge the battery from 60% to 61% while driving downhill, let’s say you dropped 4000 feet in altitude. That energy came from somewhere: You gave up potential energy by getting 4000 feet closer to the center of the Earth.
If you turn around and use the battery to drive up 4000 feet in altitude back to your original location, you’ll find that the battery goes down to below its original level — 59.5% or so.
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