Imagine you drove your (pure) electric car to a very high mountain resort. It took 3 full battery charges to get up here, but by golly, you did it. And you plugged in, so battery is topped up now. Due to your altitude, your car also has a lot of potential energy.
Going home, you start down the mountain. Two-lane road, twisty, 25-45 mph, 8-20% grades, 5000' vertical drop. Continuous braking all the way down, you can't get going fast enough for aerodynamic drag to slow you appreciably. You are a mountain-skilled driver, and know better than to ride the friction brakes - they would burn up in the first 500 vertical feet!
Normally in a gas car, you do classic downshifting, and spin the engine faster to achieve braking action. The heat is blown out the radiator, and you can do this all day. What happens in an electric?
- Regenerative braking is no use; the battery is full.
- As discussed, friction brakes would fry very quickly.
Now, locomotives regenerate into big resistor banks, and they can do that all day. Are electric cars required to have an ability like that? Are they required to have enormous, actively-cooled friction brakes that can run continuously? Or do they simply ignore this scenario?