system
Given the following post, respond as if you're commenting on Reddit:
human
Why can’t electric vehicles use total regen braking?

Here is my example. A Tesla has two regen settings and you can do one pedal driving to a point. My question relates to the need for physical disc brakes and whether they’re really needed. What is it that prevents using the motor as the brake? Is it legalities at this point? Does this generate too much electricity too fast for the battery to ingest? Is it the wiring? 

I’ve heard that an electric drive motor could easily lock up the tires of a car at high speed so I don’t think it is the motor.
gpt
Regen braking force bumps into a few limits. The maximum rate the battery can charge, the maximum power the conversion electronics can handle, and the thermal limits of everything involved. 

But the power limit is the most obvious. When the battery is full, there’s nowhere to send power from regen, so it just doesn’t work.  Drive a few miles and there’s room, but you can’t charge a very full li-ion battery very fast— so you have regen but it’s very weak. Same issue when the battery is cold. 

But even when things are ideal, and the battery is at the perfect temp for maximum charge rate and low state of charge, it can only handle about 250kW, or 335hp. (Used Tesla as an example here) That is much lower than the power of friction brakes. A car’s brakes are generally many times more powerful than its engine. 

And with a future battery that could handle the needed charge rate for emergency stops, you’d still have the problem of not being able to stop when the battery is full.

Edit: this is also why small-battery hybrids have very weak regen braking force, while big-battery EVs can produce regen braking force more like medium braking— the larger battery allows more charge power, which means more stopping force before you bump the limit.