Energy recovery from regen is minimal unless you're stuck in stop and go traffic. Usually less than 5%.

The argument made wasn't so much about getting more range from regen as about regen lessening the impact of added mass: with perfect regenerative braking, a ten ton vehicle wouldn't use much more energy than a one ton vehicle if they shared the same outer hull.

Real life doesn't have perfect regen, but on the other side of the equation real life ICE cars actually lose more efficiency to added weight than just what is converted to heat while breaking because they tend to compensate worth a bigger engine to get comparable (or better even) acceleration than a lighter counterpart and that means that during cruise where the mass is irrelevant the engine is running at all an even worse load point in terms of efficiency. ICE are terribly inefficient at partial load and when your engine is sized to get decent acceleration despite high total mass you simply can't gear long enough to get the engine to a reasonable load point in a moderate speed cruise. Electric motors don't have this problem (or a much, much smaller version of it), so they wouldn't suffer quite as hard from added mass as ICE even worth no regen at all.

> ICE are terribly inefficient at partial load

Yes, but this doesn't happen much anymore. Torque converter, electronic throttle control, variable valves, and 6+ speed gearboxes means the engine is usually near full throttle even if your foot is barely on the gas.

Turbochargers also help, along with cylinder deactivation for some V8s.

Modern engines are nearly always at a relatively efficient load point. This wasn't true until about a decade ago though.

A Prius or Ford Fusion hybrid get better MPG in city driving than highway. That's because regenerative braking recovers most of the energy needed to get back up to speed. The improved MPG has more to do with higher drag at high speeds which reduces efficiency.

Unless you're stuck in stop&go traffic, the extra energy needed to move the extra mass isn't particularly large either.

It highly, highly depends on motor setup.

Tesla low gearing induction motor is by far not the best here.

Synchronous motors have much better regen capability.

There's just not that much power available for regen. I've done a lot of dicking around with E-scooters. Even at their low speeds with lots of stop and go, regen is less than 10%.

It's only used because dumping excess energy back to batteries is cheaper than including brake hardware. The math may work out the same for EV's. Regen just to decrease the cost of brakes rather than increase range significantly. In the e-scooter world, the cheap ones use regen and more expensive models have traditional disc brakes.

Air resistance burns a ton of energy at any speeds over 20mph.

How much RPM were you getting at your scooter, and what was the vehicle mass?

You say 5% at most... The difference in between, say, 5% and 10%, a bad and good regen is huge.

The RPM's are huge. Like 16k, because smaller motors are lighter for same power. Mass is generally 40lb + rider.

The RPM doesn't matter much though. Regen efficiency is around 80% from wheel to battery. With cars you get much less regen because you lose tons of energy to air at the speeds they travel.

Mass also increases braking distance and reduces handling, no?

First off, 48 volt systems doing regen is already taking off, and there are already many new cars doing it (not just ones that have a hybrid sticker on them either: https://en.wikipedia.org/wiki/Mild_hybrid#Examples)

Second, as other comment says, the idea that braking regen is going to make up for an extra half a ton of batteries is pretty laughable