> What fraction of the energy recovered by braking is actually stored? I would have thought the battery on a non-electric or non-hybrid truck would be far too small. Surely the battery gets topped off and then the extra power is dumped as heat right?

It is always the question of what type of truck you are running. Do you just have storage goods, or do you have goods that need refrigeration? In the later you need energy to power that. As your truck does not know in advance what kind of trailer will be towed (connected, what is the right word?) the truck needs to be prepared.

Yes, extra power is dumped. But that is true for any kind of vehicle.

> Or is it because of the nature of long-haul trips? I know hybrids have much less advantage on the highway, maybe for long-haul trips the ratio of energy lost as air friction to energy lost in braking is vastly different.

Actually, Hybrid is very good concept for long-haul. Because on highways you can do coasting (sailing as we say in German) very efficient when you are in cruise control. Because with all the little hills up and down at the same speed, hybrid takes its full advantage.

There will always be situations where you can no longer recover energy, fully electric or hybrid. Your battery could be full, or overheating. At that point, you have to waste the energy.

I believe the normal way to handle this scenario is to just fall back to mechanical braking.

Why do most/all electric and hybrid cars have regenerative braking while ICE cars don't? It's because, as the GP said, the battery would be quickly topped up and extra energy wasted. Only electric vehicles doing frequent stops/downhills can make productive use of regenerative braking.

Potential energy lost by a truck going 10 m (vertically) downhill: 40 000 kg * g * 10 m = 4e6 J.

Capacity of car starter battery: 40 Ah * 12V = 400 Wh = 1e6 J. Let's guess a truck battery has 4 times the capacity of a car battery, so 4e6 J to full charge.

That's just down one small hill and all subsequent braking energy for the entire trip is wasted.

To drive the point home, a Swiss company is working on a mining truck that will be a net generator of electricity - it goes up the mountain empty, but when it goes down it is full of heavy ore, and regenerative breaking charges the batteries more than was used to climb the mountain. The extra 10kWh can then be fed back into the grid.

https://arstechnica.com/cars/2017/09/this-cement-quarry-dump...

... Also because electric regenerative braking requires a battery and electric motor (for propulsion, the starter is useless for this). Once you fit regenerative braking into an ICE vehicle it becomes a hybrid vehicle, leaving no ICE vehicles with regenerative braking. :)

(I don't think you would call a vehicle with mechanical KERS system a "hybrid", although it technically is as there are multiple propulsion systems in the vehicle.)