> 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.