Is there any NET change. Most likely and increase in total cost and energy if you really looked at it.
As you point out, nothing is free. However, electric motors have some advantages. They have instant torque from zero speed whereas ICE engines must spin up. Accelerating is where ICE engines lose their efficiency.
Submarines have used generators to run their electric engines because the ICE engine can be run in its most efficient mode as a generator, which then runs the actual electric motors driving the screws. I think this would work in ICE vehicles and you wouldn’t need the battery. But everything you do adds weight, complexity and cost. And, in the case of lithium batteries, fire risk. There’s obviously a point where some innovation makes economic sense. Beyond that point it only makes political sense.
But if replacing the battery costs more than the lifetime savings of the fuel...well...
It depends on the cost of this trailer. A 36% reduction in fuel costs, for semi-trailer trucks, is a significant amount of money. For every 8.8 miles of driving, .369 gallon of fuel is saved.
So, for every 1000 miles, 113 gallons of fuel saved, at $5 per gallon, $209 per thousand miles, or about 21 cents per mile savings. At an average of 45,000 miles per year, that is a savings of $4,309 per year.
We cannot know if there are overall savings unless we know the cost and operating cost of the units.
With the electrics in the trailer, not the tractor, it would be up to the trailer owner to ensure that the trailer was fully charged, and the trailer owner would as a result be responsible for the cost of recharging.
The trailer could provide some regenerative braking, but with the interface being a 'smart kingpin' (clever idea) the trailer would only regen at the rate the truck is slowing, which would most likely not take full advantage of regenerative braking during, say, an extended downhill run.
Since the trailer owner would be responsible for the charging and maintenance of the electronics, they would have the cost of recharging without the benefit of fuel savings, so I'm sure they would pay less per mile to the tractor owner pulling it. (Many loads are tractor owners being dispatched to pickup a loaded trailer and drop it off somewhere else.)
But this might be worthwhile for a fleet owner who also owns the tractors, the same customer base as the Tesla Semi. For that matter, this idea would be much better suited if it were paired with a Tesla semi that could intelligently communicate to the trailer's electronics in order to maximize regenerative e-braking, etc.
They’re overweight when loaded similarly.