[tablarasa]

Good points. It would be a productive use of all of the time those trucks spend idling too, going into and out of ports and in freeway traffic. Seems like there are times and circumstances when directing excess diesel energy to the trailer battery could be reasonable.

[talldatethrow]

There is no such thing as excess diesel energy. If today an idling diesel engine uses n units of fuel per minute, if you try to put any power into a battery for use later, it will use more n+x fuel per minute.

Now, if for some reason the engine was idling for no reason other than to keep a dome light on for an hour, then yes you could generate 100 watts as fast as possible and then turn the entire engine off, leaving the dome light lit for several hours off that 100watts you made. That is what a current hybrid does, again.

But if the engine has to idle for other reasons, even laziness, making power while it idles is simply using extra fuel to make that power.