Toyota Prius: plug-in batteries

Driving home tonite in commute traffic, I heard snippets of an
interview with a guy that apparently modified his Prius with additional
batteries and a plug-charging system. I didn't catch whether this was
a home-built thing, or if there is somebody doing this mod. It sounds
like a great idea, as the battery range would seem to be enough to
handle most of our around-town driving, using 90-cent per gallon
(equivalent) electricity. Anybody know anything about this?

Dand <net> wrote:
 

The first step, if you have an NHW20, 2004 or later, is to look at the
EV button modification to your car. This lets you run short distances,
about a mile, in just EV mode. However, there is some differences of
opinion about this.

Bob Wilson