There are a few _very_ simple chargers at
EV Technical Files.
I would personally only use the "
Bonn Charger". To me, bad-boy charging is a rather dodgy affair and I would want to have as many safeties in it as possible.
Also, I think you could change the output voltage by adding a variac or transformer to the input (before the GFCI). Eventually your output voltage will decrease to the point where very little/no current will move between charger and battery pack.
To my mind, these particular circuits are very much sub-optimal for use in actual electric cars, but seem like a legitimate jumping-off point for adding additional controls and stuff to make a pretty acceptable charger.