boy do I have a doozy for ya...

I am going to take a leap if faith, and believe that it is related to the manual choke. There are only two things the choke cable could do. It could serve to ground the engine in a poor way if the engine ground to fire wall connection is broke or loose. It could also short to ground the ignition feed wire that also serves as the reference voltage for the voltage regulator. If the voltage regulator see a low votage from either case, it provides more field current to regulate, hence higher voltage at battery. 18v at battery, is very bad, and could result in explosion of hydrogen gas. In either case the choke cable is a poor conductor, so the car continues to run, in a poor way. It is also possible the issue has damaged both the regulator and battery.

Since I doubt the car came with a manual choke, check the routing. And look for engine to fire wall strap. Also check all 67Dart273 suggests.