Does not sound right to me. Currently the alternator is punching out 18V. A little insight here would be appreciated. Without seeing your car, we cannot say for sure that the alternator is your problem and we cannot tell you what was damaged in the accident.
Electrical problems are nightmares to troubleshoot.
Check the wiring harnesses for damage, especially the ground wires. Also, have the battery load tested to be sure there is not a shorted cell inside. This may be due to a missing ground wire or a faulty internal voltage regulator. I am writing you today because of a problem that started directly after my car was picked up from my mechanic last week. My car was in the shop having the starter motor replaced, and I am sure you know, that on my car you have to break down most of the top of the engine to get to it - what a pain that project can be.