Battery not being charged

[subman60]

The battery on my 82 240D is not being charged by the alternator. Took the alternator off to be rebuilt. Shop called me back it's ok. Owner of the shop told me to put it back on and check voltage at the leads going into the alternator. He said two leads should have battery voltage all the time the other when the key is switched on. I have battery voltage at the two but the other one registers just over 2 volts with key in on position. Started the car to check voltage going to battery started out at 14.2 volts then went right down to 12.7 then 12.4. Battery is very low now will hardly turn over the car. If it's not the alternator and the battery has checked good at two places what else do I need to look for?