i have a 94 ej1, d15b7/d16z6 mini me, and my alternator is charging 12.55 volts, it will run with a battery cable off. checked the black/yellow stripe wire on the plug an got a good reading. 12.55 at the alt post as well. its a new alternator as the last one was doing it to.

 

The battery voltage should be up around 14V when the engine is running. If not and you are certain the alternator is okay, have your battery checked.

 

you could have a drain somewhere.. i also had a new alternator and battery and something kept draining the battery.. ended up being a short in a wire