Yes, you've probably connected something incorrectly, or the alternator has a problem. Which alternator are you running? If it's a GM 10-SI, there only needs to be two connections, a 10 gauge main charge lead going from the 1/4" terminal stud on the back of the alternator to the battery cable connection at the starter, and a jumper wire from the big alternator terminal to the #2 blade terminal on the perimeter of the alternator housing. The #1 terminal does not need anything connected to it unless you're running an idiot light on the dash. The battery terminal voltage with the engine running should be between 13 volts and 14.7 volts, depending on the state of charge of the battery, and how many acessories you're running.
The voltage will recover slightly after the battery sits awhile with no load on it. It will drop again as soon as something is turned on.
Jerry
