Charging system

Cars take about 12 - 15 volts to start up (depending on vehicle) I put a new battery in my car as well, does the same but its not draining it.

Sounds like it might be the wire from the alternator going back to the battery. Check that its bolted securely and if the wire is brittle, or broken.

Just though of another thing, could be a lose belt.

check the two red wires that go to the altenator and the battery them wires and inline fuses full on them if either stretches cut them out snd rehook up wires i had same problem.

check the serpentine belt tensioner

all the advice you got is good but there is a chance it is the built in voltage regulater, have the altanator load tested, most new are realy rebuilt and the voltage regulator is the weakest link also check your body ground just for kicks