Starting/elctrical problem

Ok, soo, when I try to start it the starter doesn't have enough juice to turn the engine over. Our battery is only a week old, because this is the second time this has done what it is doing. Our last abttery did this same thing after a couple weeks, so we got another and now the new one won't start it. The car has started fine for a week now with the new battery, and now it won't even turn over. Could we possibly have a voltage drain anywhere or something like that? With the POS cable disconected, running a voltage tester from the POS post on the battery, to the POS cable, it reads 0 amps and 0.12 V. Doesn anyone know what to make of this, or any quick tests to check for a voltage drain? Anyone have any ideas? Our starter cable is connected firmly and hasn't melted on the headers.

Thanks in advance!

Bump...Help anyone?

We read about 12.63 V after a full charge, started it up and while idling it read about 12.4 V, then cut it off and the battery only read 12.5 V Any body out there that can help? We had the alternator tested and it was putting out about 13.5 volts, had the battery fully charged, and replaced the voltage regulator. What next?