When you had the alternator tested, did they test it for voltage and amperage? How many amps did it produce?
Like the others said, get yourself a multi-meter and measure the voltage right at the battery, then measure the voltage right at the alternator.
If you have the engine running at fast idle and minimum load (stereo, pumps and blower off) then you should get at least 13.5 volts at the alternator and essentialy the same voltage at the battery.
If there is a significant difference in voltage between the alternator and the battery then there is a problem in the wiring.
If the alternator is not putting out at least 13.5 volts with an engine RPM of 1000 or so then the alternator is weak.