Suzuki Repair: electrical, rear defroster, system amplifier

Question
89' suzuki swift. auto hatchback 94,000

ive had 2 batteries die on me. they were both old. so we assumed it was the alternator. we put a new one in and it still died on me. not knowing what to do we put a brand new battery in last night. now its working fine, but is it a matter of time before this new one dies too?  if it does what could be the problem. the old alternator was tested and they said it is working fine. im out of ideas. please help.

Answer
There's a couple things to consider.  If the alternator is ok, then how about the driving cycle?  Sometimes the cycle involves having all the stuff on, lights, heater, wiper, rear defroster, and a stop and go city cycle.  When the car is at idle, no real charging takes place, because the alternator's output is getting "sucked up" to run the accessories.  The only functional charging (to replace what was taken out to start the car) happens while going at speed.  As you can see, multiple night short trips during the winter can be very hard on a battery.  
The other thing to consider is whether there is an alarm system, a giant sound system amplifier, or other things that might use power.  Just a little overnight combined with a winter short trip cycle.....well, you get the idea.
Get the alternator's output tested again, and have the tech look for a draw with everything off, and the doors closed.  I have run into glove box and trunk lights responsible for just what you describe.  
If you don't get anywhere with this train of diagnosis, contact me again, there are some more things.
                                     Scotty