more info. do you ever run the stereo without the car on? is there anything in the car that might be staying on when you turn the car off? it is possible that your amps are too large for your alternator to keep up with, but unless they're pretty ridiculous amps, i guess that would be a little unlikely. alternator might be on its way out. get a voltmeter at wal mart and check the voltage of your charging system- with everything you normally have on (including headlights) and the motor at about 3k rpm, should not drop below 13, and if it's SIGNIFICANTLY below that, you might want to invest in an alt upgrade.
I have been getting a new Sears battery about every two years since I had an aftermarket alarm installed in 1987 (Viper--no problems in 16 years). I tried an Interstate battery when I replaced the alternator two years ago. It lasted one year, I went back to Sears.
Don't forget the cables. If one of the battery or alternator cables is corroded (or worse, broken or intermittantly broken), your battery might not be properly recharging all of the time. In my Jetta, I didn't realize that one of the alternator cable terminals was bad until after replacing the alternator (d'oh!); the end attaching to the starter was loose within the terminal, although the terminal was tight to the starter and the terminal *looked* OK until you wiggled the wire while looking at it.
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