1998 Chevy Monte Carlo BATTERY KEEPS LOOSING CHARGE

1998 Chevy Monte Carlo 6 cyl

I BOUGHT THIS DURALAST BATTERY ABOUT A YEAR AGO, BUT TODAY WHEN I RAN OUT OF GAS, AFTER I WENT & GOT GAS N PUT IT IN, BUT STILL DIDNT START AND THEN BATTERY DRAINED SO I GOT A JUMP, MADE IT HOME AND THE NEXT TIME I TRY 2 START IT, NO START. TOOK THE BATTERY TO AUTO ZONE WHERE THEY CHARGED IT, DROVE A COUPLE PLACES, THEN AT LAST PLACE I WENT, NO START AGAIN (BATTERY DEAD) GOT A JUMP AND MADE IT HOME. I KEEP STARTING IT EVERY HALF HOUR OR SO JUST TO CHECK ON IT, & ITS OKAY BUT DOES ANYONE KNOW WHAT MIGHT BE THE PROBLEM? BY THE WAY, I HAVE A STEREO SYSTEM W/AMP & SPEAKERS HOOKED UP TOO.