Why is the battery gauge lighting up?

I REPLACED THE ALTERNATER, BATTERY, AND STARTER LAST YEAR.
THE CAR DIED ON ME WHILE DRIVING YESTURDAY, SO I JUMPED THE BATTERY AND IT DIDNT WORKED, SO I TOOK OFF THE ALTERNATOR AND BATTERY SINCE I HAVE A WARRANTY AND TOOK IT TO AUTOZONE AND THEY TOLD ME THAT THE ALTERNATORS BEARING WERE SHOT WHICH IT WAS MAKING NOISE BUT HEY SAID IT PASSED ALONG WITH THE BATTERY. AS KIND AS THEY ARE THEY STI REPLACED BOTH ITEMS.
WHEN I INSTALLED BOTH ALTERNATOR AND BATTERY, I TURNED THE CAR ON FOR A TEST AND MINUTES LATER THE BATTERY GAUGE CAME BACK ON?
ANY IDEAS WHAT IT MAY BE? THANKS