2003 Chevy Venture battery

Electrical problem
2003 Chevy Venture 6 cyl Automatic 71000 miles

my battery light comes on and goes off. My car starts and runs good. The light comes on some days and not others. Some days it comes on and goes off in like 5 minutes sometimes it stays on. My battery is 3 months old and when I had it tested it was 300 amps on time and the other time it cranks at 800 amps which was more than the original cranking amps o 700 something