truck dies, after battery light comes on all lights buzzers and everything works

but I know that if alternator is bad nothing works, everything works but won't turn over. starter relay turns starter over and once battery is fully charged it will run for a day or two????????? could it have anything to do with ignition? that's what mechanic said it needed. what could be electrical draining battery

2 test need to be performed .. 1 - Charging system test .. 2 - Parasitic load test .. That should lead to the culprit!

thank you

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