1998 Ford Explorer battery light stays on

About 3 days ago my battery idiot light came on & the voltage gage showed low voltage, slightly above the low mark. The truck ran OK until today. The battery completely died. I assumed it was the alternator & replaced it with a new unit I also replaced the battery due to age alone, just to be on the safe side. It started fine, but the gage still show low voltage & light is still on. I drove the truck all day & now the battery is almost completely dead. Please help.