My 2005 Ford Focus zx3 Hatchback battery light goes on and off. Could it be a sensor?

I went to Toyota because my battery light kept coming on and off plus my car would sometimes die at a red light. I went and they replaced my battery. On my way home the battery light came on and off again. I took it back and they said everything tested out fine. I just took it home but a few days ago my car shut off at a red light. I took it to Autozone thinking its my alternator but they said it was working fine but they said my tensioner belt needed replaced. I took it to Toyota again and they said they didn t know what was wrong because everything was checking out fine. They put the belt on today but the battery light came on AGAIN. I don t know what to do about it. Any suggestions?

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