1998 Nissan Sentra Battery/Brake light on and car running f

The battery/brake light has been blinking on in this car for months. I keep having it checked and it tests fine. I changed the battery cause it was old but no change. Lights keep coming on and now more frequently. Car runs fine. Starts like new. Could this be a bad sensor or could everyone be wrong when they run the diagnostics on the battery/alternator? Don't want to spend the money on having a new altenator if I don't need one and everyone tells me its fine.