The battery light is not a battery light at all, it's a Charging System Light. The light means, ALWAYS, that there is an issue with the charging system, and the issue will be the alternator nearly 100% of the time. The times it isn't the alternator, then it is in the connection of the alternator to the rest of the car. There is no light to tell you that the battery itself is unhappy with its life, or the job that is is assigned to carry out.