Normally, if the light for the battery is on, that means there is more power going out of the battery than is going in to recharge the the battery. At rest, the battery should measure about 12 volts DC. When you turn the ignition on and before you start the engine, that light should be illuminated. The light should go out when the engine starts up. You already know these things though, right? You said the Auto-zone test was ok but you don't say much about it other wise.
When your engine is running a little over idle speed, you should be able to read around 14 to 15.5 volts DC with a Volt/Ohm meter. Was this test done? Do this next informal test at night. Turn on your head lights. Now, race the engine. Do the lights get significantly brighter? Let the engine come back to idle. Do the lights significantly dim?
If you are running your engine a little faster than Idle and don't get 14 to 15.5 volts, your alternator is not functioning. If your lights go dim in the headlight test, your battery is not accepting (holding) the proper charge.
Do you know how old the battery is in your 13 year old car? You should expect 3 to 5 years depending on the quality of the battery. There is a date code on the top if you can figure it out.
If your alternator is putting out 14 to 15.5 volts and your car 'passed' the headlight test and your battery is no older than 4 years and the light is still on, I recommend you have your car looked at by your favorite mechanic. Your mechanic may discover other possible troubles which might include faulty voltage regulator, faulty diodes in your alternator or other troubles in your charging system. If you replace the battery, I suggest you get a premium battery if it is your intention to keep the car. The best battery is only a few bucks more than the standard battery, and it will serve you well.
I hope you found this information to be of some value. Thanks for your question @ FixYa.com. If this response pleases you, please remember to award the best evaluation you can.
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