Here are the most likely causes:
* Alternator Failure: This is the most common culprit. The alternator is responsible for generating electricity to charge the battery while the engine is running. A faulty alternator may not be producing enough power or any power at all.
* Alternator Belt Problems: A broken, loose, or slipping serpentine belt (the belt that drives the alternator) will prevent the alternator from spinning and charging the battery.
* Bad Battery Cable Connections: Corrosion or loose connections on the battery terminals or the wires leading to the alternator can disrupt the charging circuit. Clean and tighten these connections.
* Faulty Voltage Regulator (part of the alternator): The voltage regulator controls the alternator's output. A bad regulator can cause overcharging or undercharging. Often this is the cause in cases where the alternator tests "good" on its own.
* Faulty Battery: While you mentioned a new battery, it's possible it's faulty. Get it tested at an auto parts store.
* Wiring Problems: A short circuit or broken wire in the charging system's wiring harness could also be the cause. This is more difficult to diagnose.
What you should do:
1. Check the Battery Cables: Start by inspecting the battery cables for corrosion and tightness. Clean them thoroughly with a wire brush and baking soda and water, if necessary, and make sure the connections are secure.
2. Check the Serpentine Belt: Examine the serpentine belt for cracks, fraying, or looseness. If it looks damaged or is loose, you'll need a new belt.
3. Get the Alternator and Battery Tested: This is crucial. Take your vehicle to an auto parts store (like AutoZone, Advance Auto Parts, etc.) They can usually test your alternator and battery for free. This will quickly pinpoint if the alternator is the problem.
4. If the problem persists: If the battery and alternator test fine, you may have a more complex electrical issue that requires a professional mechanic to diagnose. This could involve faulty wiring or a problem with the voltage regulator within the alternator.
Driving with the battery light on is risky. While you might be able to drive for a short while, the battery will eventually drain, and you'll lose power. It's best to address this issue as soon as possible.
