When a brake light comes on, this means the emergency break is still engaged, or the brake fluid level is too low. When a battery light comes on, this is caused by a malfunctioning battery or a loss of power to the car. The alternator charges the battery, so a problem with the former is the most common cause of the battery indicator light turning on.
The first thing to check when the battery light comes on is the battery terminals. If the terminals have any corrosive build-up, mix equal parts water and baking soda and paint the solution on the terminals to erode the debris. If this does not fix the problem, have the battery tested by a mechanic.
If the battery is functioning properly, the most likely problem is the alternator. This keeps the battery charged, regulates the voltage and transports electricity while the engine is running. Without a properly functioning alternator, the battery cannot maintain its charge.
If the engine dies when the battery light comes on, there is a way to determine whether the problem is the battery or the alternator. If the car makes a clicking noise before turning over, the problem is most likely the battery. If there is no sound, a faulty alternator is most likely causing the battery light to come on.