The charging system, or battery, warning light, is one of the most critical warning lights on any vehicle. Whenever this light goes on, it means that the vehicle is running solely on battery power and will only be able to drive a very limited distance before it runs out of electrical power and dies.
On most vehicles the light is configured in one of three ways:
- The light is red and displays "BATT" when the key is on and the engine is off (like when you are only playing the radio)
- The light is either red or amber and displays "CHG" or "ALT"
- There is an icon or image that looks like a simple car battery; it can be red or amber
Until the 1990s, the charging systems in vehicles were controlled by some form of a voltage regulator in or near the actual alternator. The light was connected to the battery and when the key was turned, the light would ground through the alternator. When the engine was started and the alternator produced voltage, the voltage would travel throughout the vehicle's electrical system, including back up the wire that lead to the light. When the charging system light had two sources of voltage reaching it, it no longer had any ground and would be turned off. When voltage at either end of the light became too imbalanced—say from the battery being low or high or the alternator putting out too little or too much voltage—it would cause the battery light to illuminate.
As vehicles became more sophisticated and the number of computer systems kept growing, control of the charging system was moved from the alternator to the powertrain computer. This allows for a much finer attenuation in charging system operating range. This is necessary because the newer, networked on-board computer systems needed "cleaner" and "noiseless" operating voltage so that the accuracy of the sensor data and learned adaptation stayed within the specified tolerances. In these newer vehicles, the charging system light is illuminated by a command from the powertrain computer when it sees an irrational difference between the desired system voltage and the actual system voltage.
The Bulb Check: Charging System Warning Light
When the ignition key is cycled from the Off, Run, Start positions, the battery/charging system warning light runs a self test. When you start the engine, the warning light should stay on for 1 to 2 seconds and then go off and stay off for the entire time the engine is running. This means that the voltage for the charging system is meeting the requirements set either by the voltage regulator or powertrain computer, depending on the design and model year of the vehicle.
If the charging system warning light does not go off or comes on—even intermittently—while the engine is running, there is a problem with the operation of the charging system. Also, if the charging system nlght never comes on during the self test procedure described above, there is problem with the charging system and/or warning light.
What to Do: Charging System Warning Light
If the charging system warning light comes on while you are driving at night, you have very little time left before the engine dies. This is because the vehicle is operating in a "total loss" condition, meaning that the battery is not charging so all of the available voltage will drain out of the system. It is best to turn off all the unnecessary electrical devices such as the radio, air conditioning, and heater. The vehicle may only drive for a few miles, so try to find a safe place from where it can be towed to a qualified repair shop.
If the charging system light comes on during the daylight hours, there may be a bit more time because you aren't using your head lights, which are a major drain on the system. Again, turn off all the unnecessary electrical devices and look for a suitable place to stop the vehicle to have it towed to a qualified repair shop.
If during the self test procedure, the charging system light never comes on, then it would be best to have the vehicle towed to a qualified repair shop for a complete charging system inspection and diagnosis. This is because most of the charging systems need to "see" the warning light in the charging system or they will not properly allow charging system voltage into the system. Even though the warning light is out, the charging system could be in a total loss operational condition.