Battery Light on but Alternator is Charging: Common Causes

Battery Light on but Alternator is Charging. Learn the reasons why your battery warning light comes on when the alternator is charging normally. This video provides an in-depth look into the hidden causes of a false battery/charging system light, like bad battery health, loose connections, voltage regulator issues, parasistic draws, computer sensor problems, and more.

Discover how to diagnose this misleading light on your own through battery load testing, parasitic draw tests with a multimeter, computer scans for fault codes, and checking key systems like alternator output.

This conflicting situation can definitely cause some confusion and make you think the mechanics got it wrong when they say the charging system checks out fine. But in reality, there’s a few reasons why you can have a good alternator that’s putting out the proper voltage, yet still have issues that turn on the warning light indicating a bad alternator or electrical problem. I’m going to cover the most common causes, how you can test for these issues yourself, and what to check when trying to track down electrical gremlins like this.

First, the basics - that battery or charge indicator light is tied to sensors in the electrical system that monitor voltage and circuit integrity. When everything is working properly, your alternator puts out around 14 volts to charge the battery and run additional electronics. If that output voltage starts to drop or a circuit gets disrupted, the computer will turn on the indicator light to let you know.

Now in the case where the computer thinks there’s an issue, but voltage tests or a scan tool shows 14+ volts from the alternator, there’s likely something causing an intermittent fault... things like:

Bad Battery - If your battery is more than 3-4 years old, it may not be able to hold a full charge even if the alternator outputs enough power. Internal resistance develops in dying batteries which makes it difficult for them to reach a full state of charge. Since the sensors see the battery voltage fluctuating or not reaching peak levels, the computer interprets this as a fault.

Loose Battery Cables & Terminals - Over time vibration can cause the main positive and negative battery cables to loosen on the terminal ends. If that connection becomes interrupted while driving, the sensors will see wildly fluctuating voltage even if the alternator keeps pumping out power through the bad connection.

Disclaimer: This content is for educational purposes and should not replace professional advice. As an experienced mechanic, I aim to provide helpful insights, but I'm not liable for any damage or issues arising from the application of this information. Always consult a qualified mechanic for specific vehicle needs. Viewer discretion and responsibility are advised

Failing Voltage Regulator - The voltage regulator is the control unit inside the alternator that modulates power output to maintain around 14 volts. When going bad, voltage can swing higher or lower. If the sensors pick up periods of over or under-voltage, it will trigger the fault light again despite most of the time putting out the normal 14+ volts.

Short Circuits - Bare spots in wiring or broken insulation can create paths to ground. Combined with vibration while driving, this can make an otherwise working alternator setup seem intermittent. Any shorts will show up as excessive draw to the sensors.

Computer Issues - Keep in mind the PCM evaluates data from multiple sensors to determine if there’s a fault or not with the charging system voltage and circuit integrity. If any of the supporting sensors are malfunctioning due to wear or calibration issues, bad data can lead to false diagnostic errors that turn on warning lights.

Those cover some of the major players when it comes to a car battery warning light coming on despite acceptable alternator output. To give you a better idea of tracking down issues like this yourself, here is a quick 3 step process: