A car battery that won't charge is typically caused by a faulty alternator, a dead battery that can no longer hold a charge, or excessive parasitic drain from accessories. Other common culprits include corroded or loose battery terminals, a broken serpentine belt that drives the alternator, or a malfunction in the vehicle's charging system wiring.
Let's break down the most frequent causes.
A Failing Alternator
The alternator is the component that generates electricity while the engine is running to power your car's electrical systems and recharge the battery. If it fails, the battery only discharges. Signs of a bad alternator include dimming headlights, a battery warning light on the dashboard, and strange whining or grinding noises from the engine bay.
The Battery Itself
Car batteries have a finite lifespan, typically 3-5 years. An old battery can lose its ability to accept and hold a charge, a condition called sulfation where sulfate crystals build up on the lead plates inside. Even if the alternator is working, a battery at the end of its life will not recharge properly.
Excessive Parasitic Drain
This refers to an electrical component that continues to draw power from the battery after the ignition is turned off. While a small drain (50 milliamps or less) is normal for systems like the clock and alarm, a faulty glove box light, trunk light, or aftermarket stereo can create a large drain that depletes the battery overnight, making it seem like it won't charge.
Other Mechanical and Electrical Issues
Simple problems like corroded battery terminals (a white, blue, or green crusty substance) create high resistance, preventing the alternator's current from reaching the battery. A loose or snapped serpentine belt means the alternator isn't spinning. Finally, damaged wiring or a bad voltage regulator can disrupt the charging circuit.
| Common Cause | Typical Symptoms | Estimated Repair Cost (Parts & Labor) |
|---|
| Faulty Alternator | Battery warning light, dimming lights, whining noise | $500 - $1,000 |
| Dead Battery | Engine cranks slowly or not at all, old age | $150 - $300 |
| Broken Serpentine Belt | Squealing noise, loss of power steering, overheating | $100 - $250 |
| Corroded Terminals | Difficulty starting, visible corrosion on terminals | $20 - $50 (cleaning) |
| Excessive Parasitic Drain | Battery dies repeatedly overnight | $100 - $200 (diagnosis) |
The most reliable way to diagnose the issue is with a multimeter. A healthy battery should show about 12.6 volts when the car is off. With the engine running, the voltage across the battery terminals should read between 13.7 and 14.7 volts, confirming the alternator is charging.
