Yes, an alternator can absolutely overcharge a car battery. This typically happens when the alternator's internal voltage regulator fails. The regulator's job is to maintain a consistent charging voltage, usually between 13.5 and 14.8 volts. If it malfunctions, the alternator can send a continuous, uncontrolled high voltage—sometimes 15 volts or more—into the battery, leading to severe damage.
An overcharging alternator is a serious issue. The excess voltage causes the battery electrolyte (a mix of sulfuric acid and water) to overheat and boil off. You might notice a strong rotten egg smell (sulfur) coming from the battery. This rapid loss of fluid damages the internal lead plates, significantly shortening the battery's lifespan. In extreme cases, the battery can swell, leak, or even explode from the buildup of hydrogen gas.
Beyond the battery, the overvoltage can wreak havoc on your entire vehicle's electrical system. It can fry sensitive electronics like the Engine Control Unit (ECU), blow out headlights and other bulbs prematurely, and damage expensive infotainment systems. The cost of replacing these components far exceeds the cost of fixing the alternator.
Diagnosing this is straightforward with a multimeter. With the engine running, check the voltage at the battery terminals. A reading consistently above 15 volts indicates a problem. If you see this, along with symptoms like dimming or flickering lights, it's crucial to have your charging system inspected by a professional mechanic immediately to prevent further damage.
| Symptom of Overcharging | Consequence | Typical Voltage Reading (Engine Running) |
|---|
| Boiling Battery Electrolyte | Loss of fluid, internal plate damage | 15.5V+ |
| Swollen or Bulging Battery Case | Risk of crack, leak, or explosion | 15.0V+ |
| Rotten Egg Smell (Sulfur) | Electrolyte breakdown and gassing | 14.8V+ |
| Premature Headlight Bulb Failure | Overvoltage burning out filaments | 15.2V+ |
| Flickering/Dimming Lights | Erratic voltage output | Varies widely |
