what can cause a car battery not to charge

[PaisleeMarie]

A car battery not charging is typically caused by a failing alternator, a severely degraded battery, or issues within the charging circuit like bad connections or a broken serpentine belt. The alternator is the most common culprit; it's the component responsible for generating electricity while the engine runs. If it fails, the battery only discharges. A battery with a dead cell or one that's simply too old (typically 3-5 years) can also refuse to accept a charge, even from a good alternator.

Let's break down the primary causes. First, the alternator contains internal components like diodes and a voltage regulator that can wear out. When this happens, it may not produce the required voltage (usually between 13.5 and 14.8 volts) to charge the battery. You might notice dimming headlights or a battery warning light on the dashboard.

Second, the battery itself may be the problem. Extreme temperatures and frequent deep discharges can cause internal damage, leading to sulfation—a buildup of sulfate crystals that prevents the battery from holding a charge. A battery load test at an auto parts store can confirm its health.

Third, inspect the charging system's physical components. A loose or glazed serpentine belt won't spin the alternator pulley effectively. Corroded or loose battery terminals and damaged cables create high resistance, blocking the flow of current. A parasitic drain, where a component like a trunk light stays on and slowly drains the battery overnight, can also mimic a charging problem.

Common Cause	Symptoms	Typical Fix
Faulty Alternator	Battery warning light, dimming lights, whining noise	Replace alternator
Old/Damaged Battery	Slow engine crank, swollen battery case, age over 4 years	Replace battery
Bad Serpentine Belt	Squealing noise, visible cracks or glazing on the belt	Replace belt
Corroded Terminals	White/blue crusty substance on battery posts	Clean or replace terminals
Parasitic Drain	Battery dead after sitting for a day or two	Diagnose and repair faulty circuit

The most reliable way to diagnose the issue is with a multimeter. With the engine running, a voltage reading at the battery terminals below 13.5 volts strongly suggests an alternator problem. A reading above 15 volts indicates a faulty voltage regulator, which can overcharge and damage the battery.

[ElianaLee]

Check the simple stuff first. Pop the hood and look at the belt that runs the alternator—is it loose or broken? Then, check the battery terminals. If they're covered in that white, crusty stuff, that's your problem. A quick clean with a wire brush might get you back on the road. If those look good, it's probably the alternator or the battery itself, and you'll need a pro to test them.

[DiKylie]

From an electrical standpoint, the issue is a break in the charging circuit. This isn't just about the alternator or battery. The problem could be a faulty ground connection between the engine and the chassis, which is often overlooked. High resistance in the main charging cable due to internal corrosion can also prevent proper current flow. A voltage drop test across each cable and connection is the correct method to pinpoint the exact location of the high resistance.

[McGarrett]

I've found that the cause often depends on the car's age and your driving habits. If you only take short trips, the alternator never has enough time to recharge the battery fully after starting, leading to a slow death. In older cars, the wiring harness to the alternator can become brittle and crack. For newer vehicles, a failing intelligent battery sensor (IBS) can miscalculate the charge state and tell the alternator not to charge, even when it should.

[SanRemington]

It's frustrating, but diagnosing it step-by-step saves money. Start by testing the battery voltage with the engine off; it should be around 12.6 volts. Then, start the engine and check again. If the voltage doesn't jump to between 13.5 and 14.8 volts, the alternator likely isn't working. If it does, but the battery dies overnight, you have a parasitic drain. This method gives you solid information before you buy any parts or call a mechanic.