The most common things that drain a car battery are parasitic drains—small electrical loads that continue after the engine is off—and simply having an old, weak battery. Key culprits include interior lights (like trunk or glove box lights) left on, faulty charging systems, extreme temperatures, and frequent short trips that don't allow the battery to fully recharge. Modern car electronics, such as infotainment systems and keyless entry modules, also draw a small but constant amount of power.
A normal parasitic drain is typically between 20 to 50 milliamps (mA). You can measure this with a multimeter. Anything significantly higher indicates a problem that needs diagnosing. Here's a comparison of common drains:
| Electrical Component | Typical Drain (milliamps) | Notes |
|---|
| Clock / ECU Memory | 1 - 5 mA | Minimal, necessary for vehicle operation. |
| Keyless Entry System | 5 - 20 mA | Constantly "listening" for the key fob signal. |
| Aftermarket Alarm | 20 - 50+ mA | Poorly installed systems are a frequent culprit. |
| Infotainment System | 10 - 30 mA | May draw more if updating in background. |
| Glove Box Light | 100 - 400 mA | Significant drain if stuck on. |
| Dome Light | 250 - 500 mA | Will drain a battery in a day or two if left on. |
| Faulty Alternator Diode | 500 - 1000+ mA | Drains battery even while driving; requires immediate repair. |
Age and usage are major factors. A battery's capacity to hold a charge diminishes over time, especially if it's subjected to deep discharges or vibration. If your battery is over four years old, it's more susceptible to being drained by normal parasitic loads that a new battery would handle easily. Short trips are another silent killer; a 5-minute drive uses more power to start the car than it puts back, leading to a gradual discharge.
To prevent a dead battery, make sure all lights and accessories are off before exiting the vehicle. If you don't drive often, consider using a battery maintainer (trickle charger) to keep it at optimal charge. If you experience repeated dead batteries without an obvious cause like a dome light, the issue is likely a faulty component or an aging battery that needs professional testing.
