With the ignition on but the engine off, a typical healthy car battery will last between 2 to 4 hours before it's too drained to start the engine. The exact time depends heavily on your car's age, the battery's condition, and what electrical features you're using. Leaving headlights or the stereo on significantly accelerates battery drain.
This scenario is often called "parasitic draw" or "key-off load," where the vehicle's electronics continue to consume power. A modern car's computer, infotainment screen, and other modules need a small amount of electricity even when the car is off. With the ignition in the "on" or "accessory" position, this draw increases dramatically as more systems activate.
The primary factor is the battery's reserve capacity (RC), which indicates how many minutes it can supply a 25-amp load before voltage drops below 10.5 volts. A higher RC means a longer-lasting battery. The table below shows estimated times for a battery in good condition, but real-world times can be shorter.
| Electrical Load Scenario | Estimated Battery Life (Engine Off, Ignition On) |
|---|
| Ignition On Only (No lights, radio, or A/C) | 3 - 4 hours |
| Headlights (Low Beam) + Radio | 1.5 - 2.5 hours |
| Headlights (High Beam) + Radio + Heated Seats | 45 - 90 minutes |
| Maximum Load (All accessories, A/C blower on high) | 30 - 60 minutes |
If you accidentally drain your battery, a jump-start is the immediate solution. To prevent this, make it a habit to do a quick visual check before exiting the car. Ensure all interior lights, headlights, and the infotainment screen are off. For situations where you need to sit with the ignition on for extended periods, it's much safer for the battery to let the engine idle for 10-15 minutes every hour to allow the alternator to recharge it.
