The simple answer is that it depends heavily on your car's battery and the subwoofer's power demands, but for a typical setup, you risk draining your battery to the point where the car won't start in under an hour. Continuously playing a subwoofer with the engine off is a fast way to get a dead battery. The key factors are the amplifier's power draw and your battery's reserve capacity.
The main spec to look for is your car battery's Reserve Capacity (RC), measured in minutes. It indicates how long the battery can supply a minimal electrical load (usually 25 amps) before voltage drops too low to start the engine. A subwoofer amp draws significantly more than this. For example, a 500-watt RMS amplifier might draw around 35-40 amps from the battery at full power.
Here’s a rough estimate for a healthy car battery with a 100-minute RC rating:
| Subwoofer Amplifier Power (RMS) | Estimated Max Safe Runtime (Engine Off) | Key Considerations |
|---|
| 200-300 Watts | 45 - 70 minutes | Lower power draw; safer for shorter listening sessions but still risky. |
| 500-600 Watts | 20 - 35 minutes | High drain. Battery voltage will drop noticeably within this timeframe. |
| 1000+ Watts | 10 - 20 minutes | Extremely high drain. Strong likelihood of a dead battery if pushed to the limit. |
These times are approximate and assume the amplifier is playing at high volume with bass-heavy music. To extend your listening time, keep the volume low, as amplifier power draw increases exponentially with volume. The age and health of your battery are also critical; an old battery will fail much faster. The only safe way to play a powerful system for extended periods with the engine off is to use a dedicated, secondary deep-cycle battery isolated from your starting battery.
