A car battery can technically discharge to 0 volts, but it will suffer permanent damage long before that. The critical threshold is around 10.5 volts. When a battery's voltage drops to this level while the engine is off, it is considered deeply discharged. At this point, starting the car is highly unlikely, and the battery's internal components may be permanently damaged, significantly shortening its lifespan.
A healthy, fully charged car battery should measure approximately 12.6 to 12.8 volts when the engine is off. As it discharges, the voltage drops. Once it falls below 12.0 volts, it's already about 50% discharged, and leaving it in this state can lead to sulfation—a process where sulfate crystals form on the battery plates, reducing its ability to hold a charge. When voltage reaches 11.9 volts, the battery is almost completely drained and is at high risk of damage.
The following table outlines key voltage readings and their implications:
| Battery Voltage (Engine Off) | State of Charge | Implications |
|---|
| 12.6V - 12.8V | 100% | Fully charged, optimal condition. |
| 12.4V | 75% | Acceptable, but should be recharged soon. |
| 12.2V | 50% | Half-charged; prolonged state risks sulfation. |
| 12.0V | 25% | Very low; avoid using electrical accessories. |
| 11.9V | 0% | Effectively dead; high probability of permanent damage. |
| 10.5V & Below | 0% | Deeply discharged; severe internal damage likely. |
If you find your battery below 12 volts, the best course of action is to recharge it slowly with a smart battery charger. A quick jump-start might get the car running, but the alternator is not designed to fully recharge a deeply discharged battery; it primarily maintains the charge. A slow, proper recharge can help reverse minor sulfation. However, if the battery has been below 10.5 volts for an extended period, it's probably time for a replacement.
