How long does it take to charge a car battery when it's dead?
3 Answers
It takes 20 hours to charge a dead car battery. Precautions for charging a car battery are: 1. When the battery voltage is 10% lower than the rated voltage, it will lead to battery depletion. 2. Avoid water entering the battery during charging. 3. Connect directly to the battery for charging, avoiding removal of the battery to prevent data loss. Methods for maintaining a car battery include: 1. Start the car regularly to charge the battery. 2. Add distilled water or special replenishing fluid when the electrolyte is low. 3. Frequently check if the vent holes on the battery cover are clear during daily driving. 4. Inspect the battery's positive and negative terminals for oxidation. 5. Check all parts of the circuit for aging or short circuits. 6. Turn off all electrical devices when leaving the car.
Last time my car battery was completely dead, I took out the portable charger and connected it. It charged from 6 PM until 9 AM the next morning, a full 15 hours to fully recharge. This happened in winter, with an aging battery, and deep discharge requiring a longer trickle charge. Typical passenger car batteries usually have a capacity between 50 to 70 ampere-hours. If using a standard slow charge at 6 amps, theoretically it should take around 10 hours, but real-world conditions like low temperatures or battery aging can extend it to over 12 hours. I don’t recommend frequently using fast chargers with high current, even though they advertise reaching 80% in two to three hours, as fast charging generates heat that can damage the battery plates and shorten lifespan. I suggest setting a slow charge mode overnight like I did, while also identifying the cause of the discharge—such as leaving lights on or alternator failure—and checking all electrical devices are turned off before parking next time to avoid recurrence.
Having driven for decades, I deeply understand that charging time depends on specific conditions. The standard rule is dividing battery capacity by charging current to get hours—for example, a common 60Ah battery takes about 10 hours to fully charge at 6 amps. But in reality, after deep discharge or in cold weather, increased internal resistance requires an extra 1-2 hours of trickle charging, potentially totaling 12 hours. I patiently use an old-fashioned slow charger, avoiding fast-charge modes; high current saves time but risks overheating and damaging internals, especially with older batteries. After jump-starting, promptly slow-charge to stabilize power, safely charging overnight in the garage without disruption. Crucially, diagnose why the battery died—like unclosed door lights or circuit faults—and perform weekly voltage checks as preventive maintenance to avoid surprises.
