Can the car charge the battery when idling after starting?
3 Answers
Can the car charge the battery when idling after starting? Yes, it can. Once the car engine starts, the alternator begins to work, transferring power to the generator through the engine belt, which then charges the battery. However, it is advisable not to let the car idle for extended periods, as prolonged idling can lead to carbon buildup. The car battery, also known as a storage battery, is a type of battery that converts chemical energy into electrical energy. Car batteries are categorized into conventional lead-acid batteries, dry-charged batteries, and maintenance-free batteries. Typically, when people refer to a car battery, they mean a lead-acid battery. The normal service life of a car battery ranges from 1 to 8 years, depending largely on the condition of the vehicle.
I've been running an auto repair shop for almost ten years, and I often get asked whether idling a car can charge the battery. While the alternator does work at idle and can replenish the battery a bit, the efficiency is extremely low. When the engine RPM is below 2000, the alternator can only output about 20% of its peak power. Once, a customer idled their car for two hours, and the battery voltage only rose by 0.3 volts. If the battery is severely depleted, charging this way is just a waste of fuel. The most effective method is to drive for 20-30 minutes, keeping the RPM above 2500—that’s when the charging speed really kicks in. By the way, idling continuously for over half an hour can also lead to carbon buildup, and fixing that can cost several hundred bucks—totally not worth it.
This reminds me of an embarrassing incident from my last road trip. I forgot to turn off the headlights while camping, and the next day the battery was completely dead. A friend suggested I start the car and rev the engine in neutral to charge it, but after half an hour it was still struggling to start. Later I learned that a typical family car's alternator only outputs around 500 watts at idle—just enough to maintain the stereo and AC. To fully recharge a depleted battery would take five or six hours. A mechanic taught me a practical trick: for emergency charging, keep the RPM steady at 1500, which improves charging efficiency by 30%. Though nowadays I always carry a jump starter—much more reliable than revving the engine in place.
