Can the car battery be charged without driving after starting the engine?
4 Answers
Yes, the car battery can be charged without driving after starting the engine, because when the engine is running, it drives the alternator to generate electricity. As long as the electrical connections are normal, the battery can be charged properly. However, using the car to start without driving for battery charging is relatively inefficient, and the charging time is also quite long. If a battery charger is used for charging, the time will be shorter. Most people do not purchase chargers for home use, but can charge the battery at nearby auto repair shops. If the battery is already depleted and the car cannot start, it is recommended to use another car for jump-start charging.
I recently encountered a similar situation where my car battery was drained after sitting idle for too long. After jump-starting, I let it idle for half an hour, but it still wouldn’t start after shutting off. A mechanic later explained that while the engine can indeed charge the battery via the alternator when running, the charging efficiency is extremely low at idle—especially with the AC or headlights on. The power generated might only cover basic electrical needs, leaving almost no net charge. I once tested it: the charging current at idle was only about one-third of that when driving. Since then, I’ve learned to drive for at least 15 minutes after jump-starting. Higher RPMs make the alternator more effective—20 minutes of driving charges more than an hour of idling. Plus, prolonged idling can cause carbon buildup, which isn’t worth it.
This is a practical issue, and I've helped friends with dead battery rescues several times. In theory, once the engine starts running, the alternator begins working and can charge the battery. However, the key factor is the operating condition: at idle speed, the alternator runs at low RPM, producing only about 20-30 amps, while driving can generate over 70 amps. Last week, a friend's car had a dead battery - after jump-starting, they idled for half an hour with the AC on, only to stall again because the charging couldn't keep up with the power consumption. Therefore, idling alone provides very poor charging, especially with aged batteries that have high internal resistance. If you must charge while stationary, turn off all electrical devices and let it run for at least an hour. But the most reliable solution is to go for a drive. Another benefit of driving immediately after a cold start is that it helps warm up the battery faster, as charging efficiency decreases in low temperatures.
As someone who frequently tinkers with older cars, my experience is that idling can charge the battery but it's inefficient. When the engine is idling, the alternator only generates enough electricity to maintain basic circuit operations, leaving very little surplus current to charge the battery. Once, my battery was down to 30%, and after idling for two hours, it only increased to 50%. However, after driving for half an hour, it reached 80%. The key factor is RPM—when the car is moving and the RPM exceeds 1500, the alternator can truly deliver high current. Additionally, if the battery is over five or six years old and has severe internal sulfation, the charging effect while idling is even worse. My solution is simple: after jump-starting a dead battery, drive directly to buy a new one, charging it along the way. This solves the problem and prevents potential hazards.
