How Long Should You Idle to Charge a Dead Car Battery?
2 Answers
To charge a dead car battery at idle, it typically takes 10 to 15 hours. Severely depleted batteries require slow charging, as this directly affects the vehicle's driving performance. During winter when starting difficulties occur, it is recommended not to crank the engine for more than 3 seconds at a time, as continuous starter operation can lead to excessive battery discharge and damage. Methods to determine if a car battery is fully charged include: 1. The electrolyte boils and bubbles appear simultaneously, indicating charging completion; 2. Measure individual cell voltage - when the reading stabilizes at 2.7 volts without fluctuation for 3 seconds, charging is complete; 3. Measure the relative density of the electrolyte - if the density remains stable for 3 hours, charging is complete.
I've driven many cars and personally experienced dead batteries. The idling charging time isn't fixed—generally 20 to 40 minutes can provide enough power for an emergency start, but it depends on the battery size and vehicle model. For example, my old sedan could restart after idling for half an hour, while a larger SUV might need up to 40 minutes. The reason is that the alternator's output is low, providing only a slow charge to the battery. However, this isn't a good long-term solution, as prolonged idling increases engine carbon buildup and wastes fuel. If the battery is old, it might not start even after charging. So, I recommend keeping jumper cables handy or going straight to a repair shop for a professional charger. Regularly check your battery's health to avoid this issue.
