The advice to “let it run for a while” is weaker than it sounds. What the alternator needs is a drive, not an idle.
Drive continuously rather than idling, and give it a proper run rather than a short hop — a commonly given figure is twenty to thirty minutes. There is no universal number, because how much charge goes back depends on how deeply the battery was discharged and on the vehicle's charging system.
An alternator produces more output as engine speed rises. At idle it produces relatively little, and much of that goes to running the vehicle's own electrical load — ignition, lights, climate control, electronics. What is left over for recharging the battery is modest.
Driving raises engine speed and keeps it there, which is why a drive replaces charge meaningfully faster than sitting in a parking space with the engine running.
Twenty to thirty minutes is the figure most often given, and it is a reasonable rule of thumb. It is not a specification. How much charge actually returns depends on:
Each restart draws a large amount of charge. A thirty-minute journey with four stops puts back far less than thirty uninterrupted minutes.
An alternator is designed to maintain a healthy battery during normal driving, not to rescue one that has been run completely flat. A deeply discharged battery usually needs a proper charger to recover fully, and in some cases will not recover at all.
This is why a car can start on a jump, drive for half an hour, and still be flat the following morning. The drive did what it could; the battery could not hold what it was given.
If the charging system is the fault, driving does not restore anything — the engine is running on stored charge and will stop when it runs out. The signs of that are covered in battery or alternator.
And if the battery has failed internally, no amount of driving will change that. In Arizona, where heat shortens battery life, that case is common enough to be worth ruling out: see how long car batteries last in Arizona.
No. An alternator produces much less output at idle, and part of that goes to running the vehicle's own systems. Driving restores charge considerably faster.
You can, but the battery may not have enough charge to start the car again. It is better to drive first and park somewhere convenient for a second attempt if needed.
Often not. An alternator maintains charge rather than performing a deep recovery, so a battery that was run completely flat may need a proper charger.
Either the battery no longer holds charge, or something is drawing power while the car is parked. In both cases the next step is testing rather than another jump.
If you are in the Valley and need someone to come to the vehicle rather than finding another car for a jump, RoadsideHelpPro provides mobile battery jump start across its Phoenix-area service area for $59 per jump start, 24 hours a day, 7 days a week. See jump start service in Phoenix or the full list of cities.
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A jump start solves a one-off discharge. Replacement is for a battery that no longer holds charge. How to tell which situation you are in.
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