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How Long Does It Take for the Alternator to Charge the Battery?

For a battery that’s in perfect condition but has been drawn down for some reason, it will take a bit of time to rejuvenate its capacities. That can be from several engine starts with only short drives in between or using accessories without the engine on, for example. How quickly the alternator takes to charge the battery tends to vary widely based on a few factors. 

  • If the battery isn’t 100% healthy, it can take longer to charge. 
  • If your car’s accessories, heater fan, radio, or other power-consuming systems are running, there’s less power available to charge the battery. 
  • A battery recharges slower when it’s near full capacity compared to drawn down. 
  • The alternator’s amperage dictates its maximum charging rate. 

As an example, an 80-amp alternator might be recharging a battery with a capacity of 75 amp-hours (stated as Ah) from 80%. However, the alternator requires some power for the car’s other electrical functions, so only 50 amps are available for charging the battery. Theoretically, it would take 90 minutes to fill the battery from empty. Topping up 20% would require one-fifth of that time, or about 17 minutes. 

Again, that assumes all parts and systems are operating at full capacity and doesn’t consider the slower charging rate for the near-capacity battery. The good news?  Your car will start easily if it’s at 80% charge, and the alternator will have no trouble maintaining it at that level if it’s operating properly.

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Be sure to consult your owner’s manual, a repair guide, an AutoZoner at a store near you, or a licensed, professional mechanic for vehicle-specific repair information. Refer to the service manual for specific diagnostic, repair and tool information for your particular vehicle. Always chock your wheels prior to lifting a vehicle. Always disconnect the negative battery cable before servicing an electrical application on the vehicle to protect its electrical circuits in the event that a wire is accidentally pierced or grounded. Use caution when working with automotive batteries. Sulfuric acid is caustic and can burn clothing and skin or cause blindness. Always wear gloves and safety glasses and other personal protection equipment, and work in a well-ventilated area. Should electrolyte get on your body or clothing, neutralize it immediately with a solution of baking soda and water. Do not wear ties or loose clothing when working on your vehicle.

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