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  1. #1
    Join Date
    Dec 2004
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    Amherst, Mass.
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    3,132

    Car Q: How Long a Drive for Alternator to Recharge Battery?

    So my understanding is:

    1. With the engine off, a car battery is slowly losing its charge, both b/c any battery will do so, and b/c various car electronics have a constant draw on the battery.
    2. Starting the engine also drains a battery.
    3. Once the engine is running, then the alternator is constantly recharging the battery.

    The question then is, how long does the engine have to be running in order for #3 above to more then offset #2? (If more specifics are necessary, then assume 2007 Subaru Legacy with city driving.)

    Thanks in advance for any feedback, including any corrections to my [mis?]understandings!

  2. #2
    Join Date
    Sep 2004
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    Fac 51
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    Very general rule of thumb: figure on 30 minutes of higher-RPM (highway, constant RPM) driving to top up a car battery that is a bit low.

    *a bit* = not precise whatsoever, but let's say reading at 12.1 to 12.3V at the battery terminals, ignition off.

    If your battery is really drained, say, reading under 12.0V, then use a charger to bring it back up to full. Or a long road trip.

    If you drive the car 30 minutes every week or two at a minimum, and the electrical system in the car is functioning normally, that should keep the battery charged.
    Quote Originally Posted by powder11 View Post
    if you have to resort to taking advice from the nitwits on this forum, then you're doomed.

  3. #3
    Join Date
    Nov 2007
    Location
    Seattle
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    Many more specifics are necessary because it depends on numerous factors, including, inter alia, how long the car sits between starts, the capacity of the battery, the age of the battery, the load on the battery when the car is sitting, the rpms when driving and the load of devices on the charging system while the engine is running. For most cars, the non-operational load is very small, i.e., running a few microprocessors which could be powered with a few dime-size watch cells for months. I would guess that, assuming a good battery, a sound charging system and no voltage leaks, the slight discharge resulting from a few days of sitting still would be refreshed in a few minutes. Maybe more modern cars with fancy security systems, etc., might draw more power in non-op mode. I dunno about that.

  4. #4
    Join Date
    Sep 2010
    Location
    Pyongyang
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    648
    It could take longer if you play your music real loud with the air conditioner on and cigarette lighter plugged in.

  5. #5
    Join Date
    Dec 2003
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    Seattle
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    Aren't most alternators set up to produce ample/maximum charging at not much more than a couple thousand rpm?
    Quote Originally Posted by Downbound Train View Post
    And there will come a day when our ancestors look back...........

  6. #6
    Join Date
    Sep 2010
    Location
    Pyongyang
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    648
    Quote Originally Posted by PNWbrit View Post
    Aren't most alternators set up to produce ample/maximum charging at not much more than a couple thousand rpm?
    Yeah, usually >1000 rpm. There is a voltage regulator/rectifier in the alt that maintains a ~14V charge on the battery. If the battery is discharged the battery charge rate is high initally and then slows to a 1-2 amps once the battery is fully charged. But a rated 100 amp alt can't run at 100 amp for very long, so if you've got a big draw on it (accessory lights, big ass stereo etc.) some folks have to upgrade. As others have said, all else equal you should be charged back up in a half-hour or so, but if the battery is not holding the charge, its time to bite the bullet on a new one. As Big Steve said, on high end SUV's loaded with electronic gizmos, the resting discharge rate of the battery can be quite high, but I'm not sure a 2007 Suby falls into a 2011 Range Rover category on battery draw. Those things won't start if you leave them sit for 3 weeks.
    Last edited by Kim Jong-un; 07-12-2011 at 12:16 PM.

  7. #7
    Join Date
    Dec 2003
    Location
    Seattle
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    17,036
    ^ yep

    If the OP's battery is going flat in less than many months of standing still then he needs a new battery or there is a problem with the Alternator/voltage regulator/current leak.
    Quote Originally Posted by Downbound Train View Post
    And there will come a day when our ancestors look back...........

  8. #8
    Join Date
    Jun 2009
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    9,413
    30 is overkill. 10-15 minutes, even for a completely drained battery, should be enough to get another start under normal conditions

  9. #9
    Join Date
    Nov 2003
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    Quote Originally Posted by Big Steve View Post
    Maybe more modern cars with fancy security systems, etc., might draw more power in non-op mode. I dunno about that.
    Yep. I had an old stripped down CJ that I used to leave in Aspen. I never had to jump it. I replaced it with a new 4runner and I had to put in a trickle charger for the off seasons when it might not get driven for a couple of months. CJ had a lot less wire in it!

  10. #10
    Join Date
    Mar 2011
    Location
    SW Jongistan
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    345
    I have a 98 subaru (computer, electronic engine this and that, but no gizmos/gps/etc) with a battery from 2005 or so and it has started fine after sitting for a month. I hope it does so again at the end of this month. In general I'd think that if a car w/o gizmos has trouble starting after sitting for a couple of weeks, the battery or charging system is on the way out.

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