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07-12-2011, 06:53 AM #1Gu-powered Tech bindings
- Join Date
- Dec 2004
- Amherst, Mass.
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!
07-12-2011, 11:02 AM #2
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.
07-12-2011, 11:03 AM #3
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.
07-12-2011, 11:32 AM #4
It could take longer if you play your music real loud with the air conditioner on and cigarette lighter plugged in.
07-12-2011, 11:41 AM #5
Aren't most alternators set up to produce ample/maximum charging at not much more than a couple thousand rpm?
07-12-2011, 12:05 PM #6
Last edited by Kim Jong-un; 07-12-2011 at 12:16 PM.
07-12-2011, 12:08 PM #7
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.
07-13-2011, 03:18 AM #8Registered User
- Join Date
- Jun 2009
30 is overkill. 10-15 minutes, even for a completely drained battery, should be enough to get another start under normal conditions
07-13-2011, 09:15 AM #9Registered User
- Join Date
- Nov 2003
07-13-2011, 11:27 AM #10not awesome
- Join Date
- Mar 2011
- SW Jongistan
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.