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!
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.
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.
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!
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.