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Old 01-31-2012, 08:21 PM   #1
duragezic
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Default Does running engine charge car battery more than I thought?

When I first started driving, I believed a car battery got charged fairly well by a running vehicle. So if you had a dead battery, after a jump start, running the vehicle for say 45 minutes would charge it up fine. But over the past few years I have been reading that in reality it charges the battery very little. And that to really charge the battery to its previous state requires a dedicated charger.

Last week I was out of state and left a dome light on in my car for 5.5 days. The battery seemed completely discharged by then because even the power door locks didn't work.

A friend gave me a jump start and I drove the car about 35 minutes home. Ever since then I've taken standard ~10-15 mile trips and the car has started up just fine, which is pretty surprising given the 30° F avg temps.

It seems to me that the battery is being charged more than I understood it to be. Or maybe I'm pushing it and going to be stuck with an inadequately charged battery any day now? If I'm not back in the thread in 30 minutes then you guys can be like "duh!".
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Old 01-31-2012, 08:24 PM   #2
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I'm not sure where you got your information but the alternator does a much better job of charging your battery than a plug in the wall charger. I have NEVER had to charge a dead battery with a charger if I got it jump started and drove around for a while and I have been driving for over 30 years.
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Old 01-31-2012, 08:29 PM   #3
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It'll charge it up, but it puts a big load on that alternator for a long time while doing so, and can fry it. If you can charge it with a charger, you should IMO. The system was sized to maintain a battery, not charge a dead one.
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Old 01-31-2012, 08:30 PM   #4
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Quote:
Originally Posted by Ronstang View Post
I have been driving for over 30 years.
When do you stop to go to the restroom? Eat? Sleep? I know you must take a shower.....
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Old 01-31-2012, 08:31 PM   #5
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I'm assuming you are talking about running the car in the daytime.
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Old 01-31-2012, 08:36 PM   #6
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if you want answer from car guys, i can move this to the garage for you
otherwise
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Old 01-31-2012, 08:43 PM   #7
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Just driving the car is enough to recharge your battery.

My Outback wouldn't start after getting home from a weekend having the interior lights on. I put it on a battery charger just long enough to get it started. It's been fine since.
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Old 01-31-2012, 08:49 PM   #8
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Quote:
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It'll charge it up, but it puts a big load on that alternator for a long time while doing so, and can fry it. If you can charge it with a charger, you should IMO. The system was sized to maintain a battery, not charge a dead one.
If recharging a healthy battery fries your alternator, your alternator was already dying.
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Old 01-31-2012, 08:50 PM   #9
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It is supposed to maintain what the battery can charge too....so if you have a fresh battery, and you killed it, you alternator will work harder to completely charge it to the "fresh" state.
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Old 01-31-2012, 09:49 PM   #10
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Idling the car charges the battery slower. Driving the car charges the battery much quicker. The output of the alternator is lower at idle.

Driving the car for 20 minutes or more with a healthy battery and a good alternator, will recharge the battery just fine.

The alternator was meant to maintain the battery's charge, not repeatedly bring a dead battery back to full.

It is not a good idea to repeatedly kill your battery, and then use your alternator to recharge it.

Not good for the battery or the alternator.

It's okay to do it if you have to, but do not make it a habit.
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Old 01-31-2012, 09:54 PM   #11
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Well mainly being lead acid they really don't like the deep discharge.

As for how much the alternator charges, it, maybe it just charged it enough for the next start
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Old 01-31-2012, 10:51 PM   #12
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Quote:
Originally Posted by FoBoT View Post
if you want answer from car guys, i can move this to the garage for you
otherwise
what do these lines and numbers mean?
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Old 01-31-2012, 10:52 PM   #13
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Quote:
Originally Posted by Ronstang View Post
I'm not sure where you got your information but the alternator does a much better job of charging your battery than a plug in the wall charger. I have NEVER had to charge a dead battery with a charger if I got it jump started and drove around for a while and I have been driving for over 30 years.
Normally I'm right with you, Ron, but I can't agree with everything in your post here.

Yes, an alternator will absolutely charge a mild-to-moderately discharged battery, but it won't do it "better" than a bench charger. A bench charger (well, a good one) can adjust the charge rate specifically for the battery's condition and can even reverse minor sulfation. A car's charging system will simply dump as much as it can into the battery, there's no real intelligence to it. Running the alternator will work, but a bench charger is going to be "better."

As for the OP, one thing that's likely really helping him is that many (most?) modern cars have some form of discharge prevention circuitry where they won't let a battery be drained to zero. Rather, when the battery drops below a certain state of charge, the car simply cuts parasitic power. The battery may still drop far enough to where it won't start the engine, but you won't see the same sort of complete discharge with most modern cars that you used to see on older vehicles. Chances are that the OP's battery was nowhere near completely discharged but rather simply below the threshold voltage for his car's parasitic systems.

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Old 01-31-2012, 11:18 PM   #14
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The battery is a storage for the alternator. NOT a charger but the STORAGE.
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