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Old 01-20-2009, 02:55 PM   #1 (permalink)
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Alternator charging while at idle?

So my 2003 4runner would go through batteries once a year - dealer said that I dont drive enough (6 mile round trip commute to train station a day). I finally solved the problem by installing a solar panel that was a 2W trickle charger. Currently on same battery for 3 years. For the hell of it, I installed one on the FJ (sunsei is the company that makes it). But my question is, does the battery get charged while car is in idle? Dealer is telling me car has to be in motion - which makes very little sense. 5 speed vehicles, 45 mph is no more than idle in 5th gear.

Does the FJ have a better alternator than the 4runner (4th gen)? and does it charge battery at idle?
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Old 01-20-2009, 03:35 PM   #2 (permalink)
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Re: Alternator charging while at idle?

If the alternator is turning, then the battery is charging - but, if at idle, and if you have every electronic/power accessory on that you can think of within your FJ, then yes, the charging may not be able to meed the demands of the load. Note also that the FJ will only charge the battery if it needs it, which helps to protect the battery from being overcharged. (Most vehicles will have some sort of protection for the battery to prevent overcharging. Now if they would only do something to prevent the battery from completely discharging........ )
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Old 01-20-2009, 03:45 PM   #3 (permalink)
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Re: Alternator charging while at idle?

I dont know if the alt of the Fj is better than 4runner. But i know that the alt charge youd be getting will be better than at running (12-12.5V idle vs. 13-14V running) as compared to idling. On a side note, vehicles with incorrect pulleys, electrical issues, low idle speed (one or combo) can result to very little charging to battery when idling.

If your 6mile commute doesnt involve more than 15mins of driving (considering traffic lights, traffic in gen., etc...), it may require more time to charge.

If your commute is more than 15mins and the fact that your battery still did not last longer than 1yr tells me theres something wrong with your battery or alt. I personally would take them both out and bring to your nearest auto shop to get them both tested.
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Old 01-20-2009, 03:53 PM   #4 (permalink)
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Re: Alternator charging while at idle?

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CanadaFJ previously said: View Post
If the alternator is turning, then the battery is charging - but, if at idle, and if you have every electronic/power accessory on that you can think of within your FJ, then yes, the charging may not be able to meed the demands of the load.
X2.

Like you said, the 5AT likes to keep the engine at a low rpm in each gear to maximize mpg. Sometimes that is only 1,000 rpm. The alt. in the Fj is rated at a max of 140 amps (at it's rated rpm). I haven't seen a curve for the alternator so I don't know what it puts out at idle in gear (550 rpm) but it should keep your battery charged. That is provided your not crawling along at 550 rpm, foot on the brakes (lights), headlamps on, front defroster on high, rear-defroster on, wipers going, the boombox cranked up and only a 3 mile crawl.

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Old 01-20-2009, 05:03 PM   #5 (permalink)
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Re: Alternator charging while at idle?

Battery should be charged with alt running. In small towns, trips are usually 10min or less (3mi city driving) and batteries last 5+ years just like any other battery. Your alt or voltage regulator might be broken.
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Old 01-20-2009, 06:19 PM   #6 (permalink)
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Re: Alternator charging while at idle?

I drive about 2 miles to work and shut the truck off.

It's hard on the battery and it doesn't really let it charge after the high draw of starting on cold weather.

So about every two months I hook up the charger (automatic shut down to trickle type when charged) and let it run over night.

I've done this on all my cars over the years and I get batteries to last 5+ years, even in this climate.

Maybe you have a constant draw somewhere in the 4runner system. Or maybe you tend to run more accesories or leave stuff on longer than I do. I always make sure everything is turned off before I shut off the vehicle and I never start it with things turned on.
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Old 01-20-2009, 06:29 PM   #7 (permalink)
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Re: Alternator charging while at idle?

I've got to disagree with all three of you. Alternators need to be run up to about 1400 RPM after a start to begin charging ( excite the field). At an 800 RPM idle they aren't doing any charging (no potential difference). This would certainly ruin a battery a year, especially with freezing weather and modern cars that sap voltage 24/7 for alarms, led's, clocks, electronics, etc.. I like the solar charger idea, or make your alt pulley smaller so it runs at higher RPM's.
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Old 01-20-2009, 06:35 PM   #8 (permalink)
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Re: Alternator charging while at idle?

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I've got to disagree with all three of you. Alternators need to be run up to about 1400 RPM after a start to begin charging ( excite the field). At an 800 RPM idle they aren't doing any charging (no potential difference). This would certainly ruin a battery a year, especially with freezing weather and modern cars that sap voltage 24/7 for alarms, led's, clocks, electronics, etc.. I like the solar charger idea, or make your alt pulley smaller so it runs at higher RPM's.
You're correct about the excitation of field but... It really depends on the drive ratio from the crankshaft to the alternator. The crank pulley is much larger in diameter than the alternator pulley (think 10 speed bicycle). At 800 rpm crankshaft speed, the alternator probably is at or pretty close to 1400 rpm.
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Old 01-20-2009, 06:37 PM   #9 (permalink)
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Re: Alternator charging while at idle?

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jhaydeno previously said: View Post
I've got to disagree with all three of you. Alternators need to be run up to about 1400 RPM after a start to begin charging ( excite the field). At an 800 RPM idle they aren't doing any charging (no potential difference). This would certainly ruin a battery a year, especially with freezing weather and modern cars that sap voltage 24/7 for alarms, led's, clocks, electronics, etc.. I like the solar charger idea, or make your alt pulley smaller so it runs at higher RPM's.
Boy, then we must own super duper amazing batteries that can run our cars on no charge for months or years at a time.

And those old folks that let their cars idle forever with the heat on high and then drive right home and shut it off must also have amazing reserve power batteries.

And cops must kill a battery per day.

I put on only about 400 city miles a month. Total. Rarely highway. Yet don't go through batteries left and right in any car I've owned.

Like the guy above me explains, the alternator is likely spinning faster then the crank speed and is charging.

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Old 01-20-2009, 09:12 PM   #10 (permalink)
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Re: Alternator charging while at idle?

Your altenator is what keeps your vehicle running, not the battery. The battery is their to hold a charge for when you start, leave your lights on, hazzards etc (anything where the alt is not spinning).
My old K5 Blaer ran with out a battery for a whole summer, I would just use a portable booster pack to get the thing started and I would drive on from there.

Some of the older vehicles needed the rpms brought up so the regulator would engage. I doubt that is even an issue today.
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