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Old 09-22-2009, 11:04 AM   #1 (permalink)
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Default Battery? Alt?

Hey guys,
When I start my car I notice that my voltage gauge is reading at right around 14 volts. Perfect reading. Once I start driving I notice that the voltage still reads at the proper voltage until I idle. When I idle the gauge reads at @11 volts. When I give it gas it goes back to 12-14 volts. Any ideas? I checked the connection at the battery and it seemed ok....Could this be a battery problem or an alternator problem? Thanks in advance!
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Old 09-22-2009, 04:51 PM   #2 (permalink)
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Default Re: Battery? Alt?

First off, do you have a large stereo system are anything that would cause a large power drain at idle? It's possible that the alternator just isn't keeping up at idle due to the lower spinning speed, this happens in a lot of cars even when you're just running the headlights.

You should also try and check the voltage with a multimeter at the battery when it's idling to confirm that the voltage really is that low as the stock gauges aren't always accurate.
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Old 09-23-2009, 07:14 AM   #3 (permalink)
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Default Re: Battery? Alt?

Almost all cars do this, Like andyvr4 said when the engine is at idle its not spinning as fast so the alternator doesnt produce as much voltage. The voltage will always vary up and down, its when it starts to get below 9 volts that you are most likely encountering a problem.
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Old 09-23-2009, 08:32 AM   #4 (permalink)
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Default Re: Battery? Alt?

Actually it could be a battery problem.

Your car runs on the output of the alternator, not the battery. The battery is only there to start the car and sustain the car during idle. It takes a certain RPM before the alternator puts out enough voltage to run the car... any less RPM and the alternator is out of the loop. There is a 'voltage regulator' that cuts out the alternator output when its output voltage is too low, and limits the top voltage that the alternator supplies to the battery at high RPM.

At low RPM you are seeing the battery voltage, and 11 volts is somewhat low.
Is your battery older? Have you noticed or have a different 'feeling' about the way the car cranks when starting? Lead acid batteries as used in cars put out a little over 2 volts per cell, and there are 6 cells, so 11 volts could indicate a bad cell.

Depending on the age of your battery, I would consider changing it. If the car starts 'starting funny' then I would definitely consider changing it.
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Old 09-23-2009, 10:02 AM   #5 (permalink)
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Default Re: Battery? Alt?

Quote:
Originally Posted by Awake View Post
At low RPM you are seeing the battery voltage, and 11 volts is somewhat low.
Is your battery older? Have you noticed or have a different 'feeling' about the way the car cranks when starting? Lead acid batteries as used in cars put out a little over 2 volts per cell, and there are 6 cells, so 11 volts could indicate a bad cell.
If the battery really only had 11 volts there's absolutely no way that it would start the car as that's pretty much completely dead.

Quote:
Originally Posted by Awake View Post
Depending on the age of your battery, I would consider changing it. If the car starts 'starting funny' then I would definitely consider changing it.
I strongly advise not replacing the battery unless it's actually giving problems as I've seen so many instances where people replace an old battery that's working fine just to get a defective new battery that dies on them with in the next year.
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Old 09-23-2009, 12:00 PM   #6 (permalink)
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Default Re: Battery? Alt?

On older cars like the 3S, the condition and stability of the car battery is not all that important, since like I said, all it does is start the car and sustain the car at idle. The alternator runs all the electrical systems while driving. Maybe if you are actually pumping 1000 Watts on your audio system will the battery add some juice.

Starting with cars around 2003, the condition of the battery has become much more important, since everything is electronic except for the starter and a couple of miscellaneous motors.

A bad battery is the cause of most electronic related issues in newer cars, and causes stuff like stalling, bad idle, alarm lights coming on, and many other hard to track down problems. Heck, in newer cars there isn't even a cable from the accelerator to the engine, there's just a couple of redundant electronic position sensors under the pedal.

Putting in a new battery every 2-3 years is almost a 'regular maintenance' item in newer cars because batteries do go bad. The technology of lead acid batteries goes back to 1860, and except for having a gel instead of distilled water, they haven't changed much since. They use lead plates that fairly quickly get coated with oxides (sulfates)

In any case, on a 3S an old battery won't affect much of anything, but it can leave you stranded if it doesn't have the juice to get the car started.

As to the claim of people putting in new batteries just to have them fail quickly... don't shop for your batteries and tires at Wally-World.
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Old 09-23-2009, 01:34 PM   #7 (permalink)
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Default Re: Battery? Alt?

That's the thing. The car starts every time not a problem. When I first start the car and leave it at idle it's at 12-13/14 volts. It's perfect. It doesn't start acting up until I get the car warmed up a little. That's when it starts to lose a little power at idle. When I come to a stop all lights dim...it's weird....
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Old 09-23-2009, 03:30 PM   #8 (permalink)
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Default Re: Battery? Alt?

Quote:
Originally Posted by MediaMike19063 View Post
When I first start the car and leave it at idle it's at 12-13/14 volts. It's perfect. It doesn't start acting up until I get the car warmed up a little. That's when it starts to lose a little power at idle. When I come to a stop all lights dim...it's weird....
When you first start the car, are you sure it is at idle, and not at 'fast idle'?

When an engine is cold, the idle is normally set faster so it won't stall. It is the automatic version of pulling the 'choke'. So you should be running at about 1200-1500 RPM when the engine is just starting to warm up. Later when the engine is warm the idle should drop to about 900 RPM. 1200-1500 RPM is enough to be using alternator power, 900 isn't.

This is all assuming that your car is 'stock'.
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Old 09-23-2009, 03:43 PM   #9 (permalink)
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Default Re: Battery? Alt?

ur alt is bad, specifically, the voltage regulator. they're a wear and tear item. when u rev the engine and the voltage shoots back up, that's a classic indication the alternator needs replacing. the other option is that your belt is old and when it warms up, it stretches too far. most likely, it's the alternator tho.

12.9 volts is the absolute minimum to have a decent working alternator at hot idle. even then, that would bug me. factory spec is 14.0~14.5v cold start, 12.9~13.6 hot idle, 13.7~14.2v cruising.
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Old 09-23-2009, 04:13 PM   #10 (permalink)
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Default Re: Battery? Alt?

The thing is, I don't think that the meter in the car shows alternator voltage, it shows battery voltage. In other words, it is on the battery side of the regulator, not the alternator side. And at warm idle (lower RPM), the alternator is cut out of the loop by the regulator, and the car is running on battery, hence the lower voltage. So when he sees the meter come back to normal as he accelerates, he is seeing the alternator applying voltage to the battery, which normally charges the battery.

The easiest test is to disconnect the negative terminal on the battery and measure the voltage. If it is 11 volts, there's the problem. But it could still read good (12.4 V or so) even if it is bad, because it needs a load to see if it can maintain voltage when it is supplying current. He needs a 'real' car battery tester for that test.

In any case, it's worth checking now, to avoid getting stranded for whatever reason.

Like they say, there is NOTHING dumber in the whole universe than running out of gas when you have a gas gauge that works. Same thing applies to other gauge displays.

It will be interesting to see what he finds out.
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