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battery - alternator charging questions

Hello. I suspect my alternator is not charging my battery. I haven't used this car since April of this year so the battery had run down to 2 V (I measured it with a volt meter). The battery had been connected to the car.

I got it charged about a week ago at a car repair place. When I got it back, I measured the battery voltage with my voltmeter to be 12.75 V (with the engine not running).

I haven't used it all that much since then (few trips around town) but now the battery voltage is about 12.25 V (with the car not running). When I test the voltage with the engine running, it's slightly less than when it's not running. Shouldn't it be more since the alternator is sending it current?

What I want to do is get an ammeter and hook up the ammeter between the cable from the alternator to the positive post on the battery. Then get the car running and see if any current is flowing from the alternator to the battery. The question I have is what should be the current (roughly speaking) flowing from the alternator to the battery?

Are there any easy tests that will tell me if the alternator rather than the battery is faulty (or vice-versa) such as turning on the lights when the engine is running? If the lights dim when the engine is running, does that automatically mean the alternator is not charging? Could the problem be the battery's not holding the charge that the alternator is sending it?

Thank you for any help in answering any of the questions I posed! Take care,

Bill

Reply to
Bill
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be aware that such a deep discharge is very hard on a battery. It may not live long after this...

Then your alternator is not charging. If it were charging, you'd see approximately over 13.5 volts at the battery terminals with the engine running.

Reply to
Steve

No ammeter necessary. If your battery voltage is less with the car running than it is with the car not running, your alternator isn't charging your battery. That doesn't always mean the alternator is defective (you could have a wire loose or something somewhere) but it does mean your charging system isn't working and needs to be repaired if you plan to drive the car for any length of time.

Reply to
Walter

Just use a voltmeter. With the engine off, you should see about 12.5 volts, which is considered *normal*. 12.0 volts means the battery is Low and needs to be charged.

With the engine running, my alternator delivers 13.8 volts, so that is what I read at the battery terminals. 14.0 is *normal*, 13.5 volts measured at the battery terminals with the engine running is considered *low* alternator output.

So in my situation, I have 13.8 volts ( according to my Davis Car Chip ) showing up at the battery from the alternator, I am just within a whisker of not having enough voltage delivered. I consider what I'm getting to be just right. I'm 2/10ths of a volt short of *normal* which is good enough for me.

Do these very very little voltage values matter? YES !!!

If your engine idle is very very low, I can see a problem with a

*good* alternator not putting out the proper voltage.

Lg

Reply to
Lawrence Glickman

You can measure the voltage right at the back of the alternator to see if it is putting out and you could just have a bad connection between the battery and alternator.

One other thing you could try is to give the alternator a smack with a hammer to see if it maybe just has a stuck brush from sitting.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

Bill wrote:

Reply to
Mike Romain

Your Davis car chip is reading the voltage from the computer which has shown low on all testers I have tried. Use a good digital voltmeter to test the voltage at the battery. You should get around 14.5 volts with the headlights on and running about 2000 rpm...

Reply to
Woody

Thanks Steve, Walter, Lawrence Glickman, Mike Romain, and Woody for the help! I just took it to a local car repair place that I trust and they're going to look at the alternator tomorrow.

I really appreciate the help.

Bill

"Bill" wrote

Reply to
Bill

Thanks for the tip, Woody. I am surprised at Davis, or is this a fault/anomaly common to the car computer(s) ?

I have an Actron CP7611 Batt/Alt Tester ( tool freak here ) and -that- always shows *normal* with engine off ( 12.5 vdc ) and *normal* with the engine running at 750rpm idle ( 14.0 vdc ). And when I take the vehicle out on the highway, I get a pretty _flat_ 13.8 vdc coming back from the CPU to the Davis Car Chip. Sometimes it will hit 14 vdc, but rarely.

So what you're telling me is to use the DCC as sort of a monitor, but don't depend on it for precise readings; for precise readings, go right to the =source= with a multi-meter or somesuch.

I appreciate this *tip* and will keep it in mind. Little things like this used to drive me nuts ( no pun intended ) when I was doing field work.

Lg

Reply to
Lawrence Glickman

I hope things turn out OK for you Bill. I don't know how old your vehicle is, but sometimes it turns out to be corrosion inside the insulation where you cannot see it. Good Luck.

Lg

Reply to
Lawrence Glickman

Voltage measured across the battery when the engine is running should be about 14.5 volts. If the battery is not seriously discharged, it should go to this value within a minute or so of running. If it does not rise to at least 14 volts this says either alternator or regulator is not working properly.

Bill wrote:

Reply to
Don Stauffer

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