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Battery Won't Charge...?

3 June 2002
Kanata, Ontario, Canada
I can start the car fine with a boost, and I can charge up the battery with a charger fine, but it appears the alternator is not charging the batter and it will die eventually.

So, can the problem be anything other than the alternator??
Did you check the alternator belt tension? I would do that first. After that you will need to get a digital volt meter (they are easiest to read). A fully charged battery should read about 12.5V or even a little higher.

If it does read 12.5 or so, start the car, then check the voltage again across the battery. Should be about 1.5 volts higher to charge the battery, if not and the alternator belt is tight, then either the regulator or diodes(inside the alternator housing) are toast, or the alternator itself has a problem.

All of the above, besides the belt, indicate the alternator will need to be removed and bench tested, and most likely repaired.

If it does not read 12V or higher the battery needs to be load tested, to see if it is still good, unless of course it is new:).

I would recommend you also check the terminals for tightness. especially the ground lead, since they tend to loosen, although if the car starts fine, this is less likely then the other things above.

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The surest way to check for a loose alternator belt is to read the voltmeter at idle.
If its too low but then goes higher when you increase engine speed to around 3000 rpm, it means the belt's slipping and the engine has to turn faster to compensate for this slip to turn fast enough to charge.
If the belt's tight, it turns fast enough at idle to charge the batt.

Static voltage for a lead acid cell is 2.1 volts. There are 6 of them in a 12 volt battery in series so static voltage for 12 v battery is 12.6 v. Anything higher is charging the battery.

Most regulators are tuned to a 14.0 volt setpoint for best efficiency.

/used to work in an alternator design lab.
Turned out I had a bad battery! I had not considered this a possibility since the battery had been replaced recently. I guess next time I'll check to be sure. Fortunately the batter was under a 3-year no charge replacement warranty :)

Thanks for the advice though!