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Does a voltage regulator keep/ensure that the voltage at the battery is always approx. 14.5 or does it make the voltage from the alt. bypass the battery when it is fully charged so it does not overcharge it? I have a 68 cutlass and I connected the bat term on the alt. to the battery and am not sure if this is bypassing the regulator and as a result, overcharging the battery. When the car is running, the volt meter shows approx. 14.5 at the battery. Thank you.

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  • Is it showing 14.5v while running?
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Aug 26, 2015 at 23:36
  • around there more or less.
    – user11936
    Aug 26, 2015 at 23:39

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The car has no idea if the battery is charged or not. Normal charging voltage for a car is about 13.9v to 14.4v, that's with the car running. Your voltage is not out of the ordinary.

Standard car charging theory goes like this. A car battery that is fully charged should be 12.6v nominal. In reality this can vary from 12.6v to about 13.2v. To charge a battery the charging voltage needs to be a minimum of 1v above the battery. A voltage regulators job is to keep the running voltage of the car about the same, 13.9v to 14.4v, regardless of electrical load or engine rpm. When the load is high the regulator puts more current out to keep the voltage the same. When the load is light the voltage regulator puts less current out to keep the voltage the same.

To tie everything together, when a battery is low on charge it looks like a big load to the regulator and the regulator will put out lots of current to satisfy the battery. As the battery reaches full charge the load of the battery decreases, but never to zero. A lead acid battery is difficult to overcharge because of it's internal resistance.

Finally the regulator actually does not put out current like i've been saying. The regulator senses the voltage of the car's electrical system and controls the current that is in the rotor of the alternator. This in turn produces 3 phase AC current in the alternator (hence the name) which is rectified and sent out the big wire on the back of the alternator charging your car.

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  • I would disagree with your assessment that a lead acid battery is hard to overcharge ... if a charging system is running 15v+, it will boil a battery within a short period of time. Other than that, you are spot on. +1
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Aug 27, 2015 at 0:32
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    I'll rephrase, if the car is charging properly the battery is pretty difficult to overcharge.
    – vini_i
    Aug 27, 2015 at 0:55
  • Good deal ... :-)
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Aug 27, 2015 at 1:27

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