I'm reading a car electronics textbook that claims that an alternator can maintain correct system voltage (say, 14 volts) when under load, yet not be outputting enough amperage. I can't understand how this is possible. He claims that's why it's important to check the current output even if the voltage is good. Wouldn't it be necessarily putting out enough amps in order to maintain 14 volts?


He is correct.

I tested alternators and sometimes one would have a phase down, so while it would show 14v, it would not output sufficient current.

This is why the correct testing procedures should be followed so that the “real” issue can be found.

Even following the posts on here, one can see that many have had a charging issue, jumped in and replaced the alternator as the voltage was low.. Then found they still have a problem and further testing shows a different issue... Sadly they paid for an alternator that was not needed.

So, do the correct tests.

Edit, you mention a textbook but don’t give any detail. However have a look at this link and it shows you a regulator circuit, older now, newer ones are even more complicated:

enter link description here

  • The book is "Automotive Electrical and Engine Performance" 8th edition by James Halderman. This is the quote on page 298: "An alternator output test measures the current (amperes) of the alternator. A charging circuit may be able to produce correct charging circuit voltage, but not be able to produce adequate amperage." I am not disagreeing with him, I just don't quite understand how. The only way I can see how is if thr battery is fully charged (thus presenting a lower load) and the vehicle's electrical system is presenting a low load (all accessories off). – brian farrell Jul 18 at 22:53
  • @brianfarrell re-read my answer, perhaps you need to test real alternators as that book does not seem to give adequate detail. As I said you only need one phase down or a diode pack problem. Did you figure out how the regulator works? The use of 3 transistors is classic and the zener diode ? – Solar Mike Jul 18 at 22:59
  • But, when we test voltage, we always load the system by turning on brights, radio, A/C, wipers, etc. In such a scenario, how could a defective alternator (not outputting enough current) maintain 14 volts? The regulator is complicated, but it's job is simple: to energize the rotor windings when it senses the voltage fall below the set value (say, 14 volts). The result of this is simply to put out more current. The process by which it does this is complicated, but the function is simple: cause more current to come from the alternator. – brian farrell Jul 18 at 23:02
  • So you don’t know how the regulator works... When we studied this we had to produce that diagram from memory. – Solar Mike Jul 18 at 23:04
  • We have current supplied by the alternator, we have the load (which consists of the battery in parallel with the vehicle load) and we have a voltage (say, 14 volts). How does Ohms law not apply to this circuit? What is it about the regulator that allows Ohms law to be violated? The regulator may be complicated in how it works, but its function is simple to understand: it causes the alternator to output more current when voltage drops below a certain level and cause it to stop supplying current when voltage rises about the level. – brian farrell Jul 19 at 0:19

I'm not sure about the veracity of your textbook. There are some fundamental electrical laws that govern the current and voltage (and resistance).

Specifically Ohm's law: voltage= resistance * amperage.

If the voltage readings are accurate, in order to create a 14 volt potential across the battery terminals, there must be a certain amount of current proportional to the resistance of the circuit.

The alternator doesn't "allocate" current based on a whim. The current is a function of the difference in potential between battery voltage and alternator output voltage. The current is a result proportional to that voltage delta and the resistance of the charging circuit. The "hunger" of the battery ultimately determines the current, up to the capacity of the alternator. If the alternator cannot keep up with the charging demand, it would not be able to generate a high voltage potential, unless there is some sort of high resistance in the charging circuit - which would not be a fault of the alternator itself.

Voltage regulation, either internal or external to the alternator, determines the alternator output in current to the battery up to the limit of the alternator capacity. If the regulator is working properly, it will not yield a voltage that puts a current demand on the alternator it cannot handle.

  • The regulator cannot keep up when someone boost starts a car with the engine running - and that can blow the alternator even if the regulator is working properly... – Solar Mike Jul 17 at 8:04
  • This was my thinking. Perhaps on a fully charged battery and very low load (just ignition and PCM), a faulty alternator would temporarily maintain 14 volts. But eventually the voltage would drop as the battery begins to lose charge. However, I can't mathematically conceptualize the scenario where 14 volts is showing at the battery terminals while under significant load (brights, A/C, radio) and yet the alternator is not supplying enough current, especially if the battery isn't 100% charged. – brian farrell Jul 17 at 11:55
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    @SteveRacer How does the alternator or regulator get to see the battery "open circuit voltage"? The voltage the alternator or regulator sees is always due to the battery being in a circuit... – Solar Mike Jul 17 at 12:46
  • The regulator only knows if the voltage is dropping. If the battery is undercharged (lower open circuit voltage) it draws more current because it is providing less resistance. This causes the voltage to fall which causes the regulator to increase the current output of the alternator. My question is still: how can the alternator maintain voltage of 14 volts in a loaded circuit with a battery that may not be fully charged, and yet not be supplying enough current? – brian farrell Jul 17 at 16:31
  • @brianfarrell well if you read my answer I gave you a reason... – Solar Mike Jul 17 at 17:54

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