How do standard pre2000 & newer vehicle alternators charge? Is it constant current the whole time, the amount according on engine revs?
If it tapers off the current how does it know?
On earlier vehicles, the charging system is pretty simple. There’s a voltage regulator that limits the voltage from going too high, but still the charge rate does depend quite a bit on the revs of the engine. A quick and easy way to test an alternator on one of these types of vehicles is just to rev the engine. With the engine off, a good battery will be somewhere around 12.6 volts. With the engine idling, it will go up a volt or so, so anywhere around 13.something is generally good. Rev the engine, and the voltage will go up to around 14 or 15 volts typically. If the voltage doesn’t go up as the engine revs increase, then the alternator is usually toast.
Many newer vehicles have smart charging systems. They monitor the terminal voltage on the battery, and adjust the alternator’s output (by varying the current going through the alternator’s field wiring) to charge the battery based on a much more complex charging algorithm. If the battery voltage is low, the charging system knows that the battery is low and needs a high level of charge. If the battery voltage is higher, then the charging system knows that the battery is fully charged and floats the battery with a lower charge instead.
They will also vary the charging current based on the ambient temperature, as the temperature of the battery will increase with higher charging current. If you allow the battery to get too warm, you end up with more gas loss in the electrolyte (basically, hydrogen and oxygen separating from the water) which reduces the life of the battery.
The state of the battery is easily determined by the battery voltage. Fully charged, it will be around 12.6 volts. If you let the charge drop below about 12.1 volts or so, the battery starts to get damaged with sulfation building up on the plates. If you get down below 11.5 volts or so the battery starts to chemically self-destruct.
It should be kept in mind that the voltage regulator only monitors and controls the voltage produced by the alternator - it does not monitor or adjust output current or battery charge. (At least not directly, anyway.)
Based on temperature the voltage regulator says to itself, “I want the alternator’s output voltage to be X.” As an example, the voltage regulator might say, “The temperature is 8 °C, so I want the alternator’s output voltage to be 13.9 V.” The regulator also sources current to the alternator’s field windings. If the regulator measures less than 13.9 V at the output of the alternator, it sources more current into the alternator’s field windings. If the regulator measures more than 13.9 V at the output of the alternator, it sources less current into the alternator’s field windings. In other words, it’s a closed-loop control system w/ negative feedback.
(This is an oversimplification of what goes on; the control algorithm is a bit more complex than that as noted by ecg. I’m just trying to keep it simple here.)