The Voltage regulator sends a field current to one of the alternators brushes, the other is grounded. It sends enough current to that one brush in order to achieve and maintain a certain voltage. More loads, of which a depleted battery can be a huge load, more field current is sent. If more field current is sent than required to maintain a certain voltage, then voltage increases, and in general 15 volts is as high as a lead acid battery should be taken.
Lower engine rpm, more field current is sent, I've seen my field current go as high as 8.2 amps when maxing out the alternator at low rpm and making about 60 amps and 8.2 amps of field current over 2200 engine rpm can make 120 amps, on my alternator.
Lower engine rpm, means lower alternator fan speed
lower alternator fan speed means hotter alternator.
Low vehicle speed means less underhood airflow, which means hotter alternator
Heat fries things.
The voltage regulator is inside of many alternators, but many vehicles also have it i instead, nside the engine computer.
When a battery is depleted below 80% state of charge, and still healthy, it might settle to about 15 amps of load when the voltage regulator is seeking 13.6v, but if the voltage regulator is seeking 14.7v, then this same battery at same state of charge will accept double to triple that amperage, causing the alternator to work much harder and get significantly hotter, in order to seek and perhaps maintain that higher voltage.
Voltage regulation of alternators varies widely among different vehicles and it changes as the engine and alternator heat up, and with time. So many people take a reading right after starting their engine and think that is the voltage always sought and held by the voltage regulator. In most cases it is NOT.
Many people also believe their dashboard voltage gauge. This is unwise, but if the manufacturers had it reveal the true varying voltage, the reals the owner would freak out and demand waranty service, as most understand none of how charging systems work and what they are capable of, or more accurately, what they are not capable of..
As a battery reaches 80% charged it starts requiring less and less amperage to be brought to and held at a certain voltage. The closer to 100% it gets the slower it charges. Lower voltage increases this charging time and 13.6v might never get an older battery to full charge.
Ideally the battery always wants to be fully 100% charged, and cool. It takes a long time to actually achieve this. 80% to 100% cant be accomplished in less than 3.5 hours, assuming mid 14's is held at the battery terminals that whole time, and that the battery is still healthy. if voltage is lower, sub 14.1v, or the battery less than healthy, achieving true full charge from 80% can take significantly, exponentially longer, and in some cases will not occur without even more electrical pressure (voltage) held for even longer. a healthy battery discharged to 30% will still easily start a cold engine above 40F. most people act like: it starts therefore it is fully charged, and still going strong, despite being completely ignorant of the battery state of charge or state of health.
Verifying a true full charge is something almost no one ever does, but achieving it can yield huge battery longevity gains. Those gains are more pronounced the deeper the battery is regularly discharged, the more is asked of it.
Starting a modern fuel injected gas engine uses a very tiny percentage of the battery's capacity. It is the loads, their size and duration, on the battery which remain on after the engine shuts off which determines how low the average state of charge is. The vehicle's charging system is NOT designed to quickly and fully charge the battery, it is designed to never overcharge it. The same can be said of most 'Smart ' Chargers too.
I know this is not popular information and contradicts what most people firmly believe, but if one has a voltmeter and ammeter and a hydrometer and a charging source capable of holding mid 14 volts, one will see that the battery their charging source said is full, is not. Most stop in the 92 to 95% charged range on a battery in the middle of its expected life, higher earlier in its life and significantly lower than that nearer the end of its life and while the engine will still easily start, getting a battery to 98% charged is only half as good as a true 100% charge, even if it is destined to be back at 98% the next day. Its like a balloon you have to keep stretching to near its bursting point regularly, or it will highly unlikely to be able to approach that maximum size again, and develop small leaks when it does get near that.
But ignorance is bliss and people will always believe the sticker on the battery has more effect on its longevity, as opposed to how it was cared for by the vehicle's charging system, and the owner's usage of it. Worst thing once can do is discharge a LEad acid battery, and let it sit in a warm environment. The lower its average state of charge the less lifespan it will have and no magically marketed bell and whistle charging source is going to return it to its brand new fully charged capacity, the best it can hope to do is achieve charging it to its maximum remaining potential capacity. which can be very far apart.
Often State of charge is uncontrollable, and batteries are just rented anyway and not a budget destroying expense for most anyway. But imagine if the shelves were empty when it becomes time to replace. Not too hard to imagine these days......., compared to a few weeks ago.
However the maintenance minded Bitoger who stresses their oil choice, is wise to do what they can to keep their battery at as high a state of charge as they and their charging sources can achieve, often.
I cycle the snot out of Lead acid batteries and max out my alternator often. I can spin a dial on my dashboard, and change the voltage and watch the ammeter respond as well as the temperature sensors on my battery and alternator change as well. The mystery is now gone, but I recall when it still was one, and its obvious to see to whom it is still a mystery.
In regards to a healthier battery keeping an alternator healthier, it is unlikely.
A new healthy battery, if well depleted, will ask much more of an alternator, than will an older battery will if equally depleted.
Heat is cumulatively damaging so a new depleted battery will load the alternator harder than an older one of equal or even more depleted.
At the end of a battery's life, the amperage required to maintain the same voltage when near full charge, does increase, but not by such a huge amount that it will shorten the life of the alternator.
While I have no experience charging a battery with a depleted cell, and do not know how much amperage that shorted cell will eat up, My previous AGM battery when new, amps would taper to 0.0x at almost any voltage when it was truly fully charged, but a week before it became too weak to barely start my warm engine in mild ambi8ent temps when fully charged, it would require about 6.2 amps in order to maintain 14.7 volts. Now if I left this fully charged end of life battery charging at 14.7v, it would start heating and go as high as 11 amps. I've witnessed the same pattern on starting flooded batteries and flooded marine batteries too, at the end of their life they require more amperage to be held at the same voltage when near or at full charge, but they take much less amperage to be brought to that amperage and as such take much longer to reach those high states of charge.
So in this case, will 11 extra amps of output put a weak alternator over the edge, I doubt it. Will its shorter lifespan be measurable because the near end of life battery was requiring slightly more amperage to be held at 13.6 or 14.7v, I highly doubt it.
If someone has data on how much amperage a shorted cell in a particular size battery will chew up at 13.6 or 14.7v, I'd love to see it.
I'm gonna guess it is likely around 10 to 15 amps and perhaps as high as 20 in some bigger batteries. I suspect if it were higher than this the battery casing on the bottom of the cell would melt and distort.
Your headlights on low beam are about 15 amps
your Hvac blower motor is about 10 to 18 amps.
Will using your headlights in addition to your HVAC blower motor on High speed kill your alternator?
Of course not, but the alternator which never has to power those loads, will last longer than one which does not have to all other factors being equal. Measurably longer? Perhaps in a scientific study will all variables removed.
I can and do see loads increase my alternator temperature, but only if it is getting over 180f regularly will I start worrying about it
That 180F can occur fairly quickly idling parked and maxing out the alternator, but at 65mph I cant get it over 140f maxed out.
220f is the threshold where the boaters, charging huge banks of depleted batteries, say that the damage occurs quickest, and they employ highly sohisticated voltaeg regulators with alternator temperature sensors, battery temp sensors and the ability to program a 'belt saver' and lower the voltage and thus reduce the load on the alternator when it gets too hot from feeding a hungry large depleted battery bank over thick copper cabling.
Alternators in most vehicles have a cakewalk duty compared to a boat. the hardest they will likely work is after the battery is unintentionally depleted and then jumpstarted.
I have a Newish group 31 northstar AGM with 1150CCA and 103 amp hours of capacity, and the other week depleted it to about 30% state of charge. 134 amps of plug in charging source/power supply were not enough to instantly bring it to 14.7volts. I tripped a 15 amp breaker trying and then it was 94 amps for several more minutes before battery voltaeg atained 14.7 at which point amperage required to maintain that voltaeg started tapering.
This beast of a battery can certainly max out my alternator, and once it starts hitting 165 or 170 f i will lower voltage in order to keep the alternator from getting much hotter, but at highway speeds, I don't stress it as that underhood airflow at that speed keeps it from exceeding much over 140f.
Of course other vehicles and alternators will be different, but idling parked to recharge well depleted still healthy battery is without a doubt much harder on an alternator, than highways speeds. I've found sub 30mph low rpm around town driving to be not much better than idle on my vehicle in terms of alternator heating.
