A sulfated/aged battery will not place any significant more load on the alternator than a new healthy battery.
----
If a large group 27 100Ah AGED unhealthy battery is 85% charged, and it is being held at 14 volts, it will require about 6 to 7 amps to be held at 14 volts
If a large group 27 100AH NEW Healthy battery is 85% charged, and it is being held at 14 volts, it will require 7 to 9 amps to remain held at 14 volts, and will charge faster towards 100% state of charge.
So in this example the aged battery presents less stress to the alternator than the new healthy 85% charged battery.
----
If a G27 aged battery is 99% charged it will require about 1.2 amps to be held at 14.4v and 0.4 amps to be held at 13.6v
If a G27 new battery is 99% charged it will require about 0.9 amps to be held at 14.4v. and 0.3 amps to be held at 13.6v
So in this example the aged battery is presenting slightly more load to the alternator. a tenth of an amp more. 1.20 watts more.
Most android cell phones can consume 5 watts to charge. 1 amp at 5v = 5 watts
------
If a g27 aged sulfated battery is 50% charged, and the vehicle's voltage regulator is seeking 14.4v, the aged battery will require about 30 to 40 amps to instantly be brought to 14.4v, and amperage required to maintain 14.4v will start tapering nearly instantly.
If a G27 NEW healthy battery is 50% charged, and the vehicle's voltage regulator is seeking 14.4v, 30 to 40 amps will take about 15- 20 minutes before voltage at battery terminals rises to 14.4v.
Assuming the alternator is still cool and spinning fast enough that it can deliver its 130 amp rating, the 50% charged new healthy g27 battery will require about 70 to 90 amps to instantly be brought upto 14.4v and it would take about 10 more minutes before amperage would taper to 30 to 40 amps.
So in this case of a depleted battery, the new healthy battery can present a significantly larger load to an alternator, than an old sulfated one near the end of life.
-------
These are memories of actual observations with digital ammeters and many group 27 batteries over a decade, cycled deeply regularly, observed closely. Actual data, not internet folklore which has been handed down through the generations.
The healthier the battery, the more amperage it can accept, until it is nearly fully charged, where the old sulfated battery will require slightly more amperage to be held at charging voltages than the new healthy one, and only at higher states of charge.
i have NO data on a battery with a shorted cell pertaining to how much amperage is requires to seek and maintain either 14.x absorption voltages, or at float voltages, 13.2 to 13.6v. I've not had a battery with a shorted cell to actually measure how much amperage is required to maintain the voltages the charging source is asking for. So I will not say a battery with a shorted cell does not present significant more load to an alternator, as I have no Data on that.
So idling or driving slow with a shorted cell is possible the only time i will say that the alternator is being worked hard enough to actually shorten its life. And one would be surprised that a battery with a shorted cell can still start a vehicle, as some no doubt can.
A battery with a shorted cell will read 10.5 to 10.7v warm off the old school charger( a new school one would likely be flashinng error messages), and can likely still start a warm engine in mild ambient temps.
But the aged near end of life sulfated battery presenting a bigger load to the alternator wearing it out faster, is hogwash, as unless it is nearly fully charged, it cannot accept as much/ amperage as the new healthy battery.
In only one of the three examples above is the sulfated battery requiring more amperage than the healthy battery, and that is the amount of current required to hold a nearly fully battery at absorption or float voltages, and the differences in amperages the battery can accept are inconsequential to modern alternators. 0.5 amps more load is nothing to an alternator. but to repeat I have NO data on just how much more current a battery with a shorted cell will require to be held at float or absorption voltages.
What is hardest on an alternator, is post jumpstart, idling the engine, with no underhood air movement from driving/moving.
i have actual data on this too. Idling with a maxed out alternator feeding a depleted battery and or other high DC loads has it climbing quickly towards 220F.
Driving 65MPH maxed out delivering 50 more amps than it can at idle, the max stator temperator I have observed is 135f. Different vehicles will of course have different underhood airflows at speed, but a hot alternator is not a happy alternator, and they will generate a lot of heat when asked to make a large portion of their rating, and are capable of doing so, at idle speeds, parked, not moving, and the voltage regulation is not temperature sensitive and backing off the asked for voltage, requiring less amperage, which many internally regulated alternators will.
Post jumpstart, the battery can accept huge amperages, the vehicles voltage regulator is going to be asking for high voltages, requiring the alternator make everything it can to meet that voltage.
Post jumpstart the new healthy depleted battery can and will suck up much larger amperages than the dying sulfated battery whether the voltage regulator is asking for 13.8 or 14.4.
So the only time an old battery still reading over 12v, 12 hours after shutting engine off ( indicating a battery without a shorted cell) can present more load to the alternator, is when it is 98%+ charged, and the amount of extra amperage it requires to be held at charging voltages at high states of charge, compared to a new healthy battery is insignificant.
Its like saying: 'dude, if you try and charge your phone when driving, you are going to wear out your alternator faster!!'
But even that is a huge stretch as a cell phone might require 5 watts to charge, and the sulfated battery might require 1.2 more watts top be held at charging voltages than the new healthy battery assuming both are 98%+ charged.
As for what voltage the fully charged battery shoud maintain during engine cranking, there are lots of variables, such as ambient temp, engine temp, battery temp and actual starter draw., and just how quickly the voltmeter responds to change in voltage.
If one observes a fast responding voltmeter on every engine crank through fall and into winter, they will likely not be left stranded with a No start as one will see when the battery is struggling to start and at what voltage it sounds like it is struggling.
I'll start considering replacement when mine cannot maintain 10+v when fully charged, but I am in a mild climate.
