Alternator amp ratings often mislead consumers. The higher rated alternator in the same physical case size, often has poorer output at lower rpms.
Here is one example:
The 185 amp alternator can make 80 amps at 1200 rpm while the 320 amp alternator makes zero. These two alternators are in the same exact size casing
One hoping for better battery charging with an alternator of a higher rating, will often be shooting themselves in the foot. Not always.
But lets say 2 different rated alternators have the same curve, one of them, as rpm raises, is just able to produce more, similar to the 320 vs 270 in chart above. Also lets say the discharged 300 amp/hour battery bank is the only DC load, removing the other variables.
Say the 320 amp alternators voltage regulator only allows 14.0v max, and the 270 amp alternator allows 14.7v.
The 270 amp alternator in this example, will more quickly charge the depleted battery bank in such a scenario and will return the battery bank to a significantly higher state of charge in an hour.
Voltage is electrical pressure. the lower the voltage, the slower the battery charges, and alternators are controlled by the voltage regulator. They can only make so much amperage at specific rpms, but in many cases the amperage ability of the alternator at most any rpm above hot idle, can be considered an unlimited amperage source. The limiting factor in such a case is the voltage regulator, whether it be in the alternator, or on the firewall, or inside the engine computer.
Bitog being Bitog, no one asks "what is the most adequate.......whatever"
Fact is battery longevity is most directly affected by its average state of charge. The higher the better.
It takes a long time for an 80% charged battery to be brought to 100% state of charge. No less than 3.5 hours when held at 14.4v+.
Few vehicles, even if driven for 3.5 hours, ever approach holding these higher ideal charge voltages, and their batteries have no hope of being fully inflated, and they sulfate, losing storage capacity and their ability to quickly delivering what is left to a hungry starter motor. They degrade faster.
A battery that lives in a perpetual state of undercharge, has its potential longevity reduced to a fraction of what it could be if it were returned to 100% after each and every engine start and allowed to live its life at high states of charge.
The battery owner can decide for themselves what additional effort is worth it, but they should know that Ideal battery life is achieved by keeping the battery at high states of charge, and that the best lead acid battery kept chronically undercharged, will not outlast the worse lead acid battery kept fully charged, and that their vehicle does not Shiv one Git about achieving maximum battery longevity.
There is not enough data available to say which Lead acid batteries, easily available, will withstand life in a constant partial state of charge, compared to another, in that same exact usage. Anecdotes of a 5 year old battery having performed well in a specific vehicle in a specific usage, are also meaningless, as the manufacturer of the battery has likely changed in that timespan, to whomever allows the most profit to be achieved, and comparing voltage regulator A to voltage regulator B is a huge variable..
Worrying about which battery might be better, can be negated by the owner realizing their vehicle does not treat the battery very well, and that regularly applying other charging sources to the battery will greatly extend its useful lifespan, even if those additional charging sources themselves are incapable of achieving ideal.
When one tries to achieve ideal, they realize the equipment they own is inadequate.
I've been stunned by just how long it has taken a 'still going strong' battery to achieve a 100% State of charge, by an adjustable voltage power supply, that maintains a high chosen voltage, for as long as I allow it to.
I get frustrated when people assume their vehicle takes perfect care of their battery, and the only thing affecting battery life is the quality of the battery itself.
I know this sounds like an onerous duty, using a manual charging source, but I basically just dial my 40 amp Power supply to 14.7v, hook it to the battery, and watch the ammeter. when amps taper to 0.5 per 100Ah of battery capacity, I lower voltage to float. A large healthy 50% charged battery, this is usually 5.5 to 7 hours. and 6.5 to 12 hours when nearer end of life
If I cannot be there to lower voltage when amps taper to that level, then time to reach full charge is also not important, and I just choose a lower voltage, usually 13.6 to 13.8, but them bump it back upto 14.7v when I can be there to observe amperage.
It takes a long time to truly fully charge a lead acid battery, but especially so on one which has lived most of its life at ~85% state of charge, and reduced/timid charge voltages, increase this time exponentially.
Decide for yourself what is acceptable battery lifespan.
I try to relate what is ideal and how to achieve it, as this is bitog and no body starts a thread asking:
What is the most adequate.......... xxxxxxx?