Lead acid Batteries Need to be fully charged, as often as possible, as quickly as possible after any significant diacharge.
The alternator is controlled by a voltage regulator
Voltage is electrical pressure.
A quality new Marine battery of the group 24.27/31 sizes, discharged to 50%, can, if allowed to be brought upto mid 14 volt ranges at amperages from 35 to 55 amps, can perhaps reach 80% charged in 35 to 45 minutes.
But getting from 80% to 100% charged, cannot be accomplished in less than 3.5 more hours, and that is with a new healthy battery under ideal recharging currents..
That 3.5 hours assumes that the battery is brought to and held at ~14.5 volts the entire time.
If the battery is only brought to 13.8 volts, that otherwise healthy battery will require 8 to 12 hours to get in the 95% range and 95% to 100% will take about 6 to 12 more hours. It does not matter if you have a 200 amp chromed alternator recently polished, a battery can only accept so much amperage and can only be recharged so quickly, and by quickly I mean slow.
The Best lowest resistance AGM battery, A Northstar or ODyssey AGM, discharged to 50%, cannot be recharged to full in less than 6.5 hours. Flooded marine batteries will take longer, Abused or older batteries take much longer. Ideally one should not really discharge a battery below 50% state of charge, and 50% under no load, occurs at around 12.2 volts.
100% fully charged is where the battery always wants to be. Anything less than this degrades the battery capacity, and the lower it goes and the longer it stays there the faster the battery capacity degrades, and the harder and the longer it will then take to recharge the battery to the maximum remaining capacity the battery has left.
An aging battery is like a gas tank which keeps shrinking.
To prevent that gas tank from shrinking too quickly, refill it to abslutely full as soon as possible. Do NOT rely on the alternator to do this. It is VERY poor at doing this, even if one is driving for 6.5 hours.
Or simply resign youself to replacing batteries more often and likely having them fail to power your needs when you most need them.
MOst vehicle will not hold mid 14 volt ranges for very long, which greatly slows recharging.
Those who use their batteries hard, should be able to plug the battery into a charger once they get home, if they want any sort of longevity from the battery. Those that rely only on the alternator for recharging, and use the battery hard will be dissapointed, often, in the lifespan of their battery.
The best flooded marine batteries can last about 600 to perhaps 800 deep cycles to 50% state of charge, when recharged promptly and fully after each discharge.
This same battery recharged to 80% and rarely any higher will be lucky to last 50 deep cycles in a two month span.
This is not the fault of the batttery, but the human charging it.
I would hate being a battery retailer. What other product can be destroyed through ignorance and misuse/abuse, and have the consumer demand a new one under warranty?
Gee, I drove my new car into a lake, that is covered under warranty right?
A digital Ammeter showing how many amps are flowing into a depleted battery is wise, So is a voltmeter whose leads are right on the distant battery.
I have one of these displaying amperage into/out of my battery on my dashboard:https://www.amazon.com/bayite-Digit...pons&keywords=bayite+meter&psc=1
The more amperage the battery accepts, the more discharged it is.
A single group 27 or group 31 marine battery, when near fully charged and held at 14.5ish volts will accept about one amp
If the battery is accepting 15 amps at 13.8v, it is NO where near fully charged.
A 50% charged healthy group 27 battery will easily suck up 50 amps for about 15 to 20 minutes before voltage rises to 14.5v, supposing the chrging source can produce 50 amps and is actually seeking 14.5v, and there is little voltage drop on wiring from charging source to battery, and back.
There is a lot of battery charging myths out there. These myths kill batteries and drive up the prices for all of us.
The lead acid battery wants to be full and kept cool at all times. It takes a long time under ideal conditions to fully charge a battery. The 200 amp alternator, even chromed and recently polished is not going to reduce that time, unless one is also running 150 amps of lights and stereo, but 6.5 hours from 50% to 100% cannot be reduced, no matter what.
So plug in and recharge to full when you can, as soon as you can.
I have an optimized chrging system, I can spin a dial and control the vehicle's voltage when the engine is running, I have 200 watts of solar on my roof, I have an adjustable voltage 40 amp power supply. The capability of these 3 charging sources have allowed me to achieve over 700 deep cycles over 4 years from a 90 Amp hour group 27 Northstar AGM battery, and it is my only battery for everything, and I use 35 to 65 AH of that 90AH capacity at least 3 nights a week.
I can carry 345 Ah of battery capacity, but have been using only 90AH total for over 2.5 years now, as I also know how much of the battery I am using by using an amp hour counting battery monitor.
But none of these charging sources is automatic, as automatic basically means undercharged, as automatic cannot see the variables and adjust accordingly.
One does not need to achieve charging perfection, but they shold know what perfection entails, and where to draw the line. often it is easier and cheaper to simply accept poor battery lifespan, but the cheapest weay to ensure good lifespan is by plugging the battery into a capable wall charger as soon as possible after any discharging of the battery occurred. This will also extend the life of the starting battery.
Sometimes if one plugs in as soon as they get home the surface charge on teh still well depleted battery will tick the automatic charging source into thinking the battery is already full, and it will not seek 14.5ish volts. One often has to be smarter than the smart charger and reduce battery voltage by applying loads to it before hooking up the charger and letting it do its thing.
A few restarts of the charger might be required before the battery is truly full, and an abused battery could require higher voltages than automatic chrgers will allow.
The well marketed 'desulfating pulse chargers' are largely gimmick, but do not hurt the battery.
Seek at least a 10 amp chrger for a 100 amp hour wet/flooded battery and at least a 25 amp chrger for a 100Ah AGM battery, and if one gets an Odyssey AGM battery, these Dictate 40 amps of recharging current per 100Ah of capacity when discharged to the 50% or less range.
Low and slow 'trickle charging' will tickle deeply cycled AGM batteries to death.