A Lead acid car battery discharged below 80% state of charge, can potentially be quickly brought back upto 80% state of charge range, if the alternator's voltage regultor is seeking and then holding the 77f battery's voltage in the mid 14v range. Cold batteries require higher voltages, hot batteries require lesser voltages
Getting the 80% battery to 100% state of charge however, takes no less than 3.5 more hours and only if the still relatively healthy battery is held in the mid 14v range for that entire 3.5 hours . Higher voltages will not significantly reduce that time, but lower voltages will significantly increase the time it takes for a healthy 80% charged battery to reach 100% state of charge.
A less than healthy battery might never reach 100% state of charge, even if held at 13.6v for days on end.
An 80% charged battery will have NO issues starting the engine, a healthy 30% charged battery will have no issues starting the engine unless both battery and engine are super cold. It seems people think that if the engine starts the battery is fully charged or nearly so, but this is very much false.
A modern fuel injected vehicle that starts easily and quickly, whose voltage regulator seeks mid 14 volts, can return into the battery the energy used to start the engine, in about one minute. But if the battery was at 80.225% charged when the engine was started, it will only be charged back to 80.225% charged again, a minute later, if the voltage regulator is seekig mid 14 volts, and the alternator has no other huge loads on it like a winch or blower motor on high and headlights which will suck up 40 amps or so of the alternator's available output, in which cas it ould not be able to bring the battery to 14.5ish volts anyway.
There is no getting around the 3.5 hours to get a lead acid battery from 80% charged to 100% charged. The closer to 100% charged the battery is, the slower it charges. Voltages ( electrical pressure) over 14.8v on a 77f battery become increasingly abusive to it. it will bubble very fiercly and shed positive plate material at a much higher rate, and will only charge slightly faster. AGM batteries might vent at voltages over 15. AGm venting is not instant death of the AGM but it is best avoided.
A less than healthy battery will take longer than 3.5 hours to reach full charge from 80% charged in the mid 14v range.
An AGm battery can likely charge faster from 50 to 85% charged compared to a regulr flooded starting batterym but the 85% to 100% recharge times are not reduced and AGM batteries if deeply cycled really want to be returned to 100% state of charge to retain their capacity. AGM$ price does not make them super batteries by any means, they are kind of petulant princesses when not treated well, and treating them well means returning them to full charge quickly when well depleted and keeping them at high states of charge all other times.
A lead acid battery Always wants to be at 100% state of charge, and No, vehicle charging systems are not good at returning a discharged battery to this state, and yes many newer vehicles intentionally keep the battery in the 80% charged range via voltage manipulation, as a discharged battery can accept a lot more amperage than a battery coser to fully charged, and each 25 amps an alternator is said to produce, requires 1HP from the engine due to losses through the diodes and belt drive.
Each newer vehicle will be quite different in terms of its voltage regulation, and proper battery charging is ALL about the voltage regulation, and time. Manufacturers care more about meeting fuel economy standards than they do about battery life, even though a battery that needs to be replaced much more often, likely uses more energy and wastes more resources than that which is saved by keeping it intentionally undercharged.
Maintenance minded car owners, wanting maximum battery longevity, will return the battery to a true 100% state of charge often by using a plug in charger, or perhaps a solar panel of high enough wattage and a solar controller capable of holding a high enough voltage for long enough to accomplish the true full charge. Any charging is better than no charging, but 100% charged is twice as good as getting only back to 98% charged.
Reports of any specific battery's longevity, good or bad, are entirely meaningless without knowing the battery's average state of charge, and its average temperature. PLease read that sentence again and keep it in mind when the inevitable battery threads start and people start claiming one battery lasts longer than another, in their experience. A battery kept at an average 92% charged will greatly outlive one kept at an average of 82% charged. The vehicle's voltage regulator, and how the vehicle is used can easily account for higher variances in average state of charge than this.
Defects aside, the worst constructed LEAD ACID car battery kept at a very high average state of charge will outlast, by a large margin, the most expensive 'best' lead acid battery, wet or AGm or GEl, kept chronically undercharged.
The label on the battery does not denote any resistance to chronic undercharging.
The label 'might' indicate some ability to be returned to some usable level of capacity if a prolonged intentional overcharge is performed in an attempt to revive it, once it is shown to be weak.