When one hooks up a charger, that has a state of charge display, the charger is simply reading the voltage of the battery when it was hooked up. it does not know when the last load or charging source was applied to the battery, and a load or a charging source recently on a battery can have huge effects on voltage. It has no magical powers to read the true state of charge or state of health of the battery, despite the marketing literature trying to imply this near impossibility.
Hook up such a charger while the headlights have been on for 2 minutes, engine off, and it will read alarmingly low, even though the battery could perhaps be 99% charged.
The % charged screen on chargers that provide such, is pretty much useless in most scenarios, as full charge resting voltage can be as low as 12.62 on some batteries and as high as 13.23 on others, and this will also change with battery temperature. Very few chargers have temp sensors on the battery clamp, making any % charged screen even more worthless.
Alternators are controlled by a voltage regulator. which might be located in the alternator itself, or in an engine computer, or sometimes a separate unit on the firewall.
The voltage regulator basically tells the alternator how much amperage to make in order to seek, or to maintain, a certain voltage the voltage regulator is looking for.
So While vehicle charging systems generally are poor at fully charging a battery, it is because the voltage regulator is timid for fear of overcharging and lawyers, and it is usually pretty dumb.
Some vehicles do intentionally manipulate voltage, to squeeze out minutely more MPGs, and do so at the expense of battery life.
Very few vehicles care about returning the battery to a true full charge ASAP for maximum possible battery life. Some older pre 90's vehicles that have a constant setpoint of 14.0v no matter what, in comparison to todays vehicles, would take far better care of the battery, even though long drives it essentially would be overcharging, and for short trip driving with a depleted battery, would not be charging it nearly as fast as 14.4v would.
My battery gets intentionally drained with engine off. powering refrigeration, powertools, portable office My externally regulated alternator(s) were once controlled by a voltage regulator inside engine computer and I found these voltages allowed to be infuriating for my purposes/needs.
I tricked this voltage regulator into thinking it is still connected to an alternator, with a 50 watt 10 ohm resistor, and use an external adjustable voltage regulator further modified to have a voltage dial on my dashboard, next to ammeter and voltmeter. When the battery is well depleted, and I start the engine with regulator set to ~12.4v, only 10 amps or so flow into the battery. My cold alternator at cold engine idle speeds can make 67 amps, but I have to crank up the desired voltage before the alternator makes this much juice. I can hear the note of the v8 engine change as the alternator sucks up 2+ engine HP in order to make this 67 amps to feed the depleted battery and run whatever other DC loads are on at the time.
The healthier and the more depleted the battery, the more amperage it can suck up at any charging voltage reaching the battery terminals. I generally observe 2 to 3x the amperage flowing into the depleted battery when held at 14.7v, compared to 13.6v, but as the battery ages toward the final third of its expected life, this is more like 2x.
When grid power is available, I use an Adjustable voltage power supply capable of 40 amps output to fully charge the battery. It is told to seek 14.7v, and I allow it to maintain 14.7v, until amperage the 100 amp hour group 31 TPPL AGM battery accepts, tapers to 0.5, then reduce voltage to 13.6v.
The Automatic 'smart' chargers I tried, would never hold 14.7v for long enough for the 0.5 amp per 100Ah of capacity threshold, to be reached.
Their green 'full charge' light, was so hideously inaccurate, I am still filled with disgust for them, many years after I quit using them.
I recommend users of battery chargers actually measure amperage the battery is accepting at higher voltages, if they are really trying to achieve true full charge for maximum possible battery longevity, rather than believe the lying BS artist green 'full charge' light.
the green full charge light ONLY indicates the charger has lowered the voltage it will hold the battery at, switching from a constant voltage in the mid 14's, to some float voltage somewhere in the low to mid 13's.
A green light on a smart charger does NOT indicate a truly fully charged battery, in most instances.
Any charging of a depleted battery is better than no recharging, but achieving 98% state of charge is only half as good as 100% charged, in terms of extending battery lifespan and performance during that lifespan. Most smart chargers likely stop holding an absorption voltage in the 92 to 95% charged range, then switch to float voltage. Charging does still occur at float voltage on a less than fully charged still relatively healthy battery, but basically comes to a standstill on a less than healthy battery. On the healthy battery, float voltages will take days to achieve full charge, whereas at absorption voltages in the mid 14's it might take .5 to 3 more hours.
The AGM float voltage setting is usually around 13.6, and on flooded batteries is usually 13.2. Almost all these smart chargers with battery choice buttons, will achieve higher state of charge by the next morning using the AGM setting as some portion of that 12 hours, will be at float voltage.
The AGM setting can also prevent overvoltage that some 12+ amp Schuchargers are famous for applying.
People really only start using a charger regularly on the less than healthy battery, so the premature float voltage initiation, and the lying green light is doing the battery and the battery owner no favors.
LED adjustable voltage power supplies capable of 30 amps can be had for under 25$, but they are far from automatic. They can however truly fully charge a battery back to its remaining potential capacity for the human who demands it and has the desire to operate it, and an Ammeter and or hydrometer.
Much easier to believe a green light though