I like that brick in the toilet tank analogy. An increasing amount of coarse Aquarium gravel in a vehicle's fuel tank, is likely better than than my shrinking fuel tank analogy. I think it better represents how sulfation reduces the surface area of the plates where the chemical reaction occurs, and the amount the tank can hold.
Today's gauges are often dumbed down to keep the driver from freaking out when they notice something they are not used to seeing, and have no idea why that might be.
I'd not trust any stock dash voltmeter, unless one confirms it actually represents actual battery voltage. An accurate digital voltmeter in the 12v powerport/ciggy plug is better, but often this circuit is shared with other loads and those loads can drop the voltage it is reading below actual battery voltage measured at the battery terminals.
My 30+ year old vehicle came with an ammeter needle, no voltmeter.
When I did have an alternator failure, well before I knew much of anything about batteries and battery charging, 20 years ago, I did not notice that my ammeter needle swung the 1/16" of an inch from charging, to discharging. I only noticed something was wrong when my windshield wipers moved extra slowly, and my turn signal blinkers took extra long to blink on and off. It still started easily, and I carried a jump box for when it declined to the point it could not.
So in this case, I would have been much better off if there was a voltmeter showing 11.9v with engine running, rather than the 14.9 or 13.7 that the voltage regulator inside the ECM's internal vReg allowed, before alternator failure. The tiny 1/16" swing of the stock ammeter needle from charge to discharge, went unnoticed for as long as it took the 8 to 12 amp load( during daylight hours) to depleted my battery and cause windshield wipers to wipe significantly slower. The stock ammeter has been removed and replaced with a tachometer.
Engine starts themselves of a fuel injected engine really take extremely little of the battery's capacity, especially a warm engine.
My alternator failure incident way back when, was not easily resolved, as my knowledge of how it all works was so lacking. I had the option of a 90 or 120 amp alternator. Basically the Same price, so I went for 120 assuming I had the 90 which failed.
My battery was still heavily drained, but had enough to start my engine with the newly reman'd alternator installed.
Upon starting, the ammeter needle swung higher than I had ever seen before, for about 5 seconds, then fell back to discharging.
I did not realize the fusible link had blown. I then got experienced at alternator swapping, and then assumed the voltage regulator inside engine computer was at fault. I only had an ancient analog voltmeter, and no idea how to really use it for diagnostics.
Finally I took it to a sparky who found the blown link, and replaced the link. The fusible link was not blown visually. They are supposed to look blown and stretch out when pulled upon. Mine was not. The shop used 14 awg instead of the stock 10, I later found out, and that thing was getting ridiculously hot whenever my battery was depleted.
Now, as I use far more battery power, my charging system is refined. Dual alternators feeding separate battery banks. I bypassed voltage regulator inside engine computer, and control each alternator separately, manually, with external voltage regulators that I modified with external potentiometers on my dashboard, next to my 3 wire calibrate-able digital voltmeters, next to my Ammeters. 4 ammeters actually at this moment, two showing total alternator output, two showing what each battery bank is accepting, or delivering.
I recently removed the voltmeters i had on the Field wire going from voltage regulator to field terminal on alternators, as my curiosity was sated.
I also have temperature sensors thermo epoxied to alternator casing, and also to the voltage regulators backsides which act as heatsinks, to which I added finned aluminum heatsinks, and 60mm fans, as they were hitting 170F, quickly, at idle, with depleted batteries.
So voltage is basically pressure. The ammeters show how much amperage is flowing into, or out of the batteries at that pressure.
A depleted battery will accept much much more amperage when new and healthy, compared to when older and sulfated or just worn out, when its capacity is some fraction of what it was when new.
So voltage alone is not a great indicator of anything other than the electrical pressure. It says nothing about how much juice is actually flowing into or out of a battery, at that pressure. How much amperage is flowing at that voltage gives a great picture of the state of charge of the battery as well as the state of health, but the latter requires more experience watching how much amperage is flowing into that battery from different states of charge, as the battery ages., but when one sees that a known depleted battery is accepting low amperage at high voltage, it is a sign the battery's capacity is severely compromised.
If the battery is only ever used for engine starting, and never cycled deeper, then the ammeter would be much less revealing. as it would drop from about 70 amps immediately after starting and taper to less than 5 within a minute, and less than 1amp if it were 98%+ charged at the time of engine starting.
With an Ammeter reading total alternator output, One sees just how hard the alternator works to power the engine and all the DC accessory load whether headlights or blower motor, or fuel pump, ignition, electric windows, rear defroster, seat heaters, ect. The depleted battery can be considered a load, when the alternator can't make more amperage than the vehicle's DC loads requires at that rpm, and when it cannot, system voltage falls to or below battery voltage, at which point the battery provides the extra amperage, then with more engine rpm, and the more amperage that the alternator can make at higher rpm, the depleted battery then again becomes a load instead of a source.
When one can control the voltage, and has an ammeter, one can see how choosing 14.7v, vs 13.7v, basically doubles the amperage a depleted half life battery accepts, and triples the current a healthy new depleted battery accepts.
I basically choose 14.7v until battery bank A accepts less than 0.5 amps, then lower to 13.6v during the day or 13.8 to 13.9v at night as battery bank A is usually the engine starting battery, but I can choose either or, or even both.
On battery bank B ( the Dekas) I choose 14.39, but when amperage into batteries at 14.4v tapers to 5 or so I lower voltage to 13.4 or so, as that battery bank,( 6v golf cart AGM batteries in series for 12v) at that point, will accept about the same amount of amperage at either voltage.
Either battery, when well depleted bank can easily max out their individual alternators at idle, hot idle @ ~ 50 amps output, and if well depleted either can also max out their 120 amp alternators at 2K engine rpm too. The one alternator can slightly exceed its rating at 2300 engine rpm, the other one falls short of its rating and maxes out at ~ 1800 engine rpm.
Even with my vehicle's charging system, which is now refined for maximum rate recharging of depleted batteries, 80% to 100% state of charge takes no less than 3.5 hours. 3.5 hours represents the battery banks when they banks were new and healthier. The Deka bank now takes no less than 4.5 hours and the Northstar is right about 4 hours, and this is despite their age and cycle related loss of capacity.
80% to 100% taking so long, is where an adequate amount of solar wattage and sunlight, shine, as they have the time to complete the charging, silently, and the battery bank can then start its next discharge from 100% state of charge. When solar is inadequate i try and plug in and let my adjustable voltage power supplies hold absorption voltage till amperage tapers to the desired level, then I lower it to float, though I will often just lower to float if I will not be there when amps taper to the target level.
When a cycling battery starts its next discharge cycle, only having gotten to 90% state of charge, time after time, its ability to store and deliver energy, and then to also reaccept energy and recharge as fast as it could before, is compromised. The more partial state of charge cycles accumulated, the worse the battery performs, and the faster it degrades, and the harder it becomes to actually return it to its maximum potential remaining capacity, requiring higher absorption voltages held for much much longer before that benchmark is reached.
The lead acid battery always wants to be returned to a true 100% state of charge and kept cool. How much they dislike this 'less than ideal ' depends on just how much below 100% state of charge they average, as well as their temperature. Deep discharges are also very damaging to them, but more so when they are not returned to at least relatively high states of charge relatively quickly.
A parasitic drain drawing a Starting battery to 20% state of charge over 3 weeks is very hard on that battery. An Agm battery only slowly recharged from this point is especially temperamental as it really wants a significantly higher amperage rate to force migration of the electrolyte through the glass matting and deeper into the plates. The higher rates also heat the battery from within a bit more which helps dissolve that sulfation back into the electrolyte.
Lithium batteries do not mind partial state of charge cycling, but they cannot be safely recharged if they reach freezing temperatures, and most of the current Lifepo4 drop ins, their BMS's will not allow passing starter motor current, nor what the alternator could deliver to them if its output is not throttled back, way back.