How much the voltage will sag on any given battery will also be different as it ages, and of course the size of the load.
Voltage based guesses at states of charge can be hideously inaccurate.
One can develop an idea at state of charge when they know for sire the battery is full, and then when discharging itin normal use, watching the voltmeter sag. These voltage trends and tendencies can allow one to get close enough with some practice, but as the battery ages the voltage sag will be worse, when it needs a EQ charge, it will be worse. when the battery is colder, it will be worse, the bigger the load, the worse the voltage sag will be.
The trimetric and other shunt based amp hour counting battery monitors are not 100% accurate either, but they are generally close enough as long as one occasionally rezeroes it when it is known the batteries re full, such as after an Equalization, and also reduces the total battery capacity in the meter, as the battery ages.
Without performing a true capacity test one is basically guessing how much capacity the battery has lost, but the % remaining read out screens can get wildly inaccurate after dozens of cycles without a full recharge.
Mainesail has good writeups on installing battery monitors and properly programming them, and they re not for the faint of heart.
So my recommendation is if one really wants to know an accurate State of charge, they need such a meter like a victron or trimetric, but they must view it with suspicion the farther it gets from the last time it was rezeroed. Doing this also requires knowing when the batteries are indeed full, and getting the batteries to full requires an Ammeter AND or a hydrometer, and of course a charging source capable of maintaining Absorption voltage until specific gravity rises to its maximum baseline of 1.275+. An Equalization charge might be required to get the specific gravity to the maximum and it is even harder to find a charger which can bring a battery to 16 volts and hold it there until specific gravity, compensated for rising electrolyte temperature, no longer rises.
If one can do these things, and does perform them regularly, they will get extremely good cycle life from the battery, especially if they treat the battery like this from new. Usually only when the battery performance has degraded does the user try and do a hail mary recharge, but the lead sulfate, likely has already hardened, never to redissolve back into the electrolyte, and obscuring the plate surface area where the chemical reaction is to take place.
The more often the battery is indeed returned to a true full charge, the less often the Equalization hail mary recharge will be required, but an EQ on flooded batteries should be considered necessary every so many cycles, whether it be 30 60 or 180.
The Hydrometer is the flooded battery polygraph as to state of charge. 1.275 readings on all cells are not however an indicator of capacity remaining, or battery health, only that the batteery is fully charged.
And some batteries, especially those sold for northern climates can have specific gravity at 1.310, so establishing a fully charged baseline specific gravity is fairly important when the battery is new, but with a prolonged EQ charge one can figure out the maximum, but they must be there to check SG every 15 to 30 minutes and stop the EQ when battery temperature starts approaching 120F, or just starts rising quickly.
i have a seriously capacity compromised battery whose cells I can get upto 1.275+, but its performance is so miserable during discharge I could not really keep cycling it in my Van. it also takes forever to recharge fully , wasting a lot of electricity heating up.
An Ammeter along with a Voltmeter can really help one to learn state of charge.
When a 100AH flooded battery can only accept 2 to 3 amps at 14.4 to 14.8 volts, it is in the 99% range
When a 100Ah AGM battery can only accept 0.5 amps at 14.4 to 14.7v, it is in the 99.5% range.
Good luck getting an average smart charger to hold absorption voltage for long enough for amps to taper to these levels. They won't, especially on a hard working van dweller battery. One must trick smart chargers. load the battery ( lights/blower motor. Inverter powering microwave, ect....until battery voltage sags below 12.6v, then restart charger, then remove load.
lather rinse repeat until specific gravity no longer rises. It might never get to 1.275+ without 16 volts if the battery is really punch drunk from chronic undercharging.
Usually the tools which can bring a battery upto and hold 16v, can also allow the user to not have to perform EQ charges nearly as often to maintain maximum battery capacity.
One needs to reinflate the balloon to its maximum size often, or the balloon's skin gets less flexible and crusty, with tiny pinholes and resists future attempts to reach full charge, and gets more petulant each time.
Hydrometer, Ammeter and a voltmeter. With these 3 tools, you can see how well, or more likely how poorly, any given charging source is performing its main task. When voltage prematurely drops to the mid to low 13s when the battery wanted 2 more hours at 14.4v, this is like battery teasing.
Battery blueballs.
Have a heart, hold absorption voltage for long enough to allow your battery the much needed happy ending.
