The Battery POlygraph is the hydrometer.
Do not rely on voltage, and is only accurate as a state of charge refenence on a battery shich has not seen chargeing or discharging currents for many many hours, and onlyu then when has a basis for comparision on that specific battery.
Often here one will read how 12.7v means a fully charged healthy battery when it simply does not, not unless the battery has not been charged for 24 hours, and one has previously fully charged the battery and let it rest for a week and then found it at 12.70 volts.
My Northstar AGM rests at 13.06v fully charged, so a 12.7v = 100% charged declaration, is so far off and simply wrong it is nearly criminal.
How well your vehicle charges your battery is determined by the voltage the vehicle's voltage regulator allows, for the time it allows.
The Hydrometer can;t lie, unless bubbles are sticking to the float, ot the electrolyte is significantly hotter or colder than 77F. The OTC4619 hydrometer has a thermometer which says to subtract or Add 0.00X to the reading on the float.
It is not the most accurate hydrometer available, that honor likely belongs to francis freas glass works, but it is precise and can be compared to future readings, as long as one makes sure no bubbles stick to the float ands adds or subtracts for temperature.
As far as how beneficial it would be for you to apply a smart charger monthly to the battery, impossible to say not knowing how well your vehicle's voltage regulator takes care of the battery in your usage patterns. If you do listen to radio with engine off, or charge your phone with engine off, then more often applications of a smart charger would be more beneficial to it.
But unless y0our vehicles voltage regulator does a very good job, the battery will benefit from a full charge from a good charger. Most VRs are timid, aiming to not overcharge the battery, they are not much concerned with achieving a true full chrge ASAP which is a bit of a middle finger to the battery itself.
And while most automatic smart chargers will drop from absorption voltage before the battery is fully charged, they usually drop to float voltage(13.2v to 13.4v for flooded batteries), and if left at float voltage for 12 more hours it likely will be fully charged, or very nearly so. Double check with the hydrometer. on flooded batteries. On AGM full charge is determined by an ammeter. When the battery can accept only 0.5% of its capacity(20 hour rate) At absorption voltage, then it can be considered fully charged. Easy when one can control voltage and read an Ammeter, much harder when the charger drops out of absorption voltage to float voltage before one was able to take a reading.
Or of time is an issue, one could use the AGM setting as they float at 13.6 to 13.8v, which will charge faster than 13.2v. Just do not leave on for a week floating on the AGM setting.
I prefer the AGM setting on my 10+ yer old smart schumacher for my flooded batteries, as it will actually hold 14.7v longer, and will not usually exceed 15v, as it will on the flooded settings
Or one can trick and restart the charger to squeeze in those last few percent. Turn on the lights/ high beams, blower motor on high, rear defroster until battery voltage falls to ~12.6v then restart charger, then remove loads.
How any individual charger responds to this trickery is variable, but the goal it to get it to seek 14.5+/- volts and hold it for a period of time until the hydrometer reads 1.275+, or whatever one has already established as the maximum specific gravity reading on that particulr battery when it was brand spanking new and fully charged.
But in general 1.275+ is fully charged, but a battery designed for the far north, the concentration 'might' be as high a 1.310, and in the tropics 'might' be as low as 1.260. So established baselines are needed before declaring a specific gravity reading of 1.2XX= 100% state of charge.
On an older battery when one has nothing to compare current readings to, a controlled overcharge can be used to establish how high the specific gravity readings can go, but this might require voltages as high as 16.2v which is well outside the capabilities of most chargers, and the process must be monitored closely for excessive battery heating and terminated as soon as the specific gravity, compensated for the electrolytes rising temperature, stops rising.
Each of the 6 cells can vary slightly too, but usually one always reads low. To save time in future one can just dip this low cell.
Always rinse and dry the hydrometer afterwards, and save the packaging it came in for storage.
The true battery maintenance nut job can make charts recording specific gravity, electrolyte level in each cell and battery temperature, voltage of the battery, how long the charger was applied before specific gravity was maxed out, what type of service the battery has seen.
No one needs to do this, but the maintenance minded bitoger trying to get the most from their battery, like they do from their motor oil, well it is not that crazy, and a hydrometer is a one time cost of 11 dollars, not the 35$ each used oil analysis costs
I do not have any smart charger recommendations. I have grown frustrated with their marketing and Lying green lights. I use an adjustable voltage power supply and I hold a temperature compensated absorption voltage until the hydrometer or Ammeter tells me the battery is full. This time varies greatly in my usage, and very few automatic chargers would get it right, and close but no cigar is not good enough, for my intentions.
