Batteries self discharge at different rates.
The main influencing factor is mainly average battery temperature, but impurities in the lead plate paste or electrolyte also increase the rate of self discharge.
Once the bettery is below 80% charged it starts to sulfate faster and those sulfates begin to harden, perhaps never to redissolve back into the electrolyte. The longer they remain and the lower the battery gets, state of charge wise, the worse it is for the battery.
But a long sitting without a refreshing charge battery, can still perform OK, if it is brough upto 14.7ish volts for many hours, and then perhaps bumped as high as 16.2v, or until the specific gravity of the electrolyte stop rising.
I'd not pay full price for a long sitting never charged battery, but if I was offered one for a heavily discounted price, I have the tools to bring it back up to its maximum remaining capacity, and only then can I begin to determine if it was a good deal or not.
If the long sitting battery was indeed fully charged when it arrived on its shelf, and that shelf was kept cool, the battery might not have crossed that 80% state of charge threshold, as self discharge at lower temperatures is much less.
They say self dischrge is 15% a month at 77f average temperature, for flooded, and lower$$ AGMs can be 2 to 4%, and top quality AGMS are 1 to 1.5% per month.
The maintenance free flooded no removable cap batteries, I am not sure of their self discharge rate, but these batteries especially hate being deep cycled, and if inadvertently deeply cycled should be brought upto and held at 15 volts until amperage tapers to near zero or they will lose capacity quickly after that inadvertent deep cycle.
Generally a 50% discharged battery is considered deeply cycled, and a 30% charged battery can still start the engine in mild temperatures, so a battery that required a jumpstart was very much deeply cycled, and starting batteries do not like this. Even Deep cycle batteries do not like this preferring to stay above 50% state of charge.
Always try to charge any discharged battery, as close as possible to being fully charged, as is possible with the plug in charging sources available to battery owner to do so.
If one wants verification of full charge, that green light on the smart charger is not it.
Resting voltage is not it. Voltage hot off the charger is far from it.
If the battery is flooded with removeable caps, the temperature compensated hydrometer is the battery polygraph as to state of charge. OTC4619 is a good enough one. Francis Freas is the benchmark in accuracy and precision
To determine full charge on an AGM battery, one needs to hold it at 14.4 to 14.7 volts ( assuming 77f battery temperature) until amperage tapers to 0.5% of the capacity of the battery, 0.5 amps per 100Ah of capacity.
The maintenance free flooded batteries need to be brought upto 15v and held until amps taper to very low levels.
No vehicle charging system will truly fully charge a depleted battery, no matter how long the vehicle is driven, unless the voltage regulator is holding mid 14 volts the entire time, and if it does this, it can overcharge the battery. This is precisely what the programmers of automotive voltage regulators are trying to avoid, as an overcharged battery is releasing lots of hydrogen and oxygen and taking some corrosive sulfuric acid mist with them as they exit the battery.
An undercharged battery merely fails prematurely. Not a concern of the automaker.
It is wise to seek freshly manufactured batteries or retailers which actually top charge them monthly, but it is just as wise to insure the battery regularly gets to 100% state of chrarge and to not expect the vehicle to do it, because it will not.
The best lead acid battery chronically undercharged, will not last as long as the worst L/A battery kept fully charged, all other factors being equal, and free from defects.
