Did you get it load tested? If you throw the old one on a charger, what's the highest voltage you can get out of it?
While many seem to think that voltage off the charger is indicative of state of health, it is NOT.
One can have an old sulfated battery with 20% of its original capacity remaining, and bring it to, then hold it at 14.4v for 12 hours, and it will likely settle to the 12.7v range at 77f in 12 hours or so., and it still only has 20% of its original capacity remaining.
Voltage is indicative of state of charge, not state of health, and voltage is only indicative of state of charge when the battery has not seen any charging sources, or discharging loads, for many hours, and even then the 12.7= 100% SOC, oft repeated the internet over, is not applicable to each and every battery. My Northstar AGMS rest fully charged at over 13 volts, my Deka AGMs about 12.85v and I've had some new healthy tuly fully charged flooded batteries that rested fully charged at only 12.62v.
Previous flooded batteries I could get to rest at 12.82 volts at year 7, but their capacity was so compromised that charging an old flip phone would drag their voltage to the high 11's, at which point it could not start the engine, but before plugging in the flip phone to charge, it easily could.
The Load test is a far better indicator of state of health. Voltage is only relevant as to state of charge, and only really when the battery is well rested, and compared to earlier readings on that same battery when it was newer and fully charged.
That old batteries 45f and above, are widely reported to fall to 12.3v quickly, is more indicative that the charging source is simply not holding their old sulfated battery at high enough voltages for long enough to actually reach a true 100% state of charge, as the older and more sulfated a battery, the longer time it needs at absorption voltages in the mid to high 14's at ~60 to 80F, to reach full charge, and this can exceed 12 hours held at 14.4v for specific gravity to max out on each cell.
A battery holding 12.8v 5 hours off the charger but then 12.3 12 hours later, while still in the vehicle indicates the small parasitic load on the battery was able to eaily drop the voltage of the capacity compromised, perhaps well undercharged battery that low, in the time it had.
Putting a charger on a 6 year old battery is not going to get 8 years from it. Too little, far too late but its not going to hurt, and could buy time for the owner to find a battery deal to their satisfaction.
If it were regularly topped up, to truly full, from year one, day one, and never left to slowly discharge to dead, 8 years would be be relatively easy to achieve on a battery not mounted in engine compartment, on a vehicle whose voltage regulation is mot programmed to keep the battery at ~80% state of charge in order to eek out a 0.05% increase in fuel mileage. Even on such a vehicle, the battery will greatly enjoy a true full charge, but 5+ years service life is still highly unlikely to be achieved.
The older a battery, is the longer and longer it takes to get from 95% charged to 100%, and the more important that 100% becomes in preventing the increasingly rapid loss of capacity and cranking amps. The time it takes can enter the ridiculous arena, even holding it at higher voltages, and some batteries might require the equalization charge of 15.5v to 16.2v in order for specific gravity of all cells to max out, which is outright abusive, and time at these voltages should be minimized, especially on starting batteries.
Smart Charger X might do perfectly well on a new battery, but falls woefully short on an older one as it will not hold absorption voltage for long enough. The green light is not indicative of a true full charge, only that the charger has stopped holding the battery at absorption voltages, and has reverted to float voltages. Float is simply to maintain the battery at full charge. Very little additional charging happens at float voltages, especially on old batteries and too low a float voltage on a cold battery is worse than disconnecting the battery from all parasitic loads.
An old school manual charger can also easily bring a newer healthy battery well up above safe absorption voltages, if left on too long, but the same battery at year 5 would benefit from its constant current with no voltage limitation, as its ~ 0.8 amp output on a new battery can push voltage into the high 15's or higher, but at year 5 might struggle to get it past 14.3v, and 14.3v applied for days, might never max out the specific gravity of all cells on such a battery.
Even if specific gravity is maxed out on all cells, at 1.270 or higher, 1.310 perhaps in batteries intended for cold climates, 1.260 for hot, this only means the battery is fully charged to the maximum remaining capacity. That capacity might be only 1/10th of the capacity the battery had when new.
Battery Voltage is not like a battery fuel level gauge, nor a hard and fast indicator of state of health.
There is a dozen ifs ands, and buts, ............with many variables, and potential exceptions.
