Modern vehicles have significantly larger parasitic loads then older models.
3 weeks of parasitic draw on a modern vehicle is easily enough to bring a healthy fully charged battery to below 50% charged, at which point, damage (capacity loss from sitting undercharged) accelerates rapidly.
ANY lead acid battery, ideally wants to be 100% charged at all times for maximum longevity/lifespan, and kept cool. Anything less, lessens its longevity depending on degree of discharge and how long it remains there, in combination with the temperatures it is subjected to. A hot depleted battery degrades very quickly.
Charging from 80% to 100% takes about 4 hours, even if one just got done whispering lovingly to their chromed high amperage alternator while polishing it.
A sulfated battery can easily require 8 hours, held in the mid 14 volt range, before it approaches 100% from 80%.
Even when it reaches 100% charged, this does not mean the battery capacity is the same when it is new.
An Aging battery is like a gas tank whose volume shrinks to some degree every time it is depleted. You can still fill the tank, it just holds less gas. Keep the tank full as much as possible, and it can hold more gas for a longer period of time.
At some point it does not have enough 'gas' to crank the motor, even if it is fully charged.
Allow the tank to slowly leak half its fuel, time and again, and it shrinks, and no magical desulfating pulse charger is going to stretch it back out despite the claims of marketing teams or those who believed their words and made the purchase.
Not to say these well marketed desulfating chargers can't fully charge a battery, though they will usually revert to float voltage prematurely, and what might take 2 more hours at 14.4v, will require another 12 hours at 13.6v.
These smart chargers need to be outsmarted to actually achieve 100% charged in minimum time, or when started on an abused sulfated battery. After it first stops and reverts to 'float voltage' shining that human soothing green light, UNplug charger, reduce battery voltage to below 12.6 via applying large loads to battery, restart charger on next lowest amperage setting, lather rinse repeat.
Verify with a hydrometer if not a sealed battery. Specific Gravity at 1.275+ on all six cells. lather rinse repeat until SG no longer rises, and the abused battery will be as restored as it will likely ever get, and this might not be good enough to last another season, even if it holds 12.6+ volts 12 hours after removing from charging source.
