3 weeks of sitting is enough to drain many a battery, in many a vehicle, to a no start situation.
Often, the overdischarged battery is jumpstarted, driven for an hour or less, and parked again, and the driver thinks the battery is fully charged.
2 weeks later or less, often much less, of sitting, the vehicle wont start.
lather rinse repeat until its new battery time. This will usually happen quickly and the battery or alternator is blamed, but really it had a sumo wrestler sit on its chest. It was only allowed to breathe slightly, before being sat on again.
Driving for an hour, at best, is going to get the slowly drained starting battery to 85% charged, and that final 15% is not only very important to insure the battery does not lose a huge amount of capacity forever, like a fuel tank getting physically smaller, that last 15% could equal a few more days of sitting before there is a no start the next time.
85% charged to 100% charged takes any Healthy lead acid battery, no less than 3 hours, when held at ideal voltages for that entire time.
An unhealthy battery will take significantly longer to get from 85% to 100%, and might not ever reach 100% without considerable time spent/ held int he mid 14 volt range.
Its extraordinary unlikely that the vehicle will hold elevated voltages for such a duration even if the vehicle is driven for that duration.
Starting batteries HATE being slowly drained to dead over 2 weeks, any lead acid battery hates it, but a marine battery or a true deep cycle battery will, if truly fully charged afterwards, retain more capacity, and behave better and live longer than the starting battery, all other factors being equal.
When a lead acid battery is drained to dead, relying on the alternator by itself to recharge, is unwise, even if the vehicle is going to be driven for 5 hours on the freeway.
One should really put it on a charger overnight, or longer, when a battery is drained this far, and there is no guarantee that overnight is long enough.
Returning an abused battery, such as one that is repeatedly drained to dead over 2 to 3 weeks, back to its maximum remaining potential capacity takes a long time at elevated voltages.
Even with a true full charge applied afterwards, a 2 to 3 week drain to dead is still extremely abusive to it. It can only do this so many times, and each time it will take less and less time for the no start situation to arise. Each time it happens, if driven the same duration after a jumpstart will be charged to a lesser % of its remaining capacity, which is diminished.
Also, the newly replaced battery drained to dead over 2 to 3 weeks, then jumpstarted, will put a considerable load on the alternator. if parked Idling, it can get too hot, quickly. I recommend driving 25mph plus soon after jumpstarting a newish jumpstarted battery. The older battery jumpstarted, will not be able to accept nearly as much amperage at the same voltage, as controlled by the vehicle's voltage regulator, which controls the alternator's output, and thus will not tax the alternator as hard.
an ~ 10 watt solar panel on the south facing dashboard, is a good idea on a vehicle which takes 2 weeks to drain the battery to dead.
Another option is removing the negative from the battery cable. Not a good idea on every vehicle out there, but most others The vehicle might run crusty at first and radio presets might be gone, but when the ground cable is removed, the only discharge will be the battery's self discharge itself. Which can be considerable in an elevated temperature midlife battery in summer.
There's a couple asterix's involved.