The OEM battery in my 2008 Jetta SE 2.5L has been nearly fully discharged one month back (had insufficient surface charge after a prolonged use without the engine running, but leaving it idle for 20 minutes made the car start again), then worked fine for a few days afterwards (but without being subjected to much charge), but then was discharged again after only a quarter of the prior abuse (since it was probably not fully charged since the prior discharge, me underestimating the driving time it would take to bring the battery back to fuller life, plus possibly missing out on water, as I found out later).
Subsequently, on I-40 in Arizona in November, it has failed on me several times, with 7°C being the ambient temperature. In New Mexico, I added some water to it, 20ml to all cells for prophylactics, and 70ml to the nearest-plus cell to bring to meniscus, 30ml to second, none to the middle-ones, and 20ml to the nearest-minus one, but it still failed to contain much charge after some casual city driving. Subsequently, after a 12-hour drive at -4°C and a 3-hour drive at 10°C, it did let the car start two days after the last drive (yeay!), but still failed the next day afterwards (the starter would work just fine, but the voltage would drop to a level that the electronics seem to find unacceptable, so it couldn't start by itself).
Finally, the battery has been subjected to several days of charging at a stationary smart charger, with the ambient temperature being between 8°C and 25°C. It has then been driven for a couple of short trips, then sat idle for 5 days, and read 12.1V on the fifth day of being left undisturbed, and being able to start the car just fine.
It's still working fine a few days afterwards (but without being subjected to the extra abuse). The starter is not extra quick, but there are no signs that the power drops below the level required.
Has it been resurrected? Is measuring 12.1V after 5 days of disuse at 10°C to 25°C a good indication that it has at least some 40% (source 1: 12,0V @ 25%, 12,2V @ 50%) to 50+% (source 2: 40% is 11,96V, 50% is 12,10V, 60% is 12,24V) of its original capacity available for full use?