The longer a battery sits the more opportunity for chemistry class to prevail. The sulphur in the electrolyte leaches to the lead plates coating them in a film; the longer the battery is idle the harder the coating gets. At some point the now 'sulfated' battery starts giving false positives to its charge status. At that stage you can only read the true state by measuring the specific gravity of the electrolyte. You might be able to resurrect the battery with a trickle charger; that would slowly heat the lead plates freeing the sulfur to return to the acid. A higher output charger just charges the remaining electrolyte giving those false positive voltage readings.
I randomly charge my batteries over the winter using a battery tender. I'm not convinced an inexpensive charger won't cook my batteries so I don't leave the charger attached long term. So far, so good. I got at least 7 years from the first battery but it may have been more as I routinely swapped the battery from my 01 Bonnie with the 04 when the 01 went dead and being lax didn't mark which was which.
My nephew replaced the battery in the 01 last fall, it hadn't been ridden or charged in months :-(
