How often does the battery get "cycled "- - - -partially discharged and then recharged? A battery that sits around, even on a "tender" won't last anywhere near as long as one that gets cycled periodically. Turn the headlights on for a couple of hours, then either drive a few miles or use a medium-rate charger overnight to bring it back to full charge. I've got batteries that have been in service for 5 years or more, but they get exercised regularly. 6 or 12 volts doesn't really matter- - - -the kind of use a battery gets definitely does.
Jerry
