Hopefully you're using a maintenance charger, not a trickle charger. The Tender units from Deltron are proven to maintain a battery automatically, giving the battery what it needs and keeping it fully charged while its hooked up. At least until something internal in the battery fails.
Five years out of my original batt. I have found it to be hit or miss with WM Johnson Controls made batteries. My car sized units have been great. My truck's big battery has been a nightmare. Good thing for WM's warranty coverage. I'm on my fourth truck battery in four years. I only had to pay for the first one, but the free replacement period is up on the deal. We'll see how long this one goes on the last two years of their pro-rated period. In most cases its tough to beat WM price, freshness and warranty. That being said I check around for good deals on quality batteries. Currently have a Bosch in the Stang. Also an Advanced Auto battery in another of my cars.