Something I have found that really helps the life expectancy of batteries that are not in regular use is to keep a battery maintainer on them. Preferably one with a "desulphator" feature.
I even put solar chargers on my trailer batteries, lawnmower batteries, etc. It makes a difference especially during periods of little use.
Click here for battery maintainer desulfator
http://rover.ebay.com/roverimp/1/711...1;CACHEBUSTER]