As Scot said, battery pack is useful in this case. Standard lead-acid battery has a life span of 4-5 years but you can put in another type off sealed lead-acid battery with a life span up to 10 years.
For example: http://www.vabo.be/ned/cat/NED_11589_ART.htm but this is the company where I bought my batteries.
Yuasa is the top brand of batteries! I have seen batteries from 20 years old still in top condition.
Jo from Belgium