Since you are apparently new to using this type of battery, I'll alert you to two of the most common problems that will seriously limit the lifetime of your Pb based cells; deep discharge and to high a charge rate. Lead acid batteries do not take kindly to deep discharge, as soon as your battery falls to 11.9 volts under load, recharge. Never charge a sealed lead acid battery at a higher rate than 0.1C (700 ma for a 7 amphour battery). The advice to get a larger capacity battery is a good one. While slightly heavier, most lawn mower batteries are a good choice of low cost and high capacity. They are heavier and require some maintenance such as adding distilled water occasionally and
AWAYS KEEP UPRIGHT. Check out
www.rcbatteryclinic.com for some good advice on the care and maintenance of all types of batteries.