parts for a streetfighter!

my battery keeps crapping itself..like, it'll go great for a while, but it'll wear down and then die completely. i'm currently running my brother's chopper battery in it and he wants it back..
i've been told perhaps i'm running the wrong ampere/hour (old battery was 10amp/hr and current one is 7 amp/hr) but what i know about electronics is smaller than an ant shit..
anyone else experience this problem? i have spoken to a few that say it's just what TLs do..any ideas of how to remedy it would be greatly appreciated!