Well first check the age of the battery. If its older than 6 months or a year ... then get it charged first. (It takes approx 1 or 1.5 hours to fully charge it, and most electricians will try to buzz you off by charging 10-15 mins and taking Rs.50-60 from you)
Also, if your battery charges itself after you ride your bike for a few kilometers ... but then slowly loses its charge once you stop it in your home ... then its mostly because battery needs charging. BUT if even after riding a few kms, your battery doesn't get charged on its own ... then its most probably the rectifier not charging the battery (assuming the battery is fine).