my car's battery was replaced over an year ago and the battery was fine and randomly...the battery goes down and i get confused...because it was new and i bought it just an year back...then i started my car with another battery and then i let it charged the existing battery for 30 minutes by car's alternator....after that i restarted my car after charging and car restarted well..but after 4-5 hours the battery goes dead again...what is the reason...please tell me...i beg..

Thanks..