losing bater power?

i have a 125cc Jinlun bike at the moment i am going to put a hammer to it if it carrys on, the trobble with it is the battery is losing charge, i have gone though two baterys now in the last three months, one minute there is lots of battery life and the next the battery is dead, also the main head light is very dim even when the Engine is running, can anyone help with this problem.