It's either something in the battery loop or the charging loop. Do you know of the condition of the batteries? How is the charging connection made to the batteries? Is the connection good? Can you directly test the voltage and current capacity of the batteries at least? If the batteries are good then it's whatever you're using to charge the batteries. Test the charger for proper voltage and current output. Be sure your charger is operating properly. If it is then check the batteries. Pb batteries tend to get shot if they're drained too far or too many times. They have a given shelf-life no matter what. How old is the light?