This is from my deepest memory so I may get it cockeyed. If you have a lead acid battery then much of what Paco and the others have said is true along with the possibility of coking. You have a nickel cadmium battery then it requires periodic maintenance, which involves a complete discharge and recharging. A Ni Cad battery will develop a memory and with that memory it will only accept that level of charge and no more. If this is the problem then the battery does not have the ooomph to turn the engine over to light off speed assuming coking is not involved. Turning the main rotor backwards will give an indication of coking, as it will impede your ability to rotate the blades. If the coking level is low then the rotation of the blades backwards will break the engine rotor free. In either case, check the engine and check the battery log book to determine when the last maintenance was performed assuming a Ni Cad battery.