Slow power drain...???? Help....

Yesterday my son tried to start up his 1995 VT1100 to go to work, the motor tried to turn over once or twice but that was it. It never cranked up until he put a battery charger on it and started it with the battery hooked up to the charger. It never does that in the spring, summer, or fall. In those months it usually starts right up when you choke it. But when the outside temps drop to below 40 degrees, the bike doesn't like to start unless the motor is warm. Could it be that there is a slow power drain somewhere that's causing the plugs to not spark properly? Besides a battery tender attachment so he can plug the battery up to a battery tender, there isn't anything else hooked up to the battery.