Today I went out to start my 76 camaro and the battery was dead. I hooked my charger up to it and put the charger on 225 amp (jump start). The car started right up. I let the car run about 15 minutes and turned the key in the off position but the car kept running. I pulled the negative cable off the battery and the car still kept running. Finally I pulled the ignition wire off the coil and the car shut off. What could have happened to cause this? It has been around 19-23 degrees outside the past 4 or 5 nights but don't know if the cold had anything to do with it or not. Any help on what I should check is appreciated.