Motor would not shut off (sort of)

[JGra]

I have not used my boat for about the last 4 weeks or so. I like to crank it up every so often just to keep everything going. The motor is a 89 mercury 80 HP 3 Cylinder. First off I put the charger on it for a couple of hours to give the batterys a good charge. Motor would turn over great but could not get it to fire even when the the key was pushed in to choke. I found that the enrichment hose going to the carbs had come off. Once I got it reconnected it fired on the first crank. I ran the motor for about 15 minutes and thats where the problem came in. Once I turned off the key the motor kept on running. It ran for about 15 seconds and then shut off. I cranked it back up but it did the same thing. 3 rd try I tried shutting off the battery switch and even pulled the kill switch but it kept on running. I then noticed that the battery charger was still pluged in and on charge. I figured lets unplug it and give it a try. After it started up I shut the key off and it shut right off. Did this a couple time and everything seems to be working fine. Could the charger be messing with the starting circut somehow. Dont know if I have a problem or not. Thanks