Regarding what
@Nightmare just mentioned, I ALWAYS unplug my trolling motor when I leave the lake, for two reasons.
First, it prevents the oddball chance of something weird happening and having the TM somehow turning on while I'm driving down the road. I know the chances are slim for this to happen, but I do know of at least one person who had this happen, and it chewed his TM prop up pretty good, as well as leaving some ugly scars on the his bow.
I also don't want to have the TM plugged in when I charge the batteries because I remember some old school info that somehow gave me the impression that it was a good practice. Not sure why, but I never questioned it and wasn't willing to take a chance finding out the hard way that it was correct info.
Lastly, I leave my TM unplugged when charging, just in case there is any kind of voltage/power surge. I don't want it burning out my TM. This could happen if there was a lightning strike while I was charging my batteries.
However, I typically unplug my charger once the batteries are charged up and only plug the charger back in the evening before going fishing. I also unplug it if there is even a slight chance of a T storm.
With all of that said, I have noticed a trend now that has eliminated the TM plug up front and the TM is hard wired into the boat's electrical system. So this would eliminate the ability to unplug it.