I used to always keep my batteries on the on board charger as well with maintenance mode, but reading instructions and talking to battery people, they all recommend, charging when you get back, unplug when fully charged and then recharge before you use again. I really don't know which is best. My quad and car are connected all the time and those batteries have lasted a long time, but they are used completely different as they are starter batteries.

Side note, for minkota users, I was told to never leave them plugged in while charging and it says this in the manual, which of course I didn't read until after I had to rebuild my ulterra.