I always considered external chargers to draw a bit off current from a battery so it would drain a bit if the charger was off or even unplugged. Similar to a battery in a reg mod that's not actually firing. So if my batteries aren't fully charged when I go to bed or I'm going to be away for a while, I remove the batteries. I usually remove a battery from a reg mod at night too. To my somewhat uneducated brain of thought, leaving batteries in an unplugged charger for up to 15 mins or so probably isn't too big an issue, however I wouldn't even think of leaving a battery in a charger while traveling (car, boat, train, plane or bike) or walking anywhere more than a couple hundred feet, even if I had secured the batteries to the charger like with a rubber band or something.