I have to agree with blue2000 regarding the placement of the cable, but as far as removing the battery.....although, it is the recommended way, IMHO, I think it's a bit tedious to remove the battery from the car each tiime you use the trickle charger.... I think that's fine if you're storing the car for long periods (re: winter), but leaving it in the car with the trickle charger on has not been deleterious as far as I know??? (re: unless you leave it plugged for days---although leaving the charger on for days is not good for the battery in either case) ... thawing out the battery in extreme cold conditions would be another reason I could think of at the moment that is also recommended ??? But again, ....never had a problem with leaving the battery hooked up in the car while using the trickle charger....
