I purchased a Ctek MXS 5.0 battery tender to keep my battery topped up while the car is in winter storage. Upon reading the instruction booklet and watching a few videos online from Ctek, the instructions state that if the battery is removed from the vehicle then simply connect the positive and negative cables to the corresponding battery terminal. However, if the battery is still connected in the vehicle, place the positive cable to the positive battery terminal, but connect the negative cable to a suitable ground point on the vehicle? Why do they recommend not connecting to the negative terminal if the battery is still connected?

 

Maybe to prevent the possibility of a spark in an enclosed area around the battery since there may be hydrogen gas present. I've heard the same recommendation in relation to attaching jumper cables to cars.

I think the spark problem could be avoided if you connected the battery first before you plugged in the charger.