I have a 1999 ML430 which has 80,000 km on it. Still has the original battery. I am not driving it that much these days now that winter is here and with the price of gas. I may drive it once every 2 or 3 weeks so the battery doesn't get fully charged just by the amount of driving I do now. It is garaged always and the temperature inside is typically around 0 deg c. I would like to keep the battery charged up. I have a battery charger with automatic shut off. What is proper way of charging the battery. The Mercedes manual say the battery should be disconnected before putting on the charger. I have two other Volvos that I usually connect the charger with battery still connected with no problem. Is there a reason that the ML is different? May be because of its electrical system? It just seems a lot of work to disconnect the battery every time I put on the charger.
If I have to disconnect the battery, is there a procedure I need to follow and also in reconnecting the battery?

Thank for any help.