Absolutely no need to disconnect the battery from the vehicle if using a charger such as a Ctek brand.
I give a full maintenance charge overnight every couple of months to my VZ Calais and parent's VE Berlina without disconnecting the battery of either car using my Ctek charger. I actually use the connections under the bonnet of the VE as opposed to connecting direct to the battery. Connecting directly to the battery or direct to the under bonnet terminals makes no difference except I find the under bonnet connections more convenient.