In a simple classic car 12v system there's no difference between connecting a charger or conditioner to the battery terminals, or to a ciggy lighter socket. The 12v socket is just an extension of the battery terminals.
But I don't know whether complex electronics make a difference to that, and I would ask the factory - just in case.
Portable easy starters? check the manual that comes with the kit. My gut reaction is that it would work better direct to the battery terminals.
