'67 Caravel cigarette lighter

In our '67 Caravel when we're on city power the cigarette lighter burns up phone chargers. Is that normal? I have ruined several phone chargers before I realized the lighter in the Caravel was the problem. I take it if I switch the "Lights" univolt to battery power the phone charger should work fine? I haven't tried it since I don't want to ruin another phone charger. I would think that if this is the way the cigarette lighter works, you should never use it when you are on shore power.