This is very interesting as I have many old 'Braun Hobby' units from the 1950'-60's which used to work with lead-acid accumulators and the 'Vibrator' system -- well of course all the small accumulators have long since dried up BUT they all work plugged into the mains still !! I sometimes plug them in for a while without firing a flash --- the 'charge ready' light still comes on.
