[emeritus]

I have an early 1970's 6V/12V battery charger that I still use occasionally in winter to keep the battery of my wife's car in good condition, as it spends most of its time doing short journeys. The charger is wall mounted and I made up a lamp board with a variety of 6V and 12V bulbs to select and limit the charging current, useful when charging smaller-capacity non-car batteries. For trickle charging car batteries I usually stick a 12V 21W bulb in series, which limits the current to about 1A and ensures that any excess voltage gets dropped across the bulb. It normally charges at about 3.5A when connected directly to a 12V car battery as measured on its moving iron ammeter. I have never tried checking its waveform.