All the chargers I've seen on the market recently have included non-live croc clips, so the output isn't live until the charger sees some voltage, and some need more voltage that a really flat battery will have. This is a pain if you want to use the charger as a power source, e.g. to test a headlamp. The trick to charging a completely dead battery is to parallel it with a working battery before connecting the charger until it has enough voltage for the charger to recognise it as a battery.
Like Ian, I have a really ancient (1960s) one with a selenium rectifier, and also a more recent Cha