I have inherited from my father a 5 amp battery charger (lead acid) which he made sometime around 1940. It has a Sangamo Western ammeter which was returned to the factory for a complete rebuild a couple of years ago (They asked if they could buy it from me for their museum but it has too much sentimental value to me).
By flicking a switch the battery charger is intended to charge at either 6v (actually need around 7.2v) or 12v (actually need around 13.5v - 14.2v). Unfortunately the actual voltages are 9.6v and 19 volts.
Can anyone help with a simple method to bring the voltages back into line?
In 1940 the only rectifiers available would have been selenium, which have quite a lot of resistance. It is normal to have quite a lot of excess voltage when there is no load. The current will be about the 5 A when the voltage is at around 7 V or 14 V depending on the switch position.
There is no cut-off, and chargers like that can overcharge the batteries eventually.
Modern cheap chargers are similar, and give out quite a lot more voltage than 7 or 14 V.
That charger is a great museum piece, but I would not use it on my batteries unattended. If you are sitting there with a modern DVM connected watching the battery charge, you could manually shut off the charger when the battery reaches a predetermined voltage, which is effected by temperature, the age of the battery, and several other considerations.
Yes, you could add a "cutoff" circuit, but then the charger would not be "original"/
If the cutoff circuit switches the mains power, you can keep the charger original and make it safer. I have a few safety cutoff circuits that simply switch a mains socket mounted in an electrical box.