Voltage too high

[Gavit]

Vintage Battery Charger - Voltage too high

Hi Everyone

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?

Thank you in advance.

 

[Diver300]

Those voltages sound about right.

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.

 

[MikeMl]

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"/

 

[BrownOut]

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.

 

[Gavit]

Thank you all for that. Does anyone know what the 'on load' voltages should be?

 

[MikeMl]

Thank you all for that. Does anyone know what the 'on load' voltages should be?

Go to PowerStream
Click on "Technical Resources"
Click on "How to Charge Sealed Lead Acid Batteries"

This applies to flooded cell batteries, too.

 

[Gavit]

Thank you, all, I appreciate your help.