Old automotive battery charger question

23hotrodr wrote:
I have an approx. 40 year old 12/6 volt automotive battery charger. When set for 12 volts and not connected to a battery, it reads 12.81 volts. Set to 6 volts and not connected to a battery, it reads 8.1 volts. When I connect to a 12 volt battery, and set for 12 volts and battery fairly well charged I read about 16 volts and the battery starts boiling right away.

Does this seem normal? Should the difference of charger output boltage be that much different when connect to a battery?

Thanks for any info you can provide. -- Mick

Does this seem normal.. Absolutely What you are seeing is the fault of your voltmeter I've assorted meters a Peak Reading meter will likelyu show the same voltage all the time. An RMS meter it will vary. and most meters are somewhere in between.

WHY
The old charger is a transformer and either two half wave or one full wave rectifier (Diode, kind of like a one-way check valve for electrons)
A common design had both primary and secondary windings centertapped. For six volt they fed power to the ends of the primary, For 12 volt one end and the center tap. Or two primary windings and series/parallel-ed them (Parallel for 12)

The secondary winding either went through a single diode, 4 diodes in a bridge. or was center tapped each end going through a diode. (All 3 designs work the same).

The output is rectified DC goes from zero to about about 16 (Theory is 16.8 but some loss in the diodes) eitehr 60 or 120 times a second. You meter is averaging or otherwise "Calculating" a reading based on that no load. but with the battery present and charged it sees the peak (16) volts. DO NOT leave this type of charger hooked up longer than needed. Boils battery dry .