=A0 London SW
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Not only switching power supplies!!!!!
A year or two ago we repaired a car battery charger (can't remember what the problem was). To check we then hooked up a DMM to the output switched on and got some real funny voltage readings. Seemingly ow voltage IIRC.
Oops what's going on? Until we suggested that what the digital meter was seeing from the battery charger output was completely unsmoothed and unfiltered rectified voltages varying between zero and some sort of rectified peak AC voltage. We would have been better off using an analog meter!
Sure enough when we hooked up a spare car battery charging current flowed into the battery and the voltages made sense. A suitable charging voltage for lead acid batteries is around 2.3 volts per cell (maybe a little higher). So for the so called "12 volt car battery" 6 cells at 2.3 volts each =3D 13.8 volts. That's very close to the 14 volts often mentioned.
Commercial outfits often float their batteries (just maintaining them at full charge) at either 2.15 or 2.17 volts per cell. Doesn't sound like much of a difference but avoids boiling the life out of a fully charged battery.
So 6 x 2.15 =3D 12.90 (British Telcomm.) and 6 x 2.17 =3D 13.02 (Old AT&T and Bell System Spec.) Except these were usually for telephone exchange battery strings for a nominal 48 volts. i.e. 24 x 2 =3D 48 volts.