[Shop-talk] Which diodes for a battery charger?

[Randall]

> So if I measured min/max rather than just the default voltage 
> (rms on my Fluke), I should see something like the 15 volts 
> it's currently showing, while charging a very dead battery ?

Yup.  Open circuit (without the battery), min would be zero, max probably
closer to 18 volts.

> Also - I'm still seeing a little under half a volt AC at the battery.
> Normal?

Yup again.  Remember the charger is only putting out pulses of charging
current at the peak of each 120 Hz half wave.  Those pulses are higher
voltage than your battery, so the voltage drops off between pulses.  On the
AC setting, your voltmeter is seeing the pulses as AC.

Although not strictly correct, the diagram and waveforms shown here:
http://goo.gl/iwukv
under "A Single Capacitor" are pretty close to what is happening, with your
battery playing the part of the capacitor shown in that circuit.

-- Randall