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

From: "Randall" <[email-masked]>
Date: Thu, 7 Mar 2013 08:27:55 -0800

Think of the output of the transformer as a sine wave.  It goes smoothly
from peak to peak, but spends very little time at each peak.  Diodes only
conduct when the anode is more positive than the cathode.  So in a battery
charger, the diodes only conduct (to produce output) when the voltage is
near the peak, higher than the battery voltage.  The rest of the time, you
are reading battery voltage.  Without the battery, the average voltage is
much lower.

-- Randall 
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From: "Randall" <[email-masked]>
Date: Thu, 7 Mar 2013 21:20:52 -0800

> 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 

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