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

[Randall]

> The charger is 10 amps 
> continuous, 30 amps fast boost (duty cycle 10 minutes on, 12 
> minutes off), and 50 amps motor starting (duty cycle 2 
> minutes on, 5 minutes off)

That's interesting.  I have an old Schumacher charger with very similar, if
not identical, ratings; but it only uses two diodes.  I guess they decided
that two more diodes were cheaper than the extra transformer winding.

Anyway, when the original diodes gave out (and it was given to me), I
repaired it with the diode plate from an old car alternator.  Car
alternators are 3 phase and usually only one diode fails, so you can still
get a full-wave (single phase) rectifier out of a bad alternator.  And they
are some seriously heavy duty diodes, especially if you arrange for some air
flow across the mounting plate.  I just hung it on some standoffs, and let
the air convect over it.  That had to be over 20 years ago now, and it's
still going strong.  In fact, I used it to start the TR3 yesterday (looks
like maybe my battery is going bad).

But a MR2502 is apparently only rated at 25 amps, which seems way low for a
50 amp charger.
http://goo.gl/5OBz8

Maybe that's why they didn't last very long?

If you don't want to mess with alternator diodes, here's some that should be
good enough.  60 amps continuous, over 1000 amps peak. $3 each.
http://goo.gl/MJHBO
Here's a datasheet
http://goo.gl/3iYq0

Be sure to use insulators where appropriate, and a little heat sink compound
wouldn't hurt either.

-- Randall