SP10 blocking diode [RE-wrenches]

[R. Sparks Scott]

	Pat;

	BLOCKING DIODE??!!!!! I thought those were completely dead and buried!
You'll waste many times more power in the blocking diode during daytime
charging than you could force back through the panels in a month of
complete darkness. Conisder the following:
	While it is true that there is some reverse current flow in a PV panel at
night, the value of the current flow is quite small. My 12 volt array of
400 watts has a reverse current of only .03 amps. Figuring a 10 hour dark
period, the total ampere-hour consumption overnight would be .36 Ah or
about 3.6 watt hours.
	A blocking diode in my system would have to pass about 20 amps. The
voltage drop across even the best diode would be at least .5 volts.
Calculating the power loss in the diode: 20 amps x .5 v =10 watts. Take the
10 watts times 10 hours, 10W x 10 h = 100 watt hours.
	From this we see that I would lose 27 times the useable power in the diode
than I will lose overnight in reverse current to the PV's.
	Unless you have some other reason for needing to isolate the panel from
the batteries, the conventional wisdom these days is to leave the diode
out, reap the added power during the day and let what little is lost in the
panel at night go.

	-S

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