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Rick, Rat Shack has been doing so little component business that in many stores the little stock remaining has been moved to parts bins in the back room--literally.
While they may have high power diodes, a 300W panel array means roughly 20 amps at 15 volts, and a properly chosen diode would then be at least a 30A rated part. Sure, you can get diodes with a 0.7V drop, but wouldn't you rather use diodes meant for the application, with only a 0.2 or 0.3 volt drop? The difference is that a lower-loss diode will allow you to keep charging the battery while the source voltage has dropped 4/10ths of a volt--which is a significant difference when you consider 4/10ths of a volt is also a 40% difference in battery charge state.
I don't know how long ago the college classes were, but you might want to check out the local RS stores. These days? Cell phones, noisy toys, batteries, and $5 cables priced at $21.99 are about it.
And those low-loss diodes? I'm vaguely thinking Schottky or Fast Epitaxial? 25V rated (minimum for a 17-20V raw supply) 30A or better? Sorry, RS doesn't carry low-loss power diodes at all. The only thing they currently stock is a 25A 50V regulator diode bridge. Wasting .4v at 25A all day long, if you have to go that way. Maybe that's only 6 watts for ten hours?...but it's still a waste. 21 or 22 kWh per year? (Remember, I'm not licensed to practice mathematics.)