Solar Battery Charging (2xAA)

I have searched the Internet for ways to charge batteries via solar energy. I know that I can charge a NiMH battery forever as long as the current doesn`t go over a tenth of the battery`s capacity (which is, in my case, 245mAh). Because I will be using two NiMH batteries connected in series I will use the following Photovoltaic module: I guess that I`ll connect 2 of these in parallel which should give me 44mA. This would take 55 hours to charge my two completely depleted batteries (2450/44) o_0. But I`m fine with that. Anyways, the thing is I can`t really understand what diode I should use to block reverse current. Apparently, the battery depletes itself through the solar panel during the absence of light. The solution to this is to use a 1N5817 DIODE. That`s cool but I don`t really understand why it`s there and what sort of diode it is. Is there any other way to stop the battery from depleting itself? Apart from being just another drop in the huge ocean of diodes, transistors, regulators, and whatever else, the 1N5817 diode is probably sold online (which implies postal fees for something worth barely 1-2$).

By the way there`s a cool tutorial on solar battery charging: