Solar Powered MintyBoost

Question

I've decided to try and make a Solar Powered MintyBoost. I've drawn up a very rough version of a schematic:

As you can see, I plan to use a solar cell to charge a set of 2 D batteries (in series), using a Diode to prevent the batteries from discharging through the PV module. The batteries are then used to power the MintyBoost circuit, and thus charge my device. Is this a sound design?

I was planning on using for of these cells in parallel to provide 4.5 V and max 320mA. However, this would take a long time to charge 2 10,000 mAh batteries (a long time being ~62.5 hrs). Can anyone suggest a better solution for the solar panel? I haven't been able to find many ~4V panels that can provide much current. Would it be a problem to use a module that provides more than 5V, seeing as how the LT1302 is meant to operate with an input voltage below 5V (though the datasheet says up to 10V)? Or would the batteries be forcing that node to 2.4V?

Answer 1

Since the batteries are in series, the current running through the one goes through the other, you do not get 20,000 mAh, you still only have 10,000 but at double the voltage. If you charged them at 320mA, it would only take 31.25 hrs as the 320mA runs through them both at the same time.

What you need is a Buck converter. If you don't know or want to design one, Digikey sells them for about a $1. Digikey Buck Converter. That will convert the 4 - 4.5 V of the solar cell to 2.5 volts, but at much higher current. 2.5V is good as it should charge the batteries easily.