PV Gap

Jamie Said:

How to charge a 9v rechargeable battery with a solar panel?

We Answered:

First off, you need a voltage converter, to convert the 6V to 12V or so, or get two 6V panels and wire them in series.

Once you get enough voltage to charge the panel, you just connect an anti backfeed diode to the panel, and a FET transistor hooked up to a voltage comparator of some sort, and a hysteresis circuit, so it has a bit of a gap between on and off voltage.

I have a solar charge controller which uses a TL431 (if you are a home hobbyist, you can scavenge one from a switch-mode power supply, many use them; next to the opto-isolator, on the low-voltage side). It adds hysteresis by adding a resistor in parallel with the sampling resistor, when the TL431 is "on", which brings down the cutoff voltage point.