On Wed, 04 Nov 2009 18:17:34 -0700, D Yuniskis
wrote:

Franc Zabkar wrote:
On Wed, 04 Nov 2009 15:09:47 -0700, D Yuniskis
put finger to keyboard and composed:

I've rescued several small PV panels along with a pair
of gel-cels from an electric wheelchair that still have
useful life. I'd like to put together a charge controller
to run the irrigation pump "for free".

Any pointers to suitable designs? The PVs are small
so Isc is on the order of 100-200mA (though I may
double that)

If the battery capacity is big enough, and the PV panel voltage is not
too much higher than battery voltage, then you won't need a regulator,
only a 1N5817 Schottky diode to prevent the battery from discharging
into the panel.

This is inefficient. You want to find the "sweet spot"
where you get the most power from the panel. I.e.,
at no load you get no power and at short circuit the
same applies.

Ideally, you want to coax the most from the panel by
operating it where it "wants" to be operated. Then,
move this into the battery incurring the least losses
possible in the process...

aka Maximum Power Point Tracker. Google MPPT