I've tried searching on the net for characteristics of photovoltaic cells,
and am now well informed as to their chemistry, efficiency and so forth, but
can't yet explain my observation.  I'm attempting to power a PIC
microcontroller and LCD as a clock using a memory back up capacitor (1.0F)
and two 5-cell solar panels.  They're rated at 3.0v / 50mA in full sun, but
washington weather rarely sees that.  However, the pic and LCD only requires
2mA, and the pic will turn off the LCD at 4.5v and below, so then the
current draw drops to well below 1mA... the cap will go from 4.5v to 2.0v
over about 20 hours.

Heres the problem: when i run the solar cells in series in low lighting
conditions (only 2v per cell with no load) and attach a small load (a charge
pump circuit, the charge pump only consumes a few uA once fully charged,
minus cap leakage), i'll only measure 1.5 across the two elements.  the top
one will get 2v drop across it, and the bottom one gets -.5v across!?  If i
replace with only one solar unit (either one), i get the same 1.5v drop, and
the charge pump reaches the same max voltage (i think it was about 2v max, 3
stages - i'm aiming for 6v).  Diagram:

 1.5v    +----------+          1.5v  +-------+
         |          |                |       |
        ---         |               ---      >
         - ,\       |                - ,\    >   R-load
         |          |                |       >
         |          > R-load         |       |
-0.5v    +          >           0.0v +-------+
         |          >
         |          |
        --- ,\      |
         -          |
         |          |
0.0v     +----------+

Does anyone know of any resources out there that can explain the effect i'm
seeing?  Is the solution obvious and i'm just missing it?   And third, how
do i eliminate this effect?

Thanks in advance.
-Aaron

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