BCCs: Worth noting.

> Without the diode I got ~0.7uA reverse leakage current from the batteries
> through the PV cell when the cell was completely covered.

- I have seen many many many PV designs of a very wide range of sizes.
Every one uses a blocking diode.

- When connected normally the PV cells are NOT reverse biased by the
battery when the PV panel is "dark" -  they are FORWARD biased by the
battery. It is conceivable that you have a battery voltage that is low
enough to not matter about the forward current when the panel is dark.
However, this effect is not relied on in any design I have seen.

As a guide, a 12 cell 6V, 100 mA monocrystalline silicon panel that I
just tested has the following characteristics.
Panel dark, power supply connected positive to positive, as in normal opera=
tion.

Vsupply / Current in microamps

1      7
2    27
3    80
4  300
5  1000 =3D 1 mA
6        3 ma
6.5   10 mA
7      26 mA

This panel has a rated loaded voltage of about 6V.
 It charges 3 NimH cells of about 4V Voc when fully charged.
So the battery would discharge at about 0.3 mA into the dark panel.
This level of discharge MAY be acceptable depending on other criteria.

When fully discharged the battery would be about 3V and providing about 80 =
uA.

Even a very small amount of light on the panel would bring the voltage
up to at least part of maximum and further reduce the discharge
current.

In most cases (see my prior comments) there is no great gain energy
wise in eliminating the diode as the panels run essentially as close
to constant current sources .

Battery discharge current will be reduced by any illumination of the panel.

As a guide, under fluorescent lighting on a workbench the above 6V,
100 mA panel produces 3V o/c and about 100 uA s/c. I'd estimate
lighting level (not tested) to be about 100 lux.

Increasing the number of PV cells per battery-cell would reduce the
dark discharge current but not make economic sense.


      Russell

>
> I tested the following diodes that was readily available in my supply:
>
> PMEG3050EP =A030V
> SK14B 40V
> 10BQ040 40V
> SD103AW-V-GS08 40V
>
> The PMEG3050 did not reduce the reverse leakage current. The SD103AW redu=
ced
> the leakage current to ~0.25uA and the others where somewhere inbetween.
>
> Forward voltage drop was less than 100mV at the ~4mA charge current for a=
ll
> diodes (@battery voltage ~1.3V).
>
> So I guess this means that the diode is not needed in my circuit.
>
> Perhaps the inherent diode in the cell has a larger forward voltage drop =
than
> the batteries will supply so the current is not drained from the battery =
even
> when the PV cell is not generating voltage/current?
>
> I guess it would be cruicial for the original design if the panel dischar=
ges
> the battery at the same time as the LED is on.

--=20
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