photovoltaics in parallel

R

rabiticide

Guest
I live in the woods, so my solar powered walk lights do not charge
very well in the winter months. I decided to add another solar panel
in parallel to the first -- just solder it in -- then I would be able
to charge the batteries with less light.

I checked the voltage while shining a light on it and it read 6V
(which makes sense to charge 4.5V worth of AA's) so I ordered another
6V photovoltaic cell.

Is this design sound? I was just wondering what could go wrong that I
am overlooking. Thank you for any advice / validation.

rK
 
"rabiticide" <rabiticide@gmail.com> wrote in message
news:1673c28d-d673-4f1b-9f5a-b45dc39c35dd@k1g2000prb.googlegroups.com...
I live in the woods, so my solar powered walk lights do not charge
very well in the winter months. I decided to add another solar panel
in parallel to the first -- just solder it in -- then I would be able
to charge the batteries with less light.

I checked the voltage while shining a light on it and it read 6V
(which makes sense to charge 4.5V worth of AA's) so I ordered another
6V photovoltaic cell.

Is this design sound? I was just wondering what could go wrong that I
am overlooking. Thank you for any advice / validation.

rK
As with any voltage sources in parallel you will have a condition where
Click to expand...
their outputs are not the same. Currents will circulate in their internal
resistances and may harm them or at least produce poor charging results. You
could isolate the panels with diodes and lose a diode drop in voltage which
in this case probably won't matter much.

Tom
 
That's what I was afraid of, to tell the truth. So just one diode on
one positive lead? or both? Now that I think about it, I think I
will need it on both 'cause I don't know which voltage will be
higher. I'm going to go scribble it out...


rK
 
On Sun, 09 Nov 2008 16:19:59 -0500, Tom Biasi wrote:
"rabiticide" <rabiticide@gmail.com> wrote in message
news:1673c28d-d673-4f1b-9f5a-b45dc39c35dd@k1g2000prb.googlegroups.com...
I live in the woods, so my solar powered walk lights do not charge
very well in the winter months. I decided to add another solar panel in
parallel to the first -- just solder it in -- then I would be able to
charge the batteries with less light.

I checked the voltage while shining a light on it and it read 6V (which
makes sense to charge 4.5V worth of AA's) so I ordered another 6V
photovoltaic cell.

Is this design sound? I was just wondering what could go wrong that I am
overlooking. Thank you for any advice / validation.

As with any voltage sources in parallel you will have a condition where
their outputs are not the same. Currents will circulate in their internal
resistances and may harm them or at least produce poor charging results.
You could isolate the panels with diodes and lose a diode drop in voltage
which in this case probably won't matter much.

That shouldn't be necessary as photovoltaics are already diodes. They
Click to expand...
should current-share just fine.

Why not get a coulple of (milli)ammeters and check?

Cheers!
Rich
 
"Rich Grise" <rich@example.net> wrote in message
news:pan.2008.11.10.21.51.26.178887@example.net...
On Sun, 09 Nov 2008 16:19:59 -0500, Tom Biasi wrote:
"rabiticide" <rabiticide@gmail.com> wrote in message
news:1673c28d-d673-4f1b-9f5a-b45dc39c35dd@k1g2000prb.googlegroups.com...
I live in the woods, so my solar powered walk lights do not charge
very well in the winter months. I decided to add another solar panel in
parallel to the first -- just solder it in -- then I would be able to
charge the batteries with less light.

I checked the voltage while shining a light on it and it read 6V (which
makes sense to charge 4.5V worth of AA's) so I ordered another 6V
photovoltaic cell.

Is this design sound? I was just wondering what could go wrong that I am
overlooking. Thank you for any advice / validation.

As with any voltage sources in parallel you will have a condition where
their outputs are not the same. Currents will circulate in their internal
resistances and may harm them or at least produce poor charging results.
You could isolate the panels with diodes and lose a diode drop in voltage
which in this case probably won't matter much.

That shouldn't be necessary as photovoltaics are already diodes. They
should current-share just fine.

Why not get a coulple of (milli)ammeters and check?

Cheers!
Rich

Good advice but not all panels are constructed the same. Some will even
Click to expand...
_draw_ current at night.
It is very typical to iolate the panels with Schottky Diodes.
His may not need anything, a simple test as you suggest will determine.

Tom
 
In article <pan.2008.11.10.21.51.26.178887@example.net>,
Rich Grise <rich@example.net> wrote:
On Sun, 09 Nov 2008 16:19:59 -0500, Tom Biasi wrote:
"rabiticide" <rabiticide@gmail.com> wrote in message
news:1673c28d-d673-4f1b-9f5a-b45dc39c35dd@k1g2000prb.googlegroups.com...
I live in the woods, so my solar powered walk lights do not charge
very well in the winter months. I decided to add another solar panel in
parallel to the first -- just solder it in -- then I would be able to
charge the batteries with less light.

I checked the voltage while shining a light on it and it read 6V (which
makes sense to charge 4.5V worth of AA's) so I ordered another 6V
photovoltaic cell.
As with any voltage sources in parallel you will have a condition where
their outputs are not the same. Currents will circulate in their internal
resistances and may harm them or at least produce poor charging results.
You could isolate the panels with diodes and lose a diode drop in voltage
which in this case probably won't matter much.

That shouldn't be necessary as photovoltaics are already diodes. They
should current-share just fine.
Click to expand...
They are diodes, but the orientaton is the reverse of what your isolation
diodes might be. The open circuit voltage of a pv panel is its Vf, basically.

This causes a problem if panels are parallelled and one is in shade, or even
if you have a single large panel in partial shade: some of the power
collected by one panel ends up getting consumed by the other. I assume it
acts like a big, diffuse infrared LED.

My gut feeling is that the OP's plan of adding another panel would
work fine, as long as the two panels have the same number of junctions/
open circuit voltage and are both in about the same amount of light.


--
Wim Lewis <wiml@hhhh.org>, Seattle, WA, USA. PGP keyID 27F772C1
"We learn from history that we do not learn from history." -Hegel
 
On Tue, 11 Nov 2008 08:09:12 -0000, Wim Lewis <wiml@hhhh.org> wrote:

In article <pan.2008.11.10.21.51.26.178887@example.net>,
Rich Grise <rich@example.net> wrote:
On Sun, 09 Nov 2008 16:19:59 -0500, Tom Biasi wrote:
"rabiticide" <rabiticide@gmail.com> wrote in message
news:1673c28d-d673-4f1b-9f5a-b45dc39c35dd@k1g2000prb.googlegroups.com...
I live in the woods, so my solar powered walk lights do not charge
very well in the winter months. I decided to add another solar panel in
parallel to the first -- just solder it in -- then I would be able to
charge the batteries with less light.

I checked the voltage while shining a light on it and it read 6V (which
makes sense to charge 4.5V worth of AA's) so I ordered another 6V
photovoltaic cell.
As with any voltage sources in parallel you will have a condition where
their outputs are not the same. Currents will circulate in their internal
resistances and may harm them or at least produce poor charging results.
You could isolate the panels with diodes and lose a diode drop in voltage
which in this case probably won't matter much.

That shouldn't be necessary as photovoltaics are already diodes. They
should current-share just fine.

They are diodes, but the orientaton is the reverse of what your isolation
diodes might be. The open circuit voltage of a pv panel is its Vf, basically.

This causes a problem if panels are parallelled and one is in shade, or even
if you have a single large panel in partial shade: some of the power
collected by one panel ends up getting consumed by the other. I assume it
acts like a big, diffuse infrared LED.

My gut feeling is that the OP's plan of adding another panel would
work fine, as long as the two panels have the same number of junctions/
open circuit voltage and are both in about the same amount of light.
Click to expand...
But as the sun moves across the garden, there will be a time that one is in complete shade and one is in complete sunlight.

--
http://www.petersparrots.com http://www.insanevideoclips.com http://www.petersphotos.com

A single blonde pregnant girl goes to the grocery store. A couple that she knows notices she's pregnant.
The lady asks her, "Whose baby is it?"
The blonde says, "Well, I don't know they are going to do blood tests, but I think it's mine."
 
On Nov 9, 12:31 pm, rabiticide <rabitic...@gmail.com> wrote:
I live in the woods, so my solar powered walk lights do not charge
very well in the winter months.  I decided to add another solar panel
in parallel to the first -- just solder it in -- then I would be able
to charge the batteries with less light.
Click to expand...
The usual approach is to put the cells in series, and let the
load (the battery under charge) draw the series down
in voltage. This is because a common failure mode of
solar batteries is for an individual diode to short out, and
that doesn't hurt a series connection.

You may find that parallel operation doesn't charge as well (and
that it needs more light to start charging).

Don't worry about overcharging, or making too high a voltage; the
battery will regulate that.
 
On Tue, 11 Nov 2008 19:56:29 -0000, whit3rd <whit3rd@gmail.com> wrote:

On Nov 9, 12:31 pm, rabiticide <rabitic...@gmail.com> wrote:
I live in the woods, so my solar powered walk lights do not charge
very well in the winter months.  I decided to add another solar panel
in parallel to the first -- just solder it in -- then I would be able
to charge the batteries with less light.

The usual approach is to put the cells in series, and let the
load (the battery under charge) draw the series down
in voltage. This is because a common failure mode of
solar batteries is for an individual diode to short out, and
that doesn't hurt a series connection.

You may find that parallel operation doesn't charge as well (and
that it needs more light to start charging).

Don't worry about overcharging, or making too high a voltage; the
battery will regulate that.
Click to expand...
PV cells are constant current devices. If the total is 50 volts going into a 12 volt battery, you won't get more amps. You presumably lose energy to heat in the PV cells, which is probably not good. My solar power system has an MPPT unit (maximum power point tracker), which is basically a switched mode power supply which takes whatever voltage the solar cells want to give out but still maintaining a good current, and converts it to whatever voltage the batteries are at. It gains 30% power over just connecting the PVs straight to the battery.

--
http://www.petersparrots.com http://www.insanevideoclips.com http://www.petersphotos.com

Those giraffes you sold me, they won't mate.
They just walk around, eating, and not mating.
You sold me queer giraffes! I want my money back!
 
Check this out -

http://www.national.com/analog/solarmagic

I haven't read it in detail but I suspect it does not say exactly how it
solves problems of shading etc, as it seems aimed at OEM designers who
would talk directly to them. However it will help you understand what
the issues are, and may give you some ideas on how to approach the
problems.
--
Nemo
 
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