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Paralleling Solar Panels - Can you do this? - Page 11

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Posted by m II on June 15, 2010, 8:50 pm

vaughn wrote:





My panels' current output varies DIRECTLY with the load, up to the
maximum amperage allowed by the design and solar stuff hitting them.

They do, however vary output inversely when compared to the state of
charge in the batteries. At around a 15 volt charge, all input current
stops. This is with the Morningstar 30 amp controller.

I would expect the same effect with a direct connection to the
batteries, but the panels would probably try to keep charging, slower
and slower, until something approaching 16 or 17 volts is reached. That
wouldn't be good.

The variations in output caused by changing light conditions AND
changing loads guarantees that the needs of a 'constant current'
definition can NOT be met.

I'm working on a 555 timer circuit that would act as a dump switch. When
the voltage hits over 14, the timer is triggered for a minute or two. It
turns on a triac which allows a small air compressor to run. I have a
big air tank which would take a few days to fill.

This would be using what otherwise would be wasted power. I guess the
air pressure limit switch could be used to divert power to a water
heater if the pressure is up.

I've started messing around with an Arduino microcontroller. A good
experience. Almost every bit of programming code you could ever need is
freely available. There are twenty switchable input/output ports. A
serial port is built in.


Posted by vaughn on June 15, 2010, 9:29 pm

Of course!  Mine does too.  I don't know what I was thinking.

My little controller (Trace C12 drops to some low current when the batteries are
full.  Since there is no separate dump load, I really don't know where it is

Yes, I have seen the result of that strategy...dried out batteries.

My point exactly.

I really out to get me a kit for something like that.  I can't think of any
particular application, but they sure are nest!


Posted by m II on June 15, 2010, 10:19 pm

vaughn wrote:

Six of the 20 inputs can be used in an analog read or write manner. With
a ten bit range, they can read 1023 degrees of voltage (from 0 to five
volts). A series resistor voltage divider is used to get the 5 V max. An
op amp could be used to boost the signal if way below 5 V to begin with.

A temperature sensor on one of the pins could be related to the battery
voltage and be used to make a pretty fancy self compensating battery

Another use could be using the Arduino as a sine wave generator at 60
Hz. You could drive two mosfets which switched alternate windings of a
centre fed transformer primary. 12 VDC into the centre tap and out the
mosfets to ground. AC out on the secondary.

You'd get a crude inverter for not much more than the cost of the
transformer. The Arduino output could be changed to provide everything
from almost pure sine wave to a simple square wave. The pulse width
modulated pins allow that fairly easily.

The variable pulsed output can be used as a speed control for a small
electric bike. Msfets like to be fully on. They are the  most efficient
when not asked to drop voltage too much. PWM is the way to go.

Two pins could be used to measure light activated resistors in voltage
divider circuits. That makes it fairly easy to switch some relays in a
solar tracker setup. An 'H' circuit lets you reverse a DC motor fairly

One user was regulating the air supply to a wood burning fireplace.
Another was using it to work the throttle on a diesel powered wood
cutter. The uses seem almost endless. Youtube has some interesting stuff.

These people have a really good sampling of sensors and things:


The home page...

The basic board, with a processor included is roughly thirty dollars US.
When you need more processors, they cost roughly five to six dollars
each. Three dollars if bought in bulk. They are all free standing after
being programmed. All you need is a battery.

The software interface, free, with tons of sample programs, is available
for Mac, Linux and that other operating system whose name escapes me
right now.


Posted by Johnny B Good on June 15, 2010, 3:07 pm

 I've just tested a couple of cheap solar panels designed to provide a
70mA trickle charge intended to mitigate the self discharge of a car
(automobile) battery when left unattended for any lengh of time, just to
confirm what I already 'Know'.

 The truth of the matter is that they're a combination of voltage
generator and constant current generator with a bit of ohmic resistance
thrown in for good measure.

 The figures for those cheap panels were OC voltage 22v, SC current
180mA (both panels in parallel) and feeding a 12v 300mA case fan, 9.5v
at 150mA.

 A simple calculation to work out the resistance of our 'generator'
using the short circuit current method, yields a figure of 122.2 ohms
yet the volt drop with that fan load is 12.5v at .15A which gives us
83.33 ohms. A while back I was charging an AGM 12 volt battery and
getting a charge rate of about 110mA at 14v which gives us a resistance
of about 67 ohms.

 In the case of a PV panel, you won't get maximum power transfer when
the load drops the OC voltage to half. In fact, the optimum loading is
somewhere in the region of 70% of the OC voltage (when the load
resistance is about twice the 'resistance' of the solar panel).


Posted by Josepi on June 9, 2010, 1:38 pm

The diodes are not "blocking diodes" The diodes are "bypass diodes", in
parallel for when panels are in series and one is shaded, the other panel's
currents can bypass (and not reverse current through) the shaded panel.

The PV panels are diodes anyway and will not conduct in reverse. They are
almost always in parallel at you combiner panel or equipment, anyway.

That would be much more of an issue for series panels than parallel.  Panels
usually have blocking diodes installed internally, so a shaded panel simply
contributes little or no charge current, but causes no problems.


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