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Just thinking: Charge controloers

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Posted by Gordon on November 2, 2009, 12:12 am

Just thinking here.  That's about all I can do right now.
About a future homebrew solar power system.  And the
necessary charge controler.

What is the common wisdom on charge controlers?
Is the panal just disconnected from the battery
when full charge is reached?

Or can the panal be connected to a dump (diversion)

If a diversion load is used, is the battery
disconnected at full charge?  Or is the
diversion load connected across the battery/panal
If the diversion load has the same rating as the
panal, it should keep the battery from receiving
additional charge. The only problem I can see is
if the diversion load should drop out. That would
cause the battery to over charge.

Posted by Josepi on November 2, 2009, 12:28 am

Diversion load regulation is usually only used for wind turbine applications
where the turbine must maintain a load or run amuck. Solar PV uses a series
charge control where the PV panels can stand an open circuit.

A MPPT is typically used to maximize the charge out of a PV system. Think of
it as a variable DC/DC transformer that can find the best power curve point
under different sunlight and battery charge conditions. This means your PV
panels can put out 80 volts (if that is your best power curve point) and
deliver 53 volts to your batteries. They don't typically deliver over about
105% of input voltage due to the method of DC transformation.

Posted by vaughn on November 2, 2009, 2:18 am

In a perfect world, the charge controller would be smart enough to allow the
panels to continue to supply just enough power to supply system loads while
preserving the full charge in the battery.  Sometimes residual uses are
found for the heat from a dump load (such as water heating) but other than
that, nobody has yet explained to me the reasons for a dump load in a PV
system.  The panel can hardly overspeed like an unloaded wind turbine.  I
have never heard of a panel being damaged by its own open-circuit voltage,
but perhaps someone here will educate me.

My controller seems to put a partial load on my panels when full charge has
been achieved.  I assume that the controller is just shorting out the panels
intermittently with a PWM arrangement, thus the panel's internal resistance
is serving as the dump load, but I don't (yet) know why they bother.


Posted by Josepi on November 2, 2009, 4:14 am

Perhaps the extra heat generated in the panels may melt the snow off or dry
the rain? They may have been smarter than we give them credit for.

Posted by m II on November 3, 2009, 3:52 am

Josepi wrote:

Those two conditions would act as self limiting features, eliminating
the need for shorting entirely. Shadow is mother nature's voltage regulator.


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