[RE-wrenches] Adding Charge Controller(s)

Bill Hoffer sunengser at gmail.com
Fri Oct 25 07:38:16 PDT 2013


I would say in general, if you are series stringing modules , current
mismatch is really bad and voltage mismatch is not an issue.   The current
in a series string is limited by the lowest current in the string and
voltage will simply add up.  Just the opposite when paralleling strings.
Current mismatch is what we typically see with shading and is why we use
diodes to protect the shaded cells.  As we all know a shaded cell will
dissipate the excess energy produced by the rest of the array in the form
of heat known as hot-spotting.  I have tested several modules with
decreased output with my curve tracer and thermal camera to discover
mismatched cells in the module that were running up to 20 C higher
temperatures than the rest of the module.  There is a point where the
mismatch is not enough to cause a diode to bypass, but still enough to
cause some hot-spotting, this can cause long term damage, but not
necessarily cause a meltdown of the cell.  End result that ovrheating in
that cell is going to deteriorate faster than the others over time.

Cell size is but one of the factors effecting current, even in the same
cell family the voltage will be fairly consistent and the current can vary
dramatically, I have seen +/- 15%.  This is just the reality in the
manufacturing process.  A module manufacturer has to sort the cells to make
sure there is not a mismatch in the module itself.  That is why you see
such a wide range of module outputs available in a specific module family.
Cell process is always changing in an attempt to get higher efficiencies,
that is why when you get replacement modules you will find the power output
will be higher.

Bottom line if you current mismatched modules in series the lower output
modules will become the bottle neck for current flow and become heaters.
Worse case they will act like they will activate the bypass didoes and
bypass those modules and you will have substantially reduced output (
diodes may fail early from overheating all the time) .  Best case scenario
they will simply reduce your potential output and become the weak link in
your system over time.

My rule of thumb, Series string match current specifications and parallel
strings match voltage specifications.

Bill


On Thu, Oct 24, 2013 at 10:33 PM, Roy Rakobitsch <roy at windsine.org> wrote:

> Not sure what this is worth, but with most Kyocera modules that used the
> same size cells (but different amount of cells per module) you could wire
> many different VOC modules in series as long as the total number of CELLS
> per string was the same. It wouldnt matter what the individual module
> ratings were (as long as the total array was mounted at the same tilt and
> azimuth, an MPPT charge controller wouldnt know the difference. Ive done
> this many times with grid tie inverters, and honestly it wouldnt make a
> difference with a charge controller either. Different age cells/modules
> may make a slight difference. But what Im suggesting is to count the cells
> in the strings.
>
> Hope that made sense.
>
>
> Roy Rakobitsch
> NABCEP Certified Small Wind Installer®
> NABCEP Certified Solar PV Installer®
> NYSERDA eligible Small Wind installer
> Certified Advanced Tower Climbing, Safety & Rescue
> Wind/PV Design Engineer
> Windsine Inc.
> 631-514-4166
> www.windsine.org
>
>
> On Thu, October 24, 2013 8:51 pm, toddcory at finestplanet.com wrote:
> >
> > VMP not VOC
> >
> > todd
> >
> >
> >
> > On Thursday, October 24, 2013 1:26pm, "jay peltz" <jay at asis.com> said:
> >
> >
> >
> >> Hi Jeffe,
> >>
> >> I don't know the specs on those Kyocera, but if they are the same VOC
> >> then they
> >> are fine together.
> >>
> >> As to a enclosure for the CC's, I'd look at Midnight.
> >> They have about a million different boxes, you'll find one that will do
> >> the job
> >> for you I'm sure.
> >>
> >> Depending on the area, you might look at a midnight 150v, or  200v or
> >> 250v
> >> controller, as you can then run all in series.
> >>
> >> jay
> >>
> >> peltz power
> >>
> >>
> >> On Oct 24, 2013, at 1:13 PM, Jesse Dahl wrote:
> >>
> >> > Hello,
> >> >
> >> > Looked at a system today that consisted of the following:
> >> >
> >> > 15 - Kyocera 158W modules
> >> > 3 -   Kyocera 185W modules
> >> > 1 -   Outback MX60
> >> > 8 -   New Trojan L16RE (48V system)
> >> > 1 -   Xantrex SW Plus
> >> >
> >> > 6 months ago he swapped out his old charge controller to the new MX60
> >> and
> >> purchased the new batteries at that time.  The array wiring did not
> >> change and is
> >> wired in 6 strings of 3.  (confusing to the MX60?)
> >> >
> >> > He is now adding 4 Kyocera 240W modules.  He also purchased another
> >> MX60 for
> >> this array.
> >> >
> >> > From my perspective, removing the 3 185W modules and getting them on
> >> their
> >> own controller would be a better use of the modules.  The system
> >> currently does
> >> not have any disconnects on the PV side.  There are fuses in the
> >> combiner boxes on
> >> the array, but no disconnects inside at the charge controller.  So
> >> adding 3 charge
> >> controllers, and now needing to add GFDI, means a box of some kind needs
> >> to be
> >> added to house these breakers.
> >> >
> >> > Outbacks GFPs and panel mount breakers will work, but what is the best
> >> mounting solution for these?  One DC Flexware enclosure? It will of
> >> course allow
> >> for all the DC OCPD  and allow for the landing of the output of the
> >> charge
> >> controllers.  Or is there another enclosure/OCPD solution I should look
> >> at?
> >> >
> >> >
> >> > Thanks as always!
> >> >
> >> > Jesse
> >> >
> >> >
> >> >
> >> >
> >> >
> >>
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-- 
Bill Hoffer
161 SE Fourth Ave
P.O. Box 1823
White Salmon, WA 98672
sunengser at gmail.com <bhoffer at sunergyengineeringservices.com>
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