The following is some (not comprehensive) info regarding this topic
and relevant excerpts should you be interested the HTTP has been
provided. - Mic 67

What the heck is an MPPT??
http://www.solar-electric.com/charge_controls/mppt.htm
"A MPPT, or maximum power point tracker is an electronic DC to DC
converter that optimizes the match between the solar array (PV
panels), and the battery bank, utility power, DC motor, or DC pump."

Understanding MPPT Step Down Voltage
FROM: http://www.wind-sun.com/smf/index.php?topic=1474.0

I will preface this topic by stating that I am pretty much a novice in
this area and have gained most of my knowledge from forums like this
one. Thanks to everyone that contributes, I look forward to
contributing once I know what I am talking about and have some
experiences to share. The following game plan may be way off the
mark. If so, do not be concerned about hurting my feelings!

I am preparing to outfit my sloop with solar and am faced with serious
space limitations. My array design will accommodate two large panels
up to 70" X 40", so I have been looking for the most watts I can
obtain within that type of space. Cost is always important but not
the most important issue. Maximum AMPS into my battery bank is the
most important issue. Certain 24 volt panels appear to be my best
bet. I have looked closely at two:
The BP Solar SX170B rated at 170Watts at 35.4 volts yielding up to
4.8 AMPS at 24 volts nominal
The Mitsubishi PV-MF170EB3 rated at 170Watts at 24.6 volts which
yields 7.3 AMPS at 24 volts nominal

I understand that the actual Watts produced will be significantly
lower than the ratings and have a pretty good idea of what MPPT can do
for me(thanks again to this forum!), and I will definitely use a MPPT
controller to gain all that I can gain and to provide the voltage
stepdown that will be needed. What I am having trouble understanding
is how the stepdown functionality will operate if the voltage coming
in should drop below 24 volts, which appears to be a very real
possibility with the Mitsubishi Panel.

The Mitsubishi Panel appears to be my best bet and it also appears to
be of superior construction, but will it cause problems for me when
the voltage drops below 24 volts?

Finally, I will provide the following demographics, that may prove
helpful. My battery bank consists of 4 Rolls 6 volt wet cells each
rated at 262 AMPS, giving me a 524 AMP Capacity @ 12Volts. My
cruising grounds are warm to tropical, and I have learned that those
conditions are the poorest for solar performance, however thats the
tradeoff for no ice and snow!
---------------
Yes that is how they work. The MPPT controller will also convert the
extra volts from the BP Solar SX170 in amps for you.
---------------
http://www.blueskyenergyinc.com/pdf/...ESB50Lrevd.pdf
-----------
I was watching my meters today and I am running a mixed panel system
with a total out of 33 amps at 12 volts but wired for 24 volts and
down converting to 12 volts. The input voltage from the panels was 30,
the input amps was 22.5, the out put to the battery voltage was 13.9
and the output amps was 45.2. This was in bright sun with some white
clouds around and 73 deg out side. The first thing to notice is the
input voltage was 30 and out put was 13.9. Shows they are over
producing at this voltage because the total rated panel amps is 33 yet
the battery bank is getting 45.2. This is only a .2 amps gain from the
input to the out put. So MPPT is not helping at all in this other than
letting me down convert from 24 volts to 12 volts and save on wire
size. Where the large gain is coming from is the two Evergreen 115
watt panels and maybe a little of the white clouds. Before I added the
evergreen panels I was getting with a PWM controller what the panels
said.

Don't you hate it when you buy the cheap stuff like Evergreen panels
and they out produce at a lower cost then the big name brand like
Shell. Back to the MPPT, on cloudy days the mppt will put out
normally about two amps higher than the input. In bright sun it is
pretty much one to one.
-------------
PV module voltage drops as the module cells’ temperature increases.
The combination of solar heating plus high ambient temperature in your
environment will likely negate any true MPPT benefit, especially
during mid-day high-current output.
---------

http://www.elecdesign.com/Articles/I...ArticleID=1823
"Build A Smart Battery Charger Using A Single-Transistor Circuit "
http://www.elecdesign.com/Files/29/1823/Figure_01.gif
---------
http://www.affordable-solar.com/dankoff.system.htm
"CHARGE CONTROLLER
I'm testing a new Solar Boosttm 50 charge controller with MAXIMUM
POWER POINT TRACKING. I have observed over 20% gain in charge current,
compared to a traditional controller. "
-------
http://www.affordable-solar.com/charge.controllers.htm
"An Introduction to Charge Controllers"
http://www.affordable-solar.com/univ...wire.chart.htm
http://www.affordable-solar.com/pv.troubleshooting.htm
http://www.thoughtscreate.com/artequipment.html
"Difference in Inverters Multi Stepped vs.Sine Wave Format "
------------------
http://www.wind-sun.com/smf/index.php?topic=1474.15
"Putting them in parallel results in a partially shaded panel losing
more like 100% , not 50% as in a series connection. The reason is PV
cells are not linear in power generation. for example, a typical 12
volt panels (KC120 for example) put out 0 amps at 21 volts ( this is
called VOC or Volts Open Circuit) , 7.1 amps at 17 volts and 7.4 amps
at 10 amps and below. If you shade a KC120, bypass diode kicks in and
now , it would have a maximum output of 11 volts , if its parallel
with other 12 volt panels, as just descirbed, 11 volts VOC is below
the other generating panels and the shaded panel supply's NO current.

Given what is out there, I would stick with an on/off controller like
the C40/C60 and have all the panels wired for 12 volts (most 24V
panels can be wired for 12V , like the photowatt panels). Mppt in the
available controllers will not do well with the rapid shading
differences and the idle losses will consume more than there
tracking/conversion abilities would generate.

Its simple, the lowest cost option and in your case the best solution
due to no mppt charge controllers that match well to you application.
Spend the money on the solar panels, getting the most watts you can
fit, this means Mono Silicon (BP) 12 volt panels and a C40 Charge
Controller"
---------
http://www.stormcopper.com/design/Am...uick-Chart.htm
"Bus Bar Design Considerations"
--------------
http://www.wind-sun.com/smf/index.php?topic=1034.0
"To MPPT or not to MPPT?"
"Being that I'm always in very warm to hot climates, and never in cool
to cold climates, is it better to:
a) Wire the PVs in series, and use the Solar Boost 50 MPPT charge
controller (as recommended to solve "high PV temp problems")
or
b) Wire them in parallel, and use the Solar Boost 2000E MPPT charge
controller or just a "bang-bang" controller (because an MPPT
controller won't be in MPPT mode very often because of the heat on the
panels)."???????

as far as mppt controllers go they mppt most of the time. as to the %
that can vary, but tends to be higher when your battery is more
drained(don't go beyond 50% dod). you will typically see about a 10%
boost from mppt.

Also, any efficiency gained from relatively short distance wiring of
the small system as a 24 V array feeding a 12 V battery will likely be
lost in the DC-to-DC conversion inside the MPPT controller. On a
related note, the SB50 would be overkill for your size system, and
operating it in a low-power configuration may further reduce overall
system efficiency.

If the panels are in series and one is shaded, then the current flow
of both of them is reduced to the level of the shaded panel.

If the panels are in parallel and one is shaded, then the unshaded one
will still produce its full rated power.

many think it is the point when the current produced by mppt action
exceeds that of one pv and with cost considerations as well for the
controllers. nowadays the pvs cost much more than controllers so this
is something to consider. for example say you have a pv at 12v
producing 7amps into a normal controller like a sunsaver 10. this
costs about $60 for that controller. now with the sb2000 at about $210
you will typically gain about 10% of the current or .7amps in this
case. is it cheaper with pvs to get another .7amps that is within the
controller cost difference of $150?

while I believe that MPPT technology can offer some benefit over PWM
technology under certain conditions, I don’t consider it to be a
panacea.

Yes, 200 Watts of PV and above is where you'd want to typically think
about using MPPT.

i didn't realize the mx60 drained that much power idling. i guess in
keeping up with everything else being of 3% loss or less maybe this
should be too. at 200w it will have a 5% loss so maybe 300w for the
mx60. i may have to check on the idle power of my sb50.
i checked on my sb50 and it seems this controller may be suitable for
lower power systems because of it's lower power draw.
1) standby power 30ma for .36w at 12v
2) charge on pwr 150ma for 1.8w at 12v
3) charge on pwr 90ma for 2.16w at 24v
this is more than likely without the digital display and i didn't find
the info yet on the display draw.

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(Mic) wrote in :

but will it cause problems for me when
the voltage drops below 24 volts

No, the controller simply disconnects it so it doesn't discharge backwards
through the panel. Those of us with lesser panels one human can lift use
just a diode in series with the power cable so it only conducts in
charging.