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12 volt panels....

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Posted by Merlin-7 KI4ILB on December 8, 2006, 12:20 am
Just curious...
 What happens if....

 You have 1kw of panels, wired in parallel for a 12 volt battery bank , with
no blocking diodes other than the ones built into the panels...and one has a
problem, be it a bad blocking diode, panel gets shot out by some kid with a
22 rifle, etc. ?

 Would it be better to fuse each panel or add diodes to the output of each
panel and just eat the .7 volt loss?


Posted by Solar Guppy on December 8, 2006, 12:56 am
Panels have bypass diodes, not blocking diodes. Blocking diodes are for
night-time to prevent the batteries from being drained from the dark panels
which are a load. Most charge controllers have either diodes or relays to do
this so adding a blocking diode is not needed, so no concerns with the 0.7
volt drop as you don't need the diode.

As for you thoughts on a parallel strings, it is required by code and
manufactures of the panels you must not exceed the current rating of the
panel, in any condition. This generally means you can direct wire 2 , in
some rare cases 3 panels in parallel, but beyond this you must use a fused
combiner to prevent over current fault as your discussing. So for you 1kw
example, say with 8 panels, they would all be fused at the combiner, which
can be bought off the shelf.

Posted by Ecnerwal on December 8, 2006, 1:26 am

You need the fuse per code and common sense. Diodes, unfortunately, can
fail shorted. At which point a failed diode on a failed panel would be
conducting 70+ amps from all the other panels - a thing which tends to
make for smoke, melted insulation, fire, and other not-fun events.

If you are using diodes to prevent night loss, it is worthwhile to get
Schottky diodes, which have a lower forward voltage (0.15-0.45V) than
standard silicon diodes(0.6-0.7V). However, if you are using a "real"
charge controller it should have a means to accomplish this - the better
ones seem to use relays these days, as they have lower forward drop than
a diode, which adds up to power loss for 80+ amps of charge current
flowing, and is not overly hard to manage on less power to hold the
relay than would be lost in the diode, resulting in more power into the
battery, and less blown off as heat.

One big advantage of a downconverting MPPT controller in this situation
is that you can have the panels set up in a smaller number of higher
voltage series strings, since it is only each series string that needs
to be fused (thus, less fuses). Also, with 1 KW of panels, the MPPT will
(per many reports here) give you enough more actual power out of the
panels to pay for itself. However, with a 12 V battery you may need two
MPPT controllers to handle a 1000 watt array properly. The Outback MX60,
for instance, is limited to 70 amps out (at 12, 24 or 48V battery
voltage), so it can handle roughly 800W @ 12V, 1600W @ 24V, and 3200W @
48V - but if you're faced with inverter changes, etc, it's probably more
cost effective to stick with 12V and buy two controllers, especially
since I recall that you want a lot of 12V for radios, etc.

Cats, coffee, chocolate...vices to live by

Posted by on December 8, 2006, 7:13 am
 On Fri, 08 Dec 2006 01:26:23 GMT, Ecnerwal

For 1000W @ 12V I would use a single MX60. If you're getting 1000W
input you're nowhere near 12V. At least 13V would be my guess, making
it handle 910W+. But very seldom do you get rated capacity and if the
voltage is already at 14.3V+, the MX can handle 1000W.

It won't break either, it just limits output to 70A. For the very odd
chance it would do that with a 1000W array, it's not worth the extra
cost to get the last amps.

I'd only consider 2 when getting above 1200W.

Posted by Merlin-7 KI4ILB on December 8, 2006, 10:47 pm
  Thanks all, those were the answers I was expecting.

 One more question about the MX-60.

 Someone stated that it is limited to 70amps output. So if I set up the
panels for 48 volts the mx-60 is still limited to that 70 amps and the only
way I could get more out of it is to up the battery bank voltage, correct? I
just want to be sure it is the output that is limited or it changes some
when you change the input voltage.
 Thanks joe

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