A question about bypass diodes:

"Jeff" wrote in message
...
Ian Malcolm wrote:
...

Leaving out any personal attacks *this* time, Mr Ball,
Their approach is actually questionable *FOR A MARINE APPLICATION* as
they make no provision for extracting power if the panel voltage is below
that of the battery. This frequently happens if the panel is partially
shaded and some cells bypass diodes are conducting. This wouuld be a not
uncommon situation aboard an yacht at anchor, on a swinging mooring or
underway. The panel has to deliver over 140ma at over 14V before *any*
power is transferred. Elsewhere they say it is stable down to 0.8A and
as it seems to be designed for 10A nominal output, what proportion of the
time will it not operate? They seem to be excessively concerned about
keeping the converter in continuous operation mode, *Probably* to avoid
violating a competitor's patent.
It looks like it would be advisable to disable their regulator and run a
float regulator if layed up for the winter as the trickle charging
capabilities are suspect to say the least.


I couldn't find anything on the Blue Sky or Outback sites that address the
issue of raising the voltage from a partially shaded panel so the it is at
the charging point for the battery. In theory, there's nothing to prevent
this. But the situation is complicated because many installations have
several panels in parallel (I have three) the shading would not be even.


I suspect that the manufacturer the OP was considering were rather
optimistic in their test conditions for a 30% efficiency boost, they
probably tested it normal to the sun, totally clear sky and at 10 degrees
C ambient temperatue into a very flat battery. At more normal insolation
levels and temperatures, averaged over a normal battery charging cycle,
the 10% to 15% improvement that most other sources claim is more
plausible. After all, you CANNOT get more out than you can get at the
MPP.

The literature specifically says "up to 30%" gain; they certainly don't
claim that will always be possible. In particular, it depends a lot on
the temperature, since panels will put out a much higher voltage when
cold. Thus, the gains would be less in the tropics, more in temperate
zones in colder seasons. Also, gains are more with a depleted battery,
less as the battery is charged. The web sites are pretty up front about
these issues, admitting that gains might be only 10% in some scenarios.
As I've said, the system I checked out had a 25% gain, which the owner
said was typical.

Do most panels have the bypass diodes (to allow current flow through shaded
cells)? The specs for my Shell 100W panels do not mention them. I'm not
talking about the series reverse-current blocking diodes, but the per-cell
bypass. I am considering an MPPT controller, and if the panels have these
diodes, it should be better to connect the panels in series, rather than in
parallel. My installation has three panels, and does have issues with panel
shading. Currently I have the panels in parallel, and use a non-MPPT
controller.

I will probably do some V/I curves for the panels with varying types and
degrees of shading, which should answer the bypass diode question, but until
then any advice would be appreciated.

Thanks,
Paul

Paul wrote:
A question about bypass diodes:
....

Do most panels have the bypass diodes (to allow current flow through shaded
cells)? The specs for my Shell 100W panels do not mention them. I'm not
talking about the series reverse-current blocking diodes, but the per-cell
bypass. I am considering an MPPT controller, and if the panels have these
diodes, it should be better to connect the panels in series, rather than in
parallel. My installation has three panels, and does have issues with panel
shading. Currently I have the panels in parallel, and use a non-MPPT
controller.

I will probably do some V/I curves for the panels with varying types and
degrees of shading, which should answer the bypass diode question, but until
then any advice would be appreciated.

I may be wrong but I don't think most panels come with diodes - mine
don't. In fact, diodes aren't really the right thing to use. The
Blue Sky (and I'm guessing the Outback) comes with a relay that closes
when appropriate and opens to prevent reverse current flow. This is a
much more efficient way to do it.

Paul wrote:

A question about bypass diodes:

snip

Do most panels have the bypass diodes (to allow current flow through shaded
cells)? The specs for my Shell 100W panels do not mention them. I'm not
talking about the series reverse-current blocking diodes, but the per-cell
bypass. I am considering an MPPT controller, and if the panels have these
diodes, it should be better to connect the panels in series, rather than in
parallel. My installation has three panels, and does have issues with panel
shading. Currently I have the panels in parallel, and use a non-MPPT
controller.

I will probably do some V/I curves for the panels with varying types and
degrees of shading, which should answer the bypass diode question, but until
then any advice would be appreciated.

Thanks,
Paul



IIRC once you are over a fairly low reverse voltage on an individual
cell (I belive somewhere around 5V), if any significant current is
available, long term damage tends to occur. Sources indicate that this
isnt a significant problem in simple 12V battery charging setups but
becomes an issue once the difference between the flat battery voltage
and the array OC output excedes this. Check with the supplier if the
cells are rated for series connection in 24V chaging applications as
they should have shunt diodes or better yet just ask them directly.
Also watch out that you dont exceed the maximum input rating of the MPPT
controller, even in direct normally incident sunlight with additional
light being reflected onto the panels and remember that a supply as low
as 32 volts in a wet envirioment can be a lethal shock hazard.

No doubt Mr G. Ball will be along shortly to point out my mistakes.


--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk
[at]=@, [dash]=- & [dot]=. *Warning* HTML & 32K emails -- NUL:
'Stingo' Albacore #1554 - 15' Early 60's, Uffa Fox designed,
All varnished hot moulded wooden racing dinghy.

"Paul" wrote in message
...
A question about bypass diodes:
Do most panels have the bypass diodes (to allow current flow through
shaded cells)? The specs for my Shell 100W panels do not mention them.
I'm not talking about the series reverse-current blocking diodes, but the
per-cell bypass. I am considering an MPPT controller, and if the panels
have these diodes, it should be better to connect the panels in series,
rather than in parallel. My installation has three panels, and does have
issues with panel shading. Currently I have the panels in parallel, and
use a non-MPPT controller.

I will probably do some V/I curves for the panels with varying types and
degrees of shading, which should answer the bypass diode question, but
until then any advice would be appreciated.

Thanks,
Paul



Can't speak for "most," but Unisolar flexible 35's will continue to operate
partially shaded, at reduced output.