On Tue, 26 Dec 2006 13:08:48 -0500, Larry wrote:
"BF Lake" wrote in
news:JPbkh.99541$hn.5563@edtnps82:

Don't follow. (also haven't seen previous part of this) ISTR the
purpose of the blocking diode is to prevent the battery from charging
the solar panel at night.

Stops the battery from reverse current through the solar panel when solar
panel voltage is lower, not just at night. This causes damage to the

Yup. There is also another type of diode necessary with higher-voltage
(24v, 48v, etc) series connected panels and a good idea with a single
panel connected to a battery. People often confuse a blocking diode
with a bypass diode. The bypass diode protects the panel when cells are
shaded, etc. and most higher-end panels include a bypass diode, but few
include a blocking diode.

Why? Because PV cells are by nature a diode, and usually have very low
leakage. You don't normally need a blocking diode. However as you
increase the number of cells in parallel, or increase the size of each
cell (in other words, increase cell area) the leakage increases. If
you have a reasonably modern electronic charge controller, it will
function as a super-efficient blocking diode (much less than 0.6v drop).
You don't need another one.

Kitchen theories about it reducing solar panel output are nonsense. Solar

Wrong.

Let's say you have a solar array producing 10 amps (call it effectively
120watts). If your diode drops 0.6v, you are losing 6watts in that
diode. 6watts of 120watts is a 5% loss. I think 5% is significant.

panel open circuit voltages are above 18V in bright sunlight. Six tenths

Not necessarily correct. Panels are not designed for the blocking
diode, they are designed to provide high enough voltage to fully charge
a battery (14.5v) even when the panel is much hotter than room
temperature because it is sitting in the sun on a hot, still day. The
voltage produced by each cell may drop from 0.55v typical at 20C to 0.4v
at 90C. That is barely enough to do a full charge. Some people cheap
out and buy a 33 cell panel instead of a 36 cell. Unless you live where
it never gets hot, the 33 cell panels are proven to not have enough
oomph when they get hot.

sdb
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