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Is a blocking diode really needed ?

D

daveem Dave M

Jan 1, 1970
0
Hi all,
I was just measuring the current flow between the solar panel and my
batteries at night, to see what kind of loss I was dealing with, because
I don't use a charge controller, and I was surprised to see it was only
about 10 milliamps, When I considered placing a blocking diode in the
system to pervent this small loss, it appeared that the .3 to .6 volt
drop, depending on the diode used, would create more of a loss than any
night time discharge current, and I wondered if I am missing something
here?
Thanks in advance.
Dave
 
W

Windsun

Jan 1, 1970
0
Older panels used to have a lot more leakage back. We have found the same
thing on newer panels, you lose more with the diode in daytime than you
would lose back at night.
 
W

Windsun

Jan 1, 1970
0
Uhm.. no.

A blocking diode has nothing to do with that. That is an isolation or bypass
diode.
 
J

john

Jan 1, 1970
0
daveem said:
Hi all,
I was just measuring the current flow between the solar panel and my
batteries at night, to see what kind of loss I was dealing with, because
I don't use a charge controller, and I was surprised to see it was only
about 10 milliamps, When I considered placing a blocking diode in the
system to pervent this small loss, it appeared that the .3 to .6 volt
drop, depending on the diode used, would create more of a loss than any
night time discharge current, and I wondered if I am missing something
here?
Thanks in advance.
Dave
check the panel to see if there is already some sort of bd. alot already
have them built in. the panel since it is silicon will not let much
current flow in the reverse direction however, the diode is really to
prevent the panel from shorting which will happen with the reverse polarity.
 
B

boB

Jan 1, 1970
0
Hi john;




In his case this diode is not present as he is able to
measure the current.


This is not true. The silicon cells are basically large series
connected diodes with the cathode on the minus an the anode
on the positive. So if sufficient voltage is applied high
currents can flow.

One of the primary reasons there are usually 36 cell in a
"12V" panel is to ensure that the the battery doesn't have
enough voltage to cause much current to flow through the
cells when in the dark. With less cells more current will
flow.

In electronics we would call this "forward" current flow.
The solar industry calls this leakage or "reverse" current.
But this is a misnomer.


Technically this is not correct.
If the panel doesn't have parallel or "bypass" diodes one
could apply quite high voltage. Maybe as high as 5V to 10V
per series cell. At some point real reverse leakage
current will flow and possibly cells breakdown and short.
These panels are not harmed when applied to a battery
backwards.

However, many panels do have bypass diodes. When these panels
are connected to a battery backwards the bypass diodes will
be burned out destructively. The series blocking diodes would
prevent this damage.

I have written a bit about these diodes at:
http://www.redrok.com/up980301.htm#diodes

Duane



SO, I wonder what the PV manufacturers have to say, and/or what the
real problems (if any) there are when a PV array is driven (with
limited current possibly) at a bit higher than the arrays' Voc ???????
(I think that's what you refered to it as leakage)


I would think they would just dissipate that power with their large
surface area. They do get warm in normal operation as well.

Maybe it's an avalanche or zener rating... I've never seen any
specifications or info on this.

boB
 
B

boB

Jan 1, 1970
0
Hi boB;




Duane

Yes, of course... We don't normally think of that many diodes in
series this way. That's the way to look at it. From my limited
experience (but some) the drop is temerature related just like the
Voc and should follow go up with the Voc at X mV/degress C.


boB
 
B

boB

Jan 1, 1970
0
Yes, of course... We don't normally think of that many diodes in
series this way. That's the way to look at it. From my limited
experience (but some) the drop is temerature related just like the
Voc and should follow go up with the Voc at X mV/degress C.


boB
OOps ! I meant should go down with increasing temperature.

boB
 
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