David,
Since a PV module is a constant current device, I don't believe this will take
place. For a given amount of light (plus all factors affecting power), the
current potential will be there. What force can act upon the current to reduce
it? Consequently, the lowest Vmp the inverter can trac
Modules aren't exactly constant current
devices.
They are for much of their voltage range, up to a voltage a bit
below the MPP, and then the current starts tapering down to 0 at
Voc. The entire string can only pass as much current as the
lowest current mod
So the string with the highest current AND voltage dictates where the inverter
MPP locks. Other strings are "pulled" up past their MPP which means lower
current from them. Makes sense.
I was only considering constant current under a load equal to the potential. In
that case a module is constant
I question one part of this conclusion,
but have not set up any tests. The only part I question is "the
string with the highest current AND voltage dictates where the
inverter MPP locks". Rather, it seems to me that the voltage at
which the combined current of all
Kirk:
As noted below the current will be shared by all modules in the string and the
voltage will be the sum of the operating voltage at that current.
When mixed modules are used this results in differences between the operating
voltages of each string in the system.
Having parallel strings with
Allan:
I posted an example using Kirk's original quesiton but for some reason it has
not showed up yet.
You are mostly right.
>>IF<< the MPP tracker works correctly it will pick the global MPP point.
Some MPP trackers get lost and fins a local MPP and you end up losing a bunch
of energy.
In re
Hello Wrenches,
Please provide your opinions on the proper charging voltages (e.g. bulk, float,
eq,), and recommended durations.
The bank consists of 12 - Rolls 8CS25P (820AH @ 20hr rate) (4 strings of 3).
Design based partly on budget limitations.
FYI. The system is off grid and consists of a
Wayne,
In the first place, 4 parallel strings of anything is a very bad idea. Micro
resistances in cabling and connections plus eddy currents and the like will,
over time, inherently unbalance the pack. Doing frequent EQ charges will be
required but will not overcome the inherently bad design of
Well, if had been set up at 48V instead
of 24V, it would have been a well-designed system. Would have cost
less, too - two parallel strings of batteries instead of four for
the same capacity, and three controllers instead of five. But as
it is...
Your
The system is pre-existing. This is a battery replacement only. I agree that
fewer strings are best. The 8CS25 were the most AH for the money. However, the
12 - 8CS 25's is a big step up from the 32 - S530's (8 strings! and 96 holes to
fill).
FYI. As a case study, the S530 bank was 6yrs old befo
I tried to email James but got bounced back.
The system is pre-existing. This is a battery replacement only. I agree that
fewer strings are best. The 8CS25 were the most AH for the money. However, the
12 - 8CS 25's is a big step up from the 32 - S530's (8 strings! and 96 holes to
fill).
FY
this is probably a hair brained question, but has anyone ever tried paralleling
the connections within the series strings?
yes, more cables, but might this reduce the multiple parallel string
unbalancing issues?
todd
On Monday, April 29, 2013 3:53pm, "Wayne Irwin"
said:
The system i
Todd,
I have done it, a few times in my first years of doing this work.
Intermediate parallel interconnects went out of fashion because
they didn't seem to show any benefit. Yes, it gives you more paths
for current flow - but isn't that the problem to be avoided?
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