Brian: Thanks for the clarification, it saved a
phone call to Outback. That is how I think it
works also. It seems that a current activated
relay to disconnect strings, to bring the charge
rate into line with the battery bank, could be
an answer. I don't know what equipment is
available. A device that could read the current
from a shunt and generate a signal could be a solution.
Jason: The regulation of the CC might take care
of things but I'm concerned that the C/2.5 would
occur for too long before the voltage got high
enough to activate the charge control. AC
coupling could be a problem, since the full
output of a direct grid-tie inverter goes
straight to the batter without any controls
(unless dump or other loads are present). A
coordinated system of dump loads, using a Morningstar Relay driver could work.
Todd: We just got done with an extended power
outage here, and people with direct grid tie PV
are looking for ways to have power when the sun
shines. Many people ran their generators for a
couple of hours at a time to cool their
refrigerators and freezers, then lent them to
their neighbors to do the same. Having power
during the day would have been a great help. A
bank of 4, 220 AH AGM batteries would allow for
CF lights, music and computers after dark.
Batteries are expensive and have a shelf
life. They also take up a lot of space. We are
trying to minimize these issues.
Cheers,
Drake
At 07:35 PM 7/19/2012, you wrote:
Content-Language: en-US
Content-Type: multipart/alternative;
boundary="_000_8961DF9D3453CF41A4A5B6E6BECA4C770B17874813VSSJEXCH01rec_"
Drake,
Ive been hounding a couple of the charge
controller manufactures about this issue, but so
far I just havent seen a light bulb go off in
their heads, but Ill keep trying, and maybe this is a better forum to do it.
MPPT controllers can be adjusted to current
limit at amperage values below their rating, but
if you do that, you are also limiting the output
of the array in general, and the amount of PV
power available to run the loads directly from
the array (through the inverter). Not the best use of available PV power.
Say you have an 80A controller and a 200AH
battery. With current technology, you have two choices:
Let the controller operate at 80A. If you have
loads to draw off some of the current (or sell
to the grid), great, but if not you could be
seeing a charge rate of C/2.5, which would be
pretty hard on a sealed 200AH battery, to say
the least. Granted, the battery voltage would
rise pretty quickly, and the controller would
start to taper off, but it would still see high
currents especially if the absorption time is set long. Not a happy scenario.
Or, you can set the current limiting on the
controller to 20A for a C/10 charge rate. But if
you had loads drawing 60A, you would be pulling
that additional 40A from the battery and not
using the arrays full power. Also not a happy scenario.
What we need is a controller that can read the
signal from a shunt at the battery, and use that
as the basis of current limiting control.
For example, if we have an array that can
produce 80A of current, but we want to limit the
battery to 20A of charge, there would be 60A of
potential current there to run loads without
drawing on the battery. If there are no loads
running, the controller should current limit at
20A (reading from a shunt), but if loads are
turned on, the controller should be able to let
more current through while still limiting the
battery to 20A. When loads are shut off, the
controller should go back to a 20A limit.
This doesnt all have to happen very quickly as
a battery can take a heavier charge for a short
period of time, but I think that this would be a
major improvement of controller function.
Of course, if you are grid-tied you can sell all
the excess power, but if the grid goes down, or you are off-grid
.?
Because of the low cost of PV and the high cost
of batteries these days, Im seeing more and
more requests for large arrays with smaller
batteries. I also think that PV is now cheap
enough to allow for oversizing of arrays for
better battery charging on cloudy days, which
can reduce generator run times. We need smarter controllers.
What say ye, charge controller gurus?
Brian Teitelbaum
AEE Solar
From: re-wrenches-boun...@lists.re-wrenches.org
[mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Drake
Sent: Thursday, July 19, 2012 2:21 PM
To: RE-wrenches
Subject: [RE-wrenches] Small battery bank vs too large array
Hello Wrenches,
Where can I get a device that will measure
current through a shunt and create a signal to trigger a relay?
We want to be able to use a 2 kW array with
four, 200 AH sealed batteries on an Outback
system. 2 kW of PV would be too much amperage
for the batteries. The idea is to open relays
to disengage strings in conditions of high current to the batteries.
The reason for this is to create backup systems
where power will be abundantly available when
the sun shines. The system will normally connect
to the grid, except during outages. In normal
charging conditions the power will go straight
to the grid. When the grid is down, power will
be available for loads and battery charging, but
batteries will be protected from overcharge?
Any suggestions on ways to accomplish this are welcome!
Thanks
Drake
Drake Chamberlin
Athens Electric LLC
OH License 44810
CO License 3773
NABCEP Certified Solar PV Installer
740-448-7328
<http://athens-electric.com/>http://athens-electric.com/
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