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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.<br><br>
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. <br><br>
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. <br><br>
Cheers,<br><br>
Drake <br><br>
At 07:35 PM 7/19/2012, you wrote:<br>
<blockquote type=cite class=cite cite="">Content-Language: en-US<br>
Content-Type: multipart/alternative;<br>
<x-tab> </x-tab>
boundary="_000_8961DF9D3453CF41A4A5B6E6BECA4C770B17874813VSSJEXCH01rec_"<br>
<br>
Drake,<br>
<br>
I’ve been hounding a couple of the charge controller manufactures about
this issue, but so far I just haven’t seen a light bulb go off in their
heads, but I’ll keep trying, and maybe this is a better forum to do
it.<br>
<br>
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.<br>
<br>
Say you have an 80A controller and a 200AH battery. With current
technology, you have two choices:<br>
<br>
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.<br>
<br>
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 array’s full power.
Also not a happy scenario.<br>
<br>
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. <br>
<br>
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.<br>
<br>
This doesn’t 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.<br>
<br>
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….?<br>
<br>
Because of the low cost of PV and the high cost of batteries these days,
I’m 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.<br>
<br>
What say ye, charge controller gurus?<br>
<br>
Brian Teitelbaum<br>
AEE Solar<br>
<br>
<b>From:</b> re-wrenches-bounces@lists.re-wrenches.org
[<a href="mailto:re-wrenches-bounces@lists.re-wrenches.org" eudora="autourl">
mailto:re-wrenches-bounces@lists.re-wrenches.org</a>] <b>On Behalf Of
</b>Drake<br>
<b>Sent:</b> Thursday, July 19, 2012 2:21 PM<br>
<b>To:</b> RE-wrenches<br>
<b>Subject:</b> [RE-wrenches] Small battery bank vs too large array<br>
<br>
Hello Wrenches,<br>
<br>
Where can I get a device that will measure current through a shunt and
create a signal to trigger a relay? <br><br>
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.<br><br>
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?<br><br>
Any suggestions on ways to accomplish this are welcome!<br><br>
Thanks<br><br>
Drake <br><br>
<br>
Drake Chamberlin<br>
<i>Athens Electric LLC<br>
OH License 44810<br>
CO License 3773<br>
NABCEP Certified Solar PV Installer<br>
740-448-7328<br>
</i><a href="http://athens-electric.com/">http://athens-electric.com/</a>
<br>
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