[RE-wrenches] Small battery bank vs too large array

Thu Jul 19 19:52:49 PDT 2012

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,
>
>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.
>
>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 array’s 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 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.
>
>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, 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.
>
>What say ye, charge controller gurus?
>
>Brian Teitelbaum
>AEE Solar
>
>From: re-wrenches-bounces at lists.re-wrenches.org 
>[mailto:re-wrenches-bounces at 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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