Thank you to all who responded to this post.
Larry, that sounds like a great potential solution. That will put the
frequency shift in line with where the battery cutoff voltage needs
to be. The frequency shift happens 2 volts above the absorption
charge set point. My only issue with the solution is the 1 1/2 hour
round trip to the site (it could be worse). Maybe Concorde can give
me information that will allow me to set up the charge parameters
without having to tweak.
Allan, dump loads are a great way to go, for sure. The tech I spoke
to at Magnum recommended the relay as a fail safe for the frequency
shift. When the issue of the inverter frequency shifting at 2 volts
above the absorption charge set point came up, the relay became the
primary protective device for the AGM batteries.
We didn't use diversion loads since this is a backup system, and will
probably only operate a few days per year. The inverter should
maintain the batteries rather than the solar. We used the Morningstar
relay driver, in part, to allow dump loads to be added if desired.
Another solution was offered by APRS World, LLC. They likely can
build products that would trigger relays using the temperature
compensated data from the Magnum Network.
It would sure simplify life it Magnum could add an axillary
relay. Would that require putting the inverter, or entire line of
inverters, back into UL for re-approval?
Drake
At 05:36 PM 3/7/2013, you wrote:
Meant to say, "You should tweak #3 & #4 above..."
Drake,
Charging an AGM battery does not have to occur at the maximum charge
voltage allowable; you can charge at a lower voltage for longer
periods of time and still achieve 100% SoC. Since you have grid
power to work with, slow down the charge cycle.
Assuming you have a late model ME-RC and version 5 or higher
inverter, you can custom program the charge voltages. Here's what I would do:
* Go to Custom settings for battery type (setup menu 04).
* Set the Absorb voltage for 26.8.
* Set the Float for 26.4
* Set the absorb time to 8 hours
* Be sure the charge rate( setup menu 05) is 100%.
Near the end of the absorb charge cycle, measure the current going
into the battery. If it is less than 0.2 amps/100AH capacity, the
bank is full. You should #3 & #4 above until it works out for the
low temp. comp. and charge time but it should work well.
If you are concerned about this lower voltage, set up a periodic
equalize charge to 28.8.
Larry Crutcher
Starlight Solar Power Systems
On Mar 7, 2013, at 3:00 PM, Drake wrote:
Hello Wrenches,
I had an unexpected glitch in AC coupling a Magnum 4024 PAE
inverter. The system utilizes AGM batteries, which (according to
Concorde tech support) have a very strict upper voltage limit.
The relay used to stop the current input from the direct grid tie
inverter, when battery voltage is high, is controlled by a
Morningstar Relay Driver. The relay driver is set to shut off the
charge at 28.8 volts.
The charger on the Magnum is set to charge the batteries to 28.6 volts.
The plan was to be able to charge batteries from the Magnum without
tripping the AC connection between the grid tie inverter to the
grid. The Magnum should charge to 28.6 volts, 0.2 volts below the
relay driver's programmed trigger point.
There is one catch to using this approach. The Magnum inverter has a
remote temperature sensor in the battery bank. The relay driver is
not temperature corrected. Therefore, when the batteries are cold,
the charging voltage goes up in the Magnum. This higher voltage
trips the relay controlled by the driver, and disconnects the grid
tie inverter from the grid.
I've played with the voltage set points, but the charge voltage on
the Magnum would be too low for good battery maintenance. If these
were liquid electrolyte batteries, I'd crank up the voltage on the
relay driver. Since they are AGM batteries, it doesn't seem like a
good idea to do that.
The work around for this at present is to;
* Charge batteries at night in the winter
* No problem is expected in the summer
The batteries are in an insulated box in a shed connected to the
house. The inside of the battery box has a screened hole that is
supposed to allow heated air into the box. This vent doesn't keep
the batteries warm enough to prevent the problem.
Is there a temperature sensitive, voltage controlled relay or diver
available? Has anyone used a temperature bulb with the relay driver
in a way that could solve this problem? Has anyone else come up
against this issue, and if so, what was your solution?
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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