[RE-wrenches] AC Coupled Magnum Issue

Thu Mar 7 14:32:40 PST 2013

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/ 
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