[RE-wrenches] Back feeding battery inverter with grid-tieinverter

[Steve Johnson]

(This has already been said a few times but I am testing my new email on the list and might as well put in my 2 cents. the first one bounced back so I'm checking registration again)  steve



Jeff,



Why not send DC back to the garage and set your Sunny boy there.

By sending 350VDC back you will have considerably less voltage drop to deal with than 240 VAC.  There is no charge controller as it is AC coupled.



I know the old SW4024 takes AC both ways on the AC in.



I am doing a similar installation, AC coupled, but with outback VFX instead of Xantrex.

The outback will not backfeed the grid as it is an off grid inverter.  It has an internal transfer switch.  The Aux relay has to be connected to a relay to disconnect the sunny boy ac circuit to prevent of overcharging the batteries as there is no charge controller.  So yes the sb charges the batteries and feeds the subpanel both.



When the grid fails the sunny boy disconnects.  After 5 minutes it will see the ac from the outback battery backup and reconnect (thru the emergency panel).  Outback has an ac coupling paper on their website.  I am not familiar with the new xantrex's.



Steve



-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Jeff Yago
Sent: Sunday, January 18, 2009 11:31 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] Back feeding battery inverter with grid-tieinverter



I know this subject has come up before, but there have been several different suggested ways to handle the problem and I was hoping for easiest.  We install lots of both on grid and off grid systems.  However, we have a client that will require a ground mounted array much futher away from the inverter and batteries in the garage than we normally would want to do since the voltage drop for this long distance would be excessive at low voltage even with large cables.  We have decided to locate a weather-proof sunnyboy grid-tie inverter at the array, and supply 120/240 volts to the critical loads sub-panel which is supplied by a Xanrex XW inverter and battery bank in the garage. I realize there are several design issues here when the grid is down,  since we want the grid-tie inverter to remain on but be blocked from the grid by the Xantrex inverter.



The question is, what is the best way to keep a load on the sunnyboy since it will not be grid connected during an outage.  Also, is there a best way to feed the output from the Sunnyboy to the Xantrex that does not just power the sub-panel loads during an outage, but also will re-charge the battery bank through the Xanrex?



If you back-feed the Xanrex from the sub-panel it is normally supplying with output from the Sunnyboy, will the Xanrex see this input and think the grid is up and be able to re-charge the battery bank?



Will we need any separate transfer switches or power relays?



Thanks,



Jeff Yago



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