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

[penobscotsolar at midmaine.com]

Hi Jeff,
    This isn't an answer to the questions below, but an alternate
suggestion. Apollo Solar has a charge controller which allows for
strings of up to 200 VOC (160 volts nominal) and I believe 100 amps.
Would this not be a simpler, more efficient system?
Just a thought...
Daryl DeJoy
NABCEP Certified PV installer
Penobscot Solar Design




> 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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