Adding battery backup to existing grid-tie PV system [RE-wrenches]

[Max Balchowsky]

William, we've gone back to add backup on some existing systems. I've been
very pleased with the Exeltech MX series. We simply (sometimes simply) add
an emergency panel and the appropriate number of batteries (we used MK
batteries - AGM sealed gel). I like the fact that the Exeltech system is
rack mounted and hot swappable. That's a way over simplification of the
process, but it's clean and we've found it very reliable and easily
expandable if the customer wants to add capacity at a later date. Pete, of
advance solar has installed a lot of these systems. (Think he's retired
living on an island some where by now) the SMA sunny island can also be used
in that application. 

Max Balchowsky
SEE Systems 

-----Original Message-----
From: William Korthof [mailto:wkorthof at earthlink.net] 
Sent: Thursday, May 18, 2006 11:59 PM
To: RE-wrenches at topica.com
Subject: Adding battery backup to existing grid-tie PV system [RE-wrenches]


We have a customer from a several years back who bought a larger system 
(~7kW AC system) which used 3 SMA 2500 and 1 SMA 1800 inverters... they 
always wanted battery backup, but given their high end, low maintenance 
lifestyle, I wasnot eager to throw a pile of batteries, miles of heavy 
48 volt wiring, and dual SW inverters at this application. But when they 
called a couple months ago discussing their extended and unseemly LADWP 
power outage frequency, and apparent willingness to let go of the 
whirlpool, sauna, wine chiller, outdoor light show, and multi-level HVAC 
refrigeration system (at least during power outages), I took note.

Rather than dig in and rewire the PV array (or alternately), install a 
backup power system that didn't use the PV system... we came up with an 
interesting compromise:
install (1) new Outback GVFX inverter with (4) 8D deep cycle batteries,
this inverter is directly grid-tied, like the existing SMA inverters.
On the (backed-up) load side of the inverter, we conneced the frige, a 
few lights, computers, and a couple other important items. In addition, 
we reconnected the (1) SMA 1800 inverter onto the load side of the 
outback. When the outback is online (whether grid-tied or standalone) 
the SMA inverter feeds in power to offset emergency loads (and even 
charge the batteries if the power draw is lower than solar output).

This approach requires redundant inverter capacity (extra cost), but 
allows the PV array to be wired at high voltage, enabling a larger array 
and longer electrical feeder distances if needed.

I'm thinking that this is a reasonable compromise solution (cost, labor, 
performance...) for us to use as a retrofit/upgrade on older systems, as 
well as a decent way for us to furnish battery backup power with systems 
that wouldn't otherwise be compatible (Sunpower).

I'm sure others have already done this... were you satisfied with the 
results? Any issues of note?


/wk


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