Down-sizing battery systems [RE-wrenches]

Allan Sindelar allan at positiveenergysolar.com
Sun Jan 27 07:13:15 PST 2008


Jeff,
I work in an area with a long off-grid history (thanks, Windy!) so I have
thought about this a bit. It always seemed to me that early off-grid systems
had a tremendously high ratio of storage to input: I have had more than one
old-timer ask if we still sized to one-battery-per-module. Keep in mind that
one module was typically 35W or 2.2A, and a battery was a golf cart; that's
a C/100 charging rate. I have held that a C/30 rate is a reasonable minimum
for good battery care, simply to get the oomph to fill the batteries, with
C/15 to C/20 as ideal.

I seek three days with a backup charging source and 5-6 (with an admonition
to expect shorter battery life) without. This is in sunny New Mexico, but we
still get occasional weeks of cloudy weather. I shoot for higher nominal
system voltages to minimize parallel battery strings.

With grid-tie-with-backup, it's a completely different story. The usual
objective is 2-4 hours of autonomy, to handle a typical utility outage of a
couple of hours. A 48V string of sealed group 27s (such as fit in a PS1-BE)
is 4 kW-hrs of usable energy feeding a mainstream home's backed-up loads of
heat, fridge, etc. A larger bank is either very expensive if sealed or hard
to properly maintain if flooded.

Allan at Positive Energy

----- Original Message ----- 
From: "Jeff Yago" <jryago at netscape.com>
>
> Over the years I have noticed we are designing smaller and smaller battery
banks for both grid-connected and total off-grid applications.
>
> I think this may have come from the pre-90's solar design goals that
off-grid applications should have 4 to 5 day of autonomy.  This was easy to
do when the only electrical loads were a refrigerator, a few lights, and a
small television.  However, this is harder and harder to achieve these days
as more and more homeowners consider multiple computers, a satellite
receiver, high-speed Internet, recessed lighting, a 60" television with
theater-sound, and a dish-washer as must-have  necessities.
>
> To reduce battery space requirements and costs, we are starting to install
off-grid systems with fewer days of battery-only capacity, and relying on a
back-up generator to re-charge when solar is not available. I also think
since it is now easier to sell back to the grid, perhaps there is less need
for huge battery banks in battery back-up type solar systems to absorb all
that excess power when installing a large solar array.
>
> Anybody out there doing battery based systems that are starting to notice
these market trends?
>
> Jeff Yago
>
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