Down-sizing battery systems [RE-wrenches]

Joel Davidson joel.davidson at sbcglobal.net
Sun Jan 27 14:55:21 PST 2008


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I use to design for no greater than 40% depth of discharge to allow for 
unexpected consumption or bad weather, but found that users were not fully 
charging their battery bank or not equalizing often enough. Now I design for 
no greater than 50% d.o.d. and 3 days of autonomy with enough PV to match 
average daily consumption and instructions to equalize 4 to 6 times a year. 
Still, some customers can only afford a small so I tell them to expect to 
run their fossil fuel genset more often and only get 3 years at best from 
their lead-antimony flooded batteries.
Joel Davidson

----- Original Message ----- 
From: "Allan Sindelar"
To: <RE-wrenches at topica.com>
Sent: Sunday, January 27, 2008 7:13 AM
Subject: Re: Down-sizing battery systems [RE-wrenches]


>
> 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"
>>
>> 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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