[RE-wrenches] Other's thoughts on Autonomy?

Wed Dec 2 11:35:13 PST 2009

Hi Joel,

I think we are all pretty much on the same page.

I'm speaking mainly about a full time off grid residence, commonly with
flooded lead acid and of course a fossil back up generator.

Most of my full time off gridders don't even need to their generator from
late April to early October.  They report that it's normal that by 10 or 11
am their CC is in float.  Then winter sets in with frequent 2-3 week periods
of clouds.  Like Walt said, where is the magic cloudy day number for me?  Is
it the average of 3 weeks of sun and 3 weeks of clouds for 1.5 weeks of
autonomy?  That's grossly oversized in the summer and still inadequate in
the winter.

>From my view, if you need xxx kWh's per month to live that's how many you
need, and days of autonomy aren't that meaningful when you can only generate
1/3rd of that with your array.  You have to run the generator just as many
hours per month regardless of how many days of autonomy, right?  And I
believe that if you factor in charge efficiency and self discharge the
larger the bank has a few more hrs per month of generator usage.

Thanks for sharing your thoughts everyone.

Best,

Travis Creswell

Ozark Energy Services

  _____  

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Joel
Davidson
Sent: Tuesday, December 01, 2009 6:38 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] Other's thoughts on Autonomy?
wasconcordbatteries, EQUALIZE Them!

Hello Travis,

I think that 2 days is not enough battery autonomy in the Ozarks and many
other locations unless you have a fossil fuel generator to carry you through
long cloudy periods. Battery autonomy is site and load specific. I've done
systems with as little as 1 day and as much as 3 weeks at 80% depth of
discharge. Lately, I've been generically specing 1.5 days of autonomy at 50%
d.o.d. to get the dialogue started with the customer.

I use to spec up to 4 parallel strings of T-105s or L-16s in 2, 4, and 8
batteries in series, but now I keep the number of strings down to 3 or less
and prefer 1 or 2 strings of big 2-volt cells to reduce the number of cells
and connections.

Southern California urban and suburban grid-tie PV systems are almost all
non-battery although we still get asked about emergency power - until they
hear how much it adds to the cost of a grid-tie PV system.

Joel Davidson

----- Original Message ----- 

From: Travis Creswell <mailto:tcreswell at ozarkenergyservices.com>  

To: 'RE-wrenches' <mailto:re-wrenches at lists.re-wrenches.org>  

Sent: Tuesday, December 01, 2009 2:15 PM

Subject: Re: [RE-wrenches] Other's thoughts on Autonomy? was
concordbatteries, EQUALIZE Them!

IMHO, one of the worst design boo-boo's is going past more then 2 days of
autonomy.  Personally, I no longer size much over one day because it's my
anecdotal observation that most batteries die of old age and being ignored
long before cycles get them.  Speaking mostly about quality deep cycle
flooded.

Lots of good things result;

-50%-75% smaller battery bank means a $20,000 battery bank just turned into
$5,000 bank which frees up a ton of money for more modules and now-a-days
you can buy a lot more PV with that money.  More array mean far less
reliance on autonomy.  I'll take the trade all year long.  In the summer we
have 3 to 4 weeks of sun and one day of clouds and in the winter we get 3 to
4 week stretches with 1 sunny day.  Autonomy doesn't really matter in either
case from what I've seen.  The larger the bank means more self discharge
losses, which on large battery banks gets significant as they age.  5-15
years later you'll still have all that array but no matter what you're
looking at new battery bank.

-If you study the quality deep cycle manufacturers literature you'll see
that you'll see that anything over 1 day of autonomy is too much to allow
the array to actually charge the battery bank anywhere near the recommended
amps and just like rust, sulfation never sleeps.

-Less cells to water

-Less space required

-Given that a surprisingly high percentage of off gridders totally screw up
on their first bank, no matter how much we all try we might as keep the
stupid tax of replacing a 2.5 yr old battery bank to a minimum.

-All of this discussion about cross paralleling, buss bars, TLC with a
gazillion connections and multiple strings goes away.

-And the best part is we don't have to carry all of the lead into the
basement and even better back out of the basement!

Just my .02.  Feel free to strongly disagree but let's be polite about it.

Travis Creswell

Ozark Energy Services

  _____  

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of R Ray
Walters
Sent: Tuesday, December 01, 2009 12:44 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] concord batteries, EQUALIZE Them!

I used to think that one string was optimal; until I had a single cell
failure take out an entire system for weeks. (try operating a 24 v system at
22v! )

I now think that 2 parallel strings is optimum,  3 is OK, and 4 is max.

At 4 parallel strings, we start spending more time looking to make sure all
connectors are the same exact length etc. to insure equal operation.

But of course how do you account for varying internal resistance of the
batteries......??

I've done 4 parallel strings at 144 DC of sealed batteries on an electric
vehicle, but we were very careful with our resistances, I even switched to
smaller wire, on closer strings, and calculated out the exact resistance, so
all strings were theoretically equal. This set actually just died, but
achieved its manufacturer's predicted cycle life. (B&B battery, 350 cycles
to 80% DOD)

So if you're careful, 4 strings can work well.

Worst I've seen was 20 golf carts paralleled in a 12 v system, (10 strings)
and they didn't pull the main connections from across the set, just
connected to one end.

The results were very predictable, with the furthest batteries being
chronically under charged, and the closest ones being over cycled to a
premature death.

Ray Walters

On Dec 1, 2009, at 11:28 AM, Windsun at wind-sun.com wrote:

You gotta wonder about why the customer bought such a battery layout, or why
the installer sold that kind of configuration (which ever it was) with so
many small batteries. We would never recommend going over 2 parallel banks,
but sometimes the "customer knows best...".

  _____  

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