[RE-wrenches] Other's thoughts on Autonomy? was concord batteries, EQUALIZE Them!

Walt Ratterman wratterman at sunenergypower.com
Tue Dec 1 14:22:27 PST 2009


Hello Travis,

 

Just for my $.02..

 

I gave up a while ago on the textbook 3 days of autonomy, but for a slightly
different reason (including yours).

 

We would give a battery bank 3 days of autonomy, and then place all kinds of
alarms and load shedding at 50% SOC, but nothing to tell folks when they
have used up one day of their energy.  (This is particularly relevant with
small systems with LVD charge controllers.)   Consequently, everyone (we are
talking about remote installations in the jungle with no alternative) keep
using the power until the alarm sounds.  The panels can charge back up
usually not much more than one day.  The battery lives its (shortened) life
out going between 50% SOC and 70% SOC, unless of course the ever important
generator is part of the system.

 

There is no magic to 3 cloudy days versus 6, or 8 or 12, so .. Might as well
deal with it sooner rather than later..

 

Walt

SunEnergy Power International

 

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Travis
Creswell
Sent: Tuesday, December 01, 2009 2:16 PM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] Other's thoughts on Autonomy? was concord
batteries, 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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