Battery Cycle Life Ratings [RE-wrenches]

Darryl Thayer daryl_solar at yahoo.com
Thu Nov 4 06:44:32 PST 2004


 

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John and Allan
Another reason is the older batteries will have a
faster self discharge than new cells, and this will
result in the older batteries discharging the new
cells.  Also a failure on one or two cells in an older
string can cause the new bank to discharge into the
older bank.  this heats the older bank and causes a
drop in voltage and this increases the current.  In
AGM cells the well known case of the batteries that
burned down the house.  
Daryl
--- John Raynes <john at raynes.com> wrote:
> Allan,
> 
> I have an interesting "lab" experiment going on
> presently with a 24 volt 
> L16G battery bank, that speaks to some of your
> questions.  The customer set 
> up a 3-string, 12 battery system when he installed
> his original system 
> (himself) 5-1/2 years ago.  The original system had
> an SW4024, DC well 
> pump, and 720 watts of SX panels.  Started with a
> C40, upgraded to an SB50 
> three years ago.
> 
> Last year, he added a 4th string to increase his
> battery capacity, and an 
> additional 240 watts of PV.  The system wasn't
> keeping up and the generator 
> was coming on way too often.  He does maintain the
> batteries faithfully, 
> thought.  The increased capacity took care of things
> until this past month, 
> when L16's from his original bank began to fail.  He
> asked me what to do 
> (he definitely wanted to replace as few batteries as
> possible).  I gave him 
> the standard recommendation that all 12 older cells
> should be replaced, but 
> at the very least, make sure to only replace an
> entire string at a 
> time.  Since he had 3 dead cells at that point, he
> started by replacing one 
> string.
> 
> Anyway, here's some interesting data: He wanted to
> equalize the bank after 
> we inserted the new string.   I told him that
> sounded good, but that I 
> wanted to make measurements at the start and end of
> equalization.  He uses 
> a 10KW generator for equalization, so we were
> getting a very constant 
> equalization charge.  Here are the individual string
> charging amperes measured:
> 
>                    15 min.  1hr 45 min.
> Brand new string   4.7          4.3
> 1yr old string     6.3          6.1
> 1st orig. string  10.4         10.6
> 2nd orig. string  33.1         44.0!
> 
> In the string that had the excessively high
> currents, we identified a 
> battery with a bad cell at the end of equalization -
> it must have failed 
> when we shuffled the batteries around.   The other
> measured differences are 
> not likely attributable to cable connections, as all
> battery cables were 
> completely cleaned and redressed when the batteries
> were replaced.
> 
> I'm not completely sure what to make of this data,
> but it definitely seems 
> to me from these limited observations that older
> strings will hog available 
> charge current, thereby potentially decreasing the
> number of days that the 
> newer strings will be able to obtain a full 100%
> acceptance charge.  That 
> can't be good.
> 
> I'm also wondering if equalization on older cells
> near end of life should 
> be done much less often, particularly if sustained
> undercharging is not an 
> issue.  In the newer strings above, charge currents
> decayed slightly over 
> the equalization cycle, as I'd expect.  On the other
> hand they increased 
> somewhat in the older string that had no failed
> cells.  That shouldn't 
> happen.  And in the string with the bad cell, it
> looks like the EQ cycle 
> might actually have been the thing that took it over
> the edge!
> 
> I'm certain (from some other observations that I've
> made on other systems) 
> that measured differences in charge amperes are
> observed at lower voltages 
> as well (acceptance/float potentials), they're just
> not as pronounced.  Is 
> anyone making these kind of string charging current
> observations on their 
> own (or their customer's) systems, and what are you
> finding?  We're going 
> to keep much closer watch from this point on.  I'm
> beginning to think that 
> final acceptance charge current measurements might
> be a very useful 
> predictor of battery wear and expected remaining
> life.
> 
> BTW, he's now replacing the entire bank.  Once the
> cells start going....
> 
> John Raynes
> RE Solar
> Torrey UT
> 435-325-2110
> www.re-solar.com
> 
> 
> At 10:49 AM 11/3/04 -0700, you wrote:
> >
> >
> >Wrenches,
> >As I follow this battery life thread, and we learn
> that some of our
> >long-held assumptions may be myths (such as that
> L16s inherently last longer
> >than golf carts) a couple of other questions come
> to mind:
> >
> >1. It has been commonly understood that new flooded
> batteries are never to
> >be mixed with old batteries; the limit is 6 months
> to about a year,
> >depending on use. Why? Is it, as I have believed,
> that the new batteries
> >would only perform as well as the old--that is,
> remaining cycle life,
> >charging characteristics, etc. would match the
> older cells. Is it worse than
> >this? Will the mixed bank perform worse than the
> old cells, or cause the
> >entire bank to fail prematurely? What if eight 6V
> batteries are doubled to
> >16, and the new and old units are evenly split
> among each parallel string
> >(in either 24V or 48V configuration)?
> >
> >2. I have also designed systems with smaller banks
> relative to charging
> >capacities, based on manufacturers' recommendations
> for best performance.
> >But a C/15 to C/20 rate is still hard to achieve,
> and can lead to excessive
> >discharges if the owner isn't attentive. In the
> early days of offgrid
> >system, battery banks were way oversized relative
> to arrays (I still get old
> >timers thinking in terms of one "battery" (a golf
> cart) to one module (which
> >back then was a 35 watt Arco 16-2000 or the like).
> That's about a C/100
> >charge rate.
> >
> >What I have noticed is that the grossly oversized
> battery banks batteries in
> >these old systems still tended to last about 5-8
> years; that is, about as
> >long as well-sized banks. So I could use some
> real-world discussion of how
> >others size batteries to arrays and loads, outside
> of all the textbook
> >numbers.
> >
> >Allan at Positive Energy
> 


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