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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